Hi all,
Today's linux-next build (powerpc ppc44x_defconfig) failed like this:
In file included from arch/powerpc/kvm/booke.c:31:
arch/powerpc/kvm/timing.h: In function 'kvmppc_account_exit_stat':
arch/powerpc/kvm/timing.h:51: error: bit-field '<anonymous>' width not an integer constant
In file included from arch/powerpc/kvm/booke.h:26,
from arch/powerpc/kvm/booke_emulate.c:23:
arch/powerpc/kvm/timing.h: In function 'kvmppc_account_exit_stat':
arch/powerpc/kvm/timing.h:51: error: bit-field '<anonymous>' width not an integer constant
Presumably caused by commit 8c87df457cb58fe75b9b893007917cf8095660a0
("BUILD_BUG_ON(): fix it and a couple of bogus uses of it").
I applied the following patch for today. This inline function is
only called from one place in one file ...
From: Stephen Rothwell <[email protected]>
Date: Thu, 24 Sep 2009 15:13:20 +1000
Subject: [PATCH] powerpc/kvm: build fix for new BUILD_BUG_ON
Signed-off-by: Stephen Rothwell <[email protected]>
---
arch/powerpc/kvm/timing.h | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/arch/powerpc/kvm/timing.h b/arch/powerpc/kvm/timing.h
index bb13b1f..4c34099 100644
--- a/arch/powerpc/kvm/timing.h
+++ b/arch/powerpc/kvm/timing.h
@@ -48,7 +48,7 @@ static inline void kvmppc_set_exit_type(struct kvm_vcpu *vcpu, int type) {}
static inline void kvmppc_account_exit_stat(struct kvm_vcpu *vcpu, int type)
{
/* type has to be known at build time for optimization */
- BUILD_BUG_ON(__builtin_constant_p(type));
+ //BUILD_BUG_ON(__builtin_constant_p(type));
switch (type) {
case EXT_INTR_EXITS:
vcpu->stat.ext_intr_exits++;
--
1.6.4.3
--
Cheers,
Stephen Rothwell [email protected]
On Thu, 2009-09-24 at 15:21 +1000, Stephen Rothwell wrote:
> Hi all,
>
> Today's linux-next build (powerpc ppc44x_defconfig) failed like this:
>
> In file included from arch/powerpc/kvm/booke.c:31:
> arch/powerpc/kvm/timing.h: In function 'kvmppc_account_exit_stat':
> arch/powerpc/kvm/timing.h:51: error: bit-field '<anonymous>' width not an integer constant
> In file included from arch/powerpc/kvm/booke.h:26,
> from arch/powerpc/kvm/booke_emulate.c:23:
> arch/powerpc/kvm/timing.h: In function 'kvmppc_account_exit_stat':
> arch/powerpc/kvm/timing.h:51: error: bit-field '<anonymous>' width not an integer constant
>
> Presumably caused by commit 8c87df457cb58fe75b9b893007917cf8095660a0
> ("BUILD_BUG_ON(): fix it and a couple of bogus uses of it").
First, I think there is a real bug here, and the code should read like
this (to match the comment):
/* type has to be known at build time for optimization */
- BUILD_BUG_ON(__builtin_constant_p(type));
+ BUILD_BUG_ON(!__builtin_constant_p(type));
However, I get the same build error *both* ways, i.e.
__builtin_constant_p(type) evaluates to both 0 and 1? Either that, or
the new BUILD_BUG_ON() macro isn't working...
> I applied the following patch for today. This inline function is
> only called from one place in one file ...
It's also called via kvmppc_account_exit() from a number of places.
--
Hollis Blanchard
IBM Linux Technology Center
>>> Hollis Blanchard 09/29/09 2:00 AM >>>
>First, I think there is a real bug here, and the code should read like
>this (to match the comment):
> /* type has to be known at build time for optimization */
>- BUILD_BUG_ON(__builtin_constant_p(type));
>+ BUILD_BUG_ON(!__builtin_constant_p(type));
>
>However, I get the same build error *both* ways, i.e.
>__builtin_constant_p(type) evaluates to both 0 and 1? Either that, or
>the new BUILD_BUG_ON() macro isn't working...
No, at this point of the compilation process it's neither zero nor one,
it's simply considered non-constant by the compiler at that stage
(this builtin is used for optimization, not during parsing, and the
error gets generated when the body of the function gets parsed,
not when code gets generated from it).
Jan
On Tue, Sep 29, 2009 at 11:28 AM, Jan Beulich <[email protected]> wrote:
>>>> Hollis Blanchard ?09/29/09 2:00 AM >>>
>>First, I think there is a real bug here, and the code should read like
>>this (to match the comment):
>> ? ?/* type has to be known at build time for optimization */
>>- ? ?BUILD_BUG_ON(__builtin_constant_p(type));
>>+ ? ?BUILD_BUG_ON(!__builtin_constant_p(type));
>>
>>However, I get the same build error *both* ways, i.e.
>>__builtin_constant_p(type) evaluates to both 0 and 1? Either that, or
>>the new BUILD_BUG_ON() macro isn't working...
>
> No, at this point of the compilation process it's neither zero nor one,
> it's simply considered non-constant by the compiler at that stage
> (this builtin is used for optimization, not during parsing, and the
> error gets generated when the body of the function gets parsed,
> not when code gets generated from it).
>
> Jan
then maybe
if(__builtin_constant_p(type))
BUILD_BUG_ON(1);
would work?
Roel
On Tue, 2009-09-29 at 10:28 +0100, Jan Beulich wrote:
> >>> Hollis Blanchard 09/29/09 2:00 AM >>>
> >First, I think there is a real bug here, and the code should read like
> >this (to match the comment):
> > /* type has to be known at build time for optimization */
> >- BUILD_BUG_ON(__builtin_constant_p(type));
> >+ BUILD_BUG_ON(!__builtin_constant_p(type));
> >
> >However, I get the same build error *both* ways, i.e.
> >__builtin_constant_p(type) evaluates to both 0 and 1? Either that, or
> >the new BUILD_BUG_ON() macro isn't working...
>
> No, at this point of the compilation process it's neither zero nor one,
> it's simply considered non-constant by the compiler at that stage
> (this builtin is used for optimization, not during parsing, and the
> error gets generated when the body of the function gets parsed,
> not when code gets generated from it).
I think I see what you're saying. Do you have a fix to suggest?
--
Hollis Blanchard
IBM Linux Technology Center
>>> roel kluin <[email protected]> 29.09.09 11:51 >>>
>On Tue, Sep 29, 2009 at 11:28 AM, Jan Beulich <[email protected]> wrote:
>>>>> Hollis Blanchard 09/29/09 2:00 AM >>>
>>>First, I think there is a real bug here, and the code should read like
>>>this (to match the comment):
>>> /* type has to be known at build time for optimization */
>>>- BUILD_BUG_ON(__builtin_constant_p(type));
>>>+ BUILD_BUG_ON(!__builtin_constant_p(type));
>>>
>>>However, I get the same build error *both* ways, i.e.
>>>__builtin_constant_p(type) evaluates to both 0 and 1? Either that, or
>>>the new BUILD_BUG_ON() macro isn't working...
>>
>> No, at this point of the compilation process it's neither zero nor one,
>> it's simply considered non-constant by the compiler at that stage
>> (this builtin is used for optimization, not during parsing, and the
>> error gets generated when the body of the function gets parsed,
>> not when code gets generated from it).
>>
>> Jan
>
>then maybe
>
>if(__builtin_constant_p(type))
> BUILD_BUG_ON(1);
>
>would work?
Definitely not - this would result in the compiler *always* generating an
error.
Jan
>>> Hollis Blanchard <[email protected]> 30.09.09 01:39 >>>
>On Tue, 2009-09-29 at 10:28 +0100, Jan Beulich wrote:
>> >>> Hollis Blanchard 09/29/09 2:00 AM >>>
>> >First, I think there is a real bug here, and the code should read like
>> >this (to match the comment):
>> > /* type has to be known at build time for optimization */
>> >- BUILD_BUG_ON(__builtin_constant_p(type));
>> >+ BUILD_BUG_ON(!__builtin_constant_p(type));
>> >
>> >However, I get the same build error *both* ways, i.e.
>> >__builtin_constant_p(type) evaluates to both 0 and 1? Either that, or
>> >the new BUILD_BUG_ON() macro isn't working...
>>
>> No, at this point of the compilation process it's neither zero nor one,
>> it's simply considered non-constant by the compiler at that stage
>> (this builtin is used for optimization, not during parsing, and the
>> error gets generated when the body of the function gets parsed,
>> not when code gets generated from it).
>
>I think I see what you're saying. Do you have a fix to suggest?
The one Rusty suggested the other day may help here. I don't like it
as a drop-in replacement for BUILD_BUG_ON() though (due to it
deferring the error generated to the linking stage), I'd rather view
this as an improvement to MAYBE_BUILD_BUG_ON() (which should
then be used here).
Jan
On Wed, 2009-09-30 at 07:35 +0100, Jan Beulich wrote:
> >>> Hollis Blanchard <[email protected]> 30.09.09 01:39 >>>
> >On Tue, 2009-09-29 at 10:28 +0100, Jan Beulich wrote:
> >> >>> Hollis Blanchard 09/29/09 2:00 AM >>>
> >> >First, I think there is a real bug here, and the code should read like
> >> >this (to match the comment):
> >> > /* type has to be known at build time for optimization */
> >> >- BUILD_BUG_ON(__builtin_constant_p(type));
> >> >+ BUILD_BUG_ON(!__builtin_constant_p(type));
> >> >
> >> >However, I get the same build error *both* ways, i.e.
> >> >__builtin_constant_p(type) evaluates to both 0 and 1? Either that, or
> >> >the new BUILD_BUG_ON() macro isn't working...
> >>
> >> No, at this point of the compilation process it's neither zero nor one,
> >> it's simply considered non-constant by the compiler at that stage
> >> (this builtin is used for optimization, not during parsing, and the
> >> error gets generated when the body of the function gets parsed,
> >> not when code gets generated from it).
> >
> >I think I see what you're saying. Do you have a fix to suggest?
>
> The one Rusty suggested the other day may help here. I don't like it
> as a drop-in replacement for BUILD_BUG_ON() though (due to it
> deferring the error generated to the linking stage), I'd rather view
> this as an improvement to MAYBE_BUILD_BUG_ON() (which should
> then be used here).
Can you be more specific?
I have no idea what Rusty suggested where. I can't even guess what
MAYBE_BUILD_BUG_ON() is supposed to do (sounds like a terrible name).
All I know is that this used to build...
--
Hollis Blanchard
IBM Linux Technology Center
>>> Hollis Blanchard <[email protected]> 02.10.09 17:48 >>>
>On Wed, 2009-09-30 at 07:35 +0100, Jan Beulich wrote:
>> The one Rusty suggested the other day may help here. I don't like it
>> as a drop-in replacement for BUILD_BUG_ON() though (due to it
>> deferring the error generated to the linking stage), I'd rather view
>> this as an improvement to MAYBE_BUILD_BUG_ON() (which should
>> then be used here).
>
>Can you be more specific?
>
>I have no idea what Rusty suggested where. I can't even guess what
I'm attaching Rusty's response I was referring to.
>MAYBE_BUILD_BUG_ON() is supposed to do (sounds like a terrible name).
Agreed - but presumably better than just deleting the bogus instances
altogether...
Jan
Rusty's version of BUILD_BUG_ON() does indeed fix the build break, and
also exposes the bug in kvmppc_account_exit_stat(). So to recap:
original: built but didn't work
Jan's: doesn't build
Rusty's: builds and works
Where do you want to go from here?
--
Hollis Blanchard
IBM Linux Technology Center
On Mon, 2009-10-05 at 07:58 +0100, Jan Beulich wrote:
> >>> Hollis Blanchard <[email protected]> 02.10.09 17:48 >>>
> >On Wed, 2009-09-30 at 07:35 +0100, Jan Beulich wrote:
> >> The one Rusty suggested the other day may help here. I don't like it
> >> as a drop-in replacement for BUILD_BUG_ON() though (due to it
> >> deferring the error generated to the linking stage), I'd rather view
> >> this as an improvement to MAYBE_BUILD_BUG_ON() (which should
> >> then be used here).
> >
> >Can you be more specific?
> >
> >I have no idea what Rusty suggested where. I can't even guess what
>
> I'm attaching Rusty's response I was referring to.
>
> >MAYBE_BUILD_BUG_ON() is supposed to do (sounds like a terrible name).
>
> Agreed - but presumably better than just deleting the bogus instances
> altogether...
>
> Jan
> email message attachment
> > -------- Forwarded Message --------
> > From: Rusty Russell <[email protected]>
> > To: Jan Beulich <[email protected]>
> > Cc: [email protected]
> > Subject: Re: [PATCH] fix BUILD_BUG_ON() and a couple of bogus uses
> > of it
> > Date: Wed, 23 Sep 2009 10:27:00 +0930
> >
> > On Wed, 19 Aug 2009 01:29:25 am Jan Beulich wrote:
> > > gcc permitting variable length arrays makes the current construct
> > > used for BUILD_BUG_ON() useless, as that doesn't produce any diagnostic
> > > if the controlling expression isn't really constant. Instead, this
> > > patch makes it so that a bit field gets used here. Consequently, those
> > > uses where the condition isn't really constant now also need fixing.
> > >
> > > Note that in the gfp.h, kmemcheck.h, and virtio_config.h cases
> > > MAYBE_BUILD_BUG_ON() really just serves documentation purposes - even
> > > if the expression is compile time constant (__builtin_constant_p()
> > > yields true), the array is still deemed of variable length by gcc, and
> > > hence the whole expression doesn't have the intended effect.
> > >
> > > Signed-off-by: Jan Beulich <[email protected]>
> >
> > We used to use an undefined symbol here; diagnostics are worse but it catches
> > more stuff.
> >
> > Perhaps a hybrid is the way to go?
> >
> > #ifndef __OPTIMIZE__
> > #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> > #else
> > /* If it's a constant, catch it at compile time, otherwise at link time. */
> > extern int __build_bug_on_failed;
> > #define BUILD_BUG_ON(condition) \
> > do { \
> > ((void)sizeof(char[1 - 2*!!(condition)])); \
> > if (condition) __build_bug_on_failed = 1; \
> > } while(0)
> > #endif
> >
> > Thanks,
> > Rusty.
On Fri, 2009-10-09 at 12:14 -0700, Hollis Blanchard wrote:
> Rusty's version of BUILD_BUG_ON() does indeed fix the build break, and
> also exposes the bug in kvmppc_account_exit_stat(). So to recap:
>
> original: built but didn't work
> Jan's: doesn't build
> Rusty's: builds and works
>
> Where do you want to go from here?
Jan, what are your thoughts? Your BUILD_BUG_ON patch has broken the
build, and we still need to fix it.
--
Hollis Blanchard
IBM Linux Technology Center
>>> Hollis Blanchard <[email protected]> 15.10.09 00:57 >>>
>On Fri, 2009-10-09 at 12:14 -0700, Hollis Blanchard wrote:
>> Rusty's version of BUILD_BUG_ON() does indeed fix the build break, and
>> also exposes the bug in kvmppc_account_exit_stat(). So to recap:
>>
>> original: built but didn't work
>> Jan's: doesn't build
>> Rusty's: builds and works
>>
>> Where do you want to go from here?
>
>Jan, what are your thoughts? Your BUILD_BUG_ON patch has broken the
>build, and we still need to fix it.
My perspective is that it just uncovered already existing brokenness. And
honestly, I won't be able to get to look into this within the next days. (And
btw., when I run into issues with other people's code changes, quite
frequently I'm told to propose a patch, so I'm also having some
philosophical problem understanding why I can't simply expect the same
when people run into issues with changes I made, especially in cases like
this where it wasn't me introducing the broken code.) So, if this can wait
for a couple of days, I can try to find time to look into this. Otherwise, I'd
rely on someone running into the actual issue to implement a solution.
Jan
On Thu, 2009-10-15 at 08:27 +0100, Jan Beulich wrote:
> >>> Hollis Blanchard <[email protected]> 15.10.09 00:57 >>>
> >On Fri, 2009-10-09 at 12:14 -0700, Hollis Blanchard wrote:
> >> Rusty's version of BUILD_BUG_ON() does indeed fix the build break, and
> >> also exposes the bug in kvmppc_account_exit_stat(). So to recap:
> >>
> >> original: built but didn't work
> >> Jan's: doesn't build
> >> Rusty's: builds and works
> >>
> >> Where do you want to go from here?
> >
> >Jan, what are your thoughts? Your BUILD_BUG_ON patch has broken the
> >build, and we still need to fix it.
>
> My perspective is that it just uncovered already existing brokenness. And
> honestly, I won't be able to get to look into this within the next days. (And
> btw., when I run into issues with other people's code changes, quite
> frequently I'm told to propose a patch, so I'm also having some
> philosophical problem understanding why I can't simply expect the same
> when people run into issues with changes I made, especially in cases like
> this where it wasn't me introducing the broken code.) So, if this can wait
> for a couple of days, I can try to find time to look into this. Otherwise, I'd
> rely on someone running into the actual issue to implement a solution.
Sorry, I thought it was clear, but to be more explicit: I propose the
following patch, which replaces the current BUILD_BUG_ON implementation
with Rusty's version.
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -677,18 +677,19 @@ struct sysinfo {
char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
};
-/* Force a compilation error if condition is true */
-#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
-
-/* Force a compilation error if condition is constant and true */
-#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
-
-/* Force a compilation error if condition is true, but also produce a
- result (of value 0 and type size_t), so the expression can be used
- e.g. in a structure initializer (or where-ever else comma expressions
- aren't permitted). */
-#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
-#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
+#ifndef __OPTIMIZE__
+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
+#else
+/* If it's a constant, catch it at compile time, otherwise at link time. */
+extern int __build_bug_on_failed;
+#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1)
+#define BUILD_BUG_ON(condition) \
+ do { \
+ ((void)sizeof(char[1 - 2*!!(condition)])); \
+ if (condition) __build_bug_on_failed = 1; \
+ } while(0)
+#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
+#endif
/* Trap pasters of __FUNCTION__ at compile-time */
#define __FUNCTION__ (__func__)
--
Hollis Blanchard
IBM Linux Technology Center
On Tue, 20 Oct 2009 04:49:29 am Hollis Blanchard wrote:
> On Thu, 2009-10-15 at 08:27 +0100, Jan Beulich wrote:
> > My perspective is that it just uncovered already existing brokenness.
>
> Sorry, I thought it was clear, but to be more explicit: I propose the
> following patch, which replaces the current BUILD_BUG_ON implementation
> with Rusty's version.
OK, I switched my brain back on. Yeah, I agree: we may still want
BUILD_OR_RUNTIME_BUG_ON one day, but I like this.
It's just missing the giant comment that it needs :)
/**
* BUILD_BUG_ON - break compile if a condition is true.
* @cond: the condition which the compiler should know is false.
*
* If you have some code which relies on certain constants being equal, or
* other compile-time-evaluated condition, you should use BUILD_BUG_ON to
* detect if someone changes it.
*
* The implementation uses gcc's reluctance to create a negative array, but
* gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
* to inline functions). So as a fallback we use the optimizer; if it can't
* prove the condition is false, it will cause a link error on the undefined
* "__build_bug_on_failed". This error is less neat, and can be harder to
* track down.
*/
Thanks!
Rusty.
On Tue, 2009-10-20 at 11:42 +1030, Rusty Russell wrote:
> On Tue, 20 Oct 2009 04:49:29 am Hollis Blanchard wrote:
> > On Thu, 2009-10-15 at 08:27 +0100, Jan Beulich wrote:
> > > My perspective is that it just uncovered already existing brokenness.
> >
> > Sorry, I thought it was clear, but to be more explicit: I propose the
> > following patch, which replaces the current BUILD_BUG_ON implementation
> > with Rusty's version.
>
> OK, I switched my brain back on. Yeah, I agree: we may still want
> BUILD_OR_RUNTIME_BUG_ON one day, but I like this.
>
> It's just missing the giant comment that it needs :)
>
> /**
> * BUILD_BUG_ON - break compile if a condition is true.
> * @cond: the condition which the compiler should know is false.
> *
> * If you have some code which relies on certain constants being equal, or
> * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
> * detect if someone changes it.
> *
> * The implementation uses gcc's reluctance to create a negative array, but
> * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
> * to inline functions). So as a fallback we use the optimizer; if it can't
> * prove the condition is false, it will cause a link error on the undefined
> * "__build_bug_on_failed". This error is less neat, and can be harder to
> * track down.
> */
Do you want to put together a signed-off patch Rusty? It's your code, so
I don't feel comfortable doing that.
Once we have that, can we remove the mysterious MAYBE_BUILD_BUG_ON
statements introduced in previous patches? (Does it BUG or doesn't it??)
--
Hollis Blanchard
IBM Linux Technology Center
BUILD_BUG_ON used to use the optimizer to do code elimination or fail
at link time; it was changed to first the size of a negative array (a
nicer compile time error), then (in
8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
bitfields: needs a literal constant at parse time, and can't be put under
"if (__builtin_constant_p(x))" for example.
negative array: can handle anything, but if the compiler can't tell it's
a constant, silently has no effect.
link time: breaks link if the compiler can't determine the value, but the
linker output is not usually as informative as a compiler error.
If we use the negative-array-size method *and* the link time trick,
we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
branches, and maximal ability for the compiler to detect errors at
build time.
Signed-off-by: Rusty Russell <[email protected]>
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -683,12 +683,6 @@ struct sysinfo {
char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
};
-/* Force a compilation error if condition is true */
-#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
-
-/* Force a compilation error if condition is constant and true */
-#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
-
/* Force a compilation error if condition is true, but also produce a
result (of value 0 and type size_t), so the expression can be used
e.g. in a structure initializer (or where-ever else comma expressions
@@ -696,6 +690,33 @@ struct sysinfo {
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
+/**
+ * BUILD_BUG_ON - break compile if a condition is true.
+ * @cond: the condition which the compiler should know is false.
+ *
+ * If you have some code which relies on certain constants being equal, or
+ * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
+ * detect if someone changes it.
+ *
+ * The implementation uses gcc's reluctance to create a negative array, but
+ * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
+ * to inline functions). So as a fallback we use the optimizer; if it can't
+ * prove the condition is false, it will cause a link error on the undefined
+ * "__build_bug_on_failed". This error message can be harder to track down
+ * though, hence the two different methods.
+ */
+#ifndef __OPTIMIZE__
+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
+#else
+extern int __build_bug_on_failed;
+#define BUILD_BUG_ON(condition) \
+ do { \
+ ((void)sizeof(char[1 - 2*!!(condition)])); \
+ if (condition) __build_bug_on_failed = 1; \
+ } while(0)
+#endif
+#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
+
/* Trap pasters of __FUNCTION__ at compile-time */
#define __FUNCTION__ (__func__)
On Tue, Oct 20, 2009 at 02:15:33PM +1030, Rusty Russell wrote:
>BUILD_BUG_ON used to use the optimizer to do code elimination or fail
>at link time; it was changed to first the size of a negative array (a
>nicer compile time error), then (in
>8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
>
>bitfields: needs a literal constant at parse time, and can't be put under
> "if (__builtin_constant_p(x))" for example.
>negative array: can handle anything, but if the compiler can't tell it's
> a constant, silently has no effect.
>link time: breaks link if the compiler can't determine the value, but the
> linker output is not usually as informative as a compiler error.
>
>If we use the negative-array-size method *and* the link time trick,
>we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
>branches, and maximal ability for the compiler to detect errors at
>build time.
>
>Signed-off-by: Rusty Russell <[email protected]>
>
>diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>--- a/include/linux/kernel.h
>+++ b/include/linux/kernel.h
>@@ -683,12 +683,6 @@ struct sysinfo {
> char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
> };
>
>-/* Force a compilation error if condition is true */
>-#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
>-
>-/* Force a compilation error if condition is constant and true */
>-#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
>-
> /* Force a compilation error if condition is true, but also produce a
> result (of value 0 and type size_t), so the expression can be used
> e.g. in a structure initializer (or where-ever else comma expressions
>@@ -696,6 +690,33 @@ struct sysinfo {
> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
> #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
>
>+/**
>+ * BUILD_BUG_ON - break compile if a condition is true.
>+ * @cond: the condition which the compiler should know is false.
>+ *
>+ * If you have some code which relies on certain constants being equal, or
>+ * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
>+ * detect if someone changes it.
>+ *
>+ * The implementation uses gcc's reluctance to create a negative array, but
>+ * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
>+ * to inline functions). So as a fallback we use the optimizer; if it can't
>+ * prove the condition is false, it will cause a link error on the undefined
>+ * "__build_bug_on_failed". This error message can be harder to track down
>+ * though, hence the two different methods.
>+ */
>+#ifndef __OPTIMIZE__
>+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
>+#else
>+extern int __build_bug_on_failed;
Hmm, what exactly is __build_bug_on_failed?
>+#define BUILD_BUG_ON(condition) \
>+ do { \
>+ ((void)sizeof(char[1 - 2*!!(condition)])); \
>+ if (condition) __build_bug_on_failed = 1; \
>+ } while(0)
>+#endif
>+#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
>+
> /* Trap pasters of __FUNCTION__ at compile-time */
> #define __FUNCTION__ (__func__)
>
>--
>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>the body of a message to [email protected]
>More majordomo info at http://vger.kernel.org/majordomo-info.html
>Please read the FAQ at http://www.tux.org/lkml/
--
Live like a child, think like the god.
On 10/20/09, Am?rico Wang <[email protected]> wrote:
> On Tue, Oct 20, 2009 at 02:15:33PM +1030, Rusty Russell wrote:
>>BUILD_BUG_ON used to use the optimizer to do code elimination or fail
>>at link time; it was changed to first the size of a negative array (a
>>nicer compile time error), then (in
>>8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
>>
>>bitfields: needs a literal constant at parse time, and can't be put under
>> "if (__builtin_constant_p(x))" for example.
>>negative array: can handle anything, but if the compiler can't tell it's
>> a constant, silently has no effect.
>>link time: breaks link if the compiler can't determine the value, but the
>> linker output is not usually as informative as a compiler error.
>>
>>If we use the negative-array-size method *and* the link time trick,
>>we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
>>branches, and maximal ability for the compiler to detect errors at
>>build time.
>>
>>Signed-off-by: Rusty Russell <[email protected]>
>>
>>diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>>--- a/include/linux/kernel.h
>>+++ b/include/linux/kernel.h
>>@@ -683,12 +683,6 @@ struct sysinfo {
>> char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
>> };
>>
>>-/* Force a compilation error if condition is true */
>>-#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
>>-
>>-/* Force a compilation error if condition is constant and true */
>>-#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
>>-
>> /* Force a compilation error if condition is true, but also produce a
>> result (of value 0 and type size_t), so the expression can be used
>> e.g. in a structure initializer (or where-ever else comma expressions
>>@@ -696,6 +690,33 @@ struct sysinfo {
>> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
>> #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
>>
>>+/**
>>+ * BUILD_BUG_ON - break compile if a condition is true.
>>+ * @cond: the condition which the compiler should know is false.
>>+ *
>>+ * If you have some code which relies on certain constants being equal, or
>>+ * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
>>+ * detect if someone changes it.
>>+ *
>>+ * The implementation uses gcc's reluctance to create a negative array,
>> but
>>+ * gcc (as of 4.4) only emits that error for obvious cases (eg. not
>> arguments
>>+ * to inline functions). So as a fallback we use the optimizer; if it
>> can't
>>+ * prove the condition is false, it will cause a link error on the
>> undefined
>>+ * "__build_bug_on_failed". This error message can be harder to track
>> down
>>+ * though, hence the two different methods.
>>+ */
>>+#ifndef __OPTIMIZE__
>>+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
>>+#else
>>+extern int __build_bug_on_failed;
>
> Hmm, what exactly is __build_bug_on_failed?
Well, we haven't added a definition for it in this patch. I'm sure
grep will tell you it wasn't defined before hand either. So any
reference to it is an error - which will be reported at link time.
>>+#define BUILD_BUG_ON(condition) \
>>+ do { \
>>+ ((void)sizeof(char[1 - 2*!!(condition)])); \
>>+ if (condition) __build_bug_on_failed = 1; \
If "condition" is known false at compile time, gcc -O will eliminate
the code which refers to __build_bug_on_failed. If it's not proved to
be false - it will break the build, which is exactly what we want
BUILD_BUG_ON to do.
>>+ } while(0)
>>+#endif
>>+#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
>>+
>> /* Trap pasters of __FUNCTION__ at compile-time */
>> #define __FUNCTION__ (__func__)
>>
On Tue, 2009-10-20 at 14:15 +1030, Rusty Russell wrote:
> BUILD_BUG_ON used to use the optimizer to do code elimination or fail
> at link time; it was changed to first the size of a negative array (a
> nicer compile time error), then (in
> 8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
>
> bitfields: needs a literal constant at parse time, and can't be put under
> "if (__builtin_constant_p(x))" for example.
> negative array: can handle anything, but if the compiler can't tell it's
> a constant, silently has no effect.
> link time: breaks link if the compiler can't determine the value, but the
> linker output is not usually as informative as a compiler error.
>
> If we use the negative-array-size method *and* the link time trick,
> we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
> branches, and maximal ability for the compiler to detect errors at
> build time.
>
> Signed-off-by: Rusty Russell <[email protected]>
Thanks Rusty, this indeed fixes the problem.
Acked-by: Hollis Blanchard <[email protected]>
--
Hollis Blanchard
IBM Linux Technology Center
On Tue, Oct 20, 2009 at 10:43 PM, Alan Jenkins
<[email protected]> wrote:
> On 10/20/09, Américo Wang <[email protected]> wrote:
>> On Tue, Oct 20, 2009 at 02:15:33PM +1030, Rusty Russell wrote:
>>>BUILD_BUG_ON used to use the optimizer to do code elimination or fail
>>>at link time; it was changed to first the size of a negative array (a
>>>nicer compile time error), then (in
>>>8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
>>>
>>>bitfields: needs a literal constant at parse time, and can't be put under
>>> "if (__builtin_constant_p(x))" for example.
>>>negative array: can handle anything, but if the compiler can't tell it's
>>> a constant, silently has no effect.
>>>link time: breaks link if the compiler can't determine the value, but the
>>> linker output is not usually as informative as a compiler error.
>>>
>>>If we use the negative-array-size method *and* the link time trick,
>>>we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
>>>branches, and maximal ability for the compiler to detect errors at
>>>build time.
>>>
>>>Signed-off-by: Rusty Russell <[email protected]>
>>>
>>>diff --git a/include/linux/kernel.h b/include/linux/kernel.h
>>>--- a/include/linux/kernel.h
>>>+++ b/include/linux/kernel.h
>>>@@ -683,12 +683,6 @@ struct sysinfo {
>>> char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
>>> };
>>>
>>>-/* Force a compilation error if condition is true */
>>>-#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
>>>-
>>>-/* Force a compilation error if condition is constant and true */
>>>-#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
>>>-
>>> /* Force a compilation error if condition is true, but also produce a
>>> result (of value 0 and type size_t), so the expression can be used
>>> e.g. in a structure initializer (or where-ever else comma expressions
>>>@@ -696,6 +690,33 @@ struct sysinfo {
>>> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
>>> #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
>>>
>>>+/**
>>>+ * BUILD_BUG_ON - break compile if a condition is true.
>>>+ * @cond: the condition which the compiler should know is false.
>>>+ *
>>>+ * If you have some code which relies on certain constants being equal, or
>>>+ * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
>>>+ * detect if someone changes it.
>>>+ *
>>>+ * The implementation uses gcc's reluctance to create a negative array,
>>> but
>>>+ * gcc (as of 4.4) only emits that error for obvious cases (eg. not
>>> arguments
>>>+ * to inline functions). So as a fallback we use the optimizer; if it
>>> can't
>>>+ * prove the condition is false, it will cause a link error on the
>>> undefined
>>>+ * "__build_bug_on_failed". This error message can be harder to track
>>> down
>>>+ * though, hence the two different methods.
>>>+ */
>>>+#ifndef __OPTIMIZE__
>>>+#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
>>>+#else
>>>+extern int __build_bug_on_failed;
>>
>> Hmm, what exactly is __build_bug_on_failed?
>
> Well, we haven't added a definition for it in this patch. I'm sure
> grep will tell you it wasn't defined before hand either. So any
> reference to it is an error - which will be reported at link time.
>
>>>+#define BUILD_BUG_ON(condition) \
>>>+ do { \
>>>+ ((void)sizeof(char[1 - 2*!!(condition)])); \
>>>+ if (condition) __build_bug_on_failed = 1; \
>
> If "condition" is known false at compile time, gcc -O will eliminate
> the code which refers to __build_bug_on_failed. If it's not proved to
> be false - it will break the build, which is exactly what we want
> BUILD_BUG_ON to do.
Ah, clever trick! Got it.
Thanks!
Reviewed-by: WANG Cong <[email protected]>
On Tue, 2009-10-20 at 14:15 +1030, Rusty Russell wrote:
> BUILD_BUG_ON used to use the optimizer to do code elimination or fail
> at link time; it was changed to first the size of a negative array (a
> nicer compile time error), then (in
> 8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
>
> bitfields: needs a literal constant at parse time, and can't be put under
> "if (__builtin_constant_p(x))" for example.
> negative array: can handle anything, but if the compiler can't tell it's
> a constant, silently has no effect.
> link time: breaks link if the compiler can't determine the value, but the
> linker output is not usually as informative as a compiler error.
>
> If we use the negative-array-size method *and* the link time trick,
> we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
> branches, and maximal ability for the compiler to detect errors at
> build time.
>
> Signed-off-by: Rusty Russell <[email protected]>
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -683,12 +683,6 @@ struct sysinfo {
> char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
> };
>
> -/* Force a compilation error if condition is true */
> -#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
> -
> -/* Force a compilation error if condition is constant and true */
> -#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
> -
> /* Force a compilation error if condition is true, but also produce a
> result (of value 0 and type size_t), so the expression can be used
> e.g. in a structure initializer (or where-ever else comma expressions
> @@ -696,6 +690,33 @@ struct sysinfo {
> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
> #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
>
> +/**
> + * BUILD_BUG_ON - break compile if a condition is true.
> + * @cond: the condition which the compiler should know is false.
> + *
> + * If you have some code which relies on certain constants being equal, or
> + * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
> + * detect if someone changes it.
> + *
> + * The implementation uses gcc's reluctance to create a negative array, but
> + * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
> + * to inline functions). So as a fallback we use the optimizer; if it can't
> + * prove the condition is false, it will cause a link error on the undefined
> + * "__build_bug_on_failed". This error message can be harder to track down
> + * though, hence the two different methods.
> + */
> +#ifndef __OPTIMIZE__
> +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> +#else
> +extern int __build_bug_on_failed;
> +#define BUILD_BUG_ON(condition) \
> + do { \
> + ((void)sizeof(char[1 - 2*!!(condition)])); \
> + if (condition) __build_bug_on_failed = 1; \
> + } while(0)
> +#endif
> +#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
> +
> /* Trap pasters of __FUNCTION__ at compile-time */
> #define __FUNCTION__ (__func__)
What's the state of this patch?
--
Hollis Blanchard
IBM Linux Technology Center
Hi Rusty,
On Tue, 20 Oct 2009 14:15:33 +1030 Rusty Russell <[email protected]> wrote:
>
> +#ifndef __OPTIMIZE__
> +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> +#else
> +extern int __build_bug_on_failed;
> +#define BUILD_BUG_ON(condition) \
> + do { \
> + ((void)sizeof(char[1 - 2*!!(condition)])); \
> + if (condition) __build_bug_on_failed = 1; \
> + } while(0)
> +#endif
> +#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
> +
I decided to try this in linux-next, but an x86_64 allmodconfig build
gave this (gcc 4.4.0):
ERROR: "__build_bug_on_failed" [drivers/net/virtio_net.ko] undefined!
ERROR: "__build_bug_on_failed" [drivers/block/virtio_blk.ko] undefined!
I assume that this is caused by the "MAYBE_BUILD_BUG_ON(fbit >= 32)" in
virtio_has_feature() (in include/linux/virtio_config.h) which is called
all over the place. Unfortunately, virtio_has_feature() gets uninlined
in those two files ...
I have taken the patch back out again for today.
--
Cheers,
Stephen Rothwell [email protected]
http://www.canb.auug.org.au/~sfr/
On Tue, 2009-10-20 at 14:15 +1030, Rusty Russell wrote:
> BUILD_BUG_ON used to use the optimizer to do code elimination or fail
> at link time; it was changed to first the size of a negative array (a
> nicer compile time error), then (in
> 8c87df457cb58fe75b9b893007917cf8095660a0) to a bitfield.
What's the status with this patch ? The lack of it breaks my KVM stuff
in powerpc...
Cheers,
Ben.
> bitfields: needs a literal constant at parse time, and can't be put under
> "if (__builtin_constant_p(x))" for example.
> negative array: can handle anything, but if the compiler can't tell it's
> a constant, silently has no effect.
> link time: breaks link if the compiler can't determine the value, but the
> linker output is not usually as informative as a compiler error.
>
> If we use the negative-array-size method *and* the link time trick,
> we get the ability to use BUILD_BUG_ON() under __builtin_constant_p()
> branches, and maximal ability for the compiler to detect errors at
> build time.
>
> Signed-off-by: Rusty Russell <[email protected]>
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -683,12 +683,6 @@ struct sysinfo {
> char _f[20-2*sizeof(long)-sizeof(int)]; /* Padding: libc5 uses this.. */
> };
>
> -/* Force a compilation error if condition is true */
> -#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))
> -
> -/* Force a compilation error if condition is constant and true */
> -#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))
> -
> /* Force a compilation error if condition is true, but also produce a
> result (of value 0 and type size_t), so the expression can be used
> e.g. in a structure initializer (or where-ever else comma expressions
> @@ -696,6 +690,33 @@ struct sysinfo {
> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
> #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
>
> +/**
> + * BUILD_BUG_ON - break compile if a condition is true.
> + * @cond: the condition which the compiler should know is false.
> + *
> + * If you have some code which relies on certain constants being equal, or
> + * other compile-time-evaluated condition, you should use BUILD_BUG_ON to
> + * detect if someone changes it.
> + *
> + * The implementation uses gcc's reluctance to create a negative array, but
> + * gcc (as of 4.4) only emits that error for obvious cases (eg. not arguments
> + * to inline functions). So as a fallback we use the optimizer; if it can't
> + * prove the condition is false, it will cause a link error on the undefined
> + * "__build_bug_on_failed". This error message can be harder to track down
> + * though, hence the two different methods.
> + */
> +#ifndef __OPTIMIZE__
> +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> +#else
> +extern int __build_bug_on_failed;
> +#define BUILD_BUG_ON(condition) \
> + do { \
> + ((void)sizeof(char[1 - 2*!!(condition)])); \
> + if (condition) __build_bug_on_failed = 1; \
> + } while(0)
> +#endif
> +#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
> +
> /* Trap pasters of __FUNCTION__ at compile-time */
> #define __FUNCTION__ (__func__)
>
> _______________________________________________
> Linuxppc-dev mailing list
> [email protected]
> https://lists.ozlabs.org/listinfo/linuxppc-dev
On Thu, 5 Nov 2009 10:50:20 am Stephen Rothwell wrote:
> Hi Rusty,
>
> On Tue, 20 Oct 2009 14:15:33 +1030 Rusty Russell <[email protected]> wrote:
> >
> > +#ifndef __OPTIMIZE__
> > +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> > +#else
> > +extern int __build_bug_on_failed;
> > +#define BUILD_BUG_ON(condition) \
> > + do { \
> > + ((void)sizeof(char[1 - 2*!!(condition)])); \
> > + if (condition) __build_bug_on_failed = 1; \
> > + } while(0)
> > +#endif
> > +#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
> > +
>
> I decided to try this in linux-next, but an x86_64 allmodconfig build
> gave this (gcc 4.4.0):
>
> ERROR: "__build_bug_on_failed" [drivers/net/virtio_net.ko] undefined!
> ERROR: "__build_bug_on_failed" [drivers/block/virtio_blk.ko] undefined!
>
> I assume that this is caused by the "MAYBE_BUILD_BUG_ON(fbit >= 32)" in
> virtio_has_feature() (in include/linux/virtio_config.h) which is called
> all over the place. Unfortunately, virtio_has_feature() gets uninlined
> in those two files ...
Huh? virtio_has_feature does:
if (__builtin_constant_p(fbit))
BUILD_BUG_ON(fbit >= 32);
else
BUG_ON(fbit >= 32);
So, if it's not a constant, gcc should throw away that first branch. If it
is, it should optimize it away (and no, these are not real bugs AFAICT).
I did a 4.3.3 allmodconfig with this patch on 64-bit a few days back and all
was fine. Will try 4.4.0 now.
Thanks,
Rusty.
On Thu, 5 Nov 2009 04:58:36 pm Rusty Russell wrote:
> I did a 4.3.3 allmodconfig with this patch on 64-bit a few days back and all
> was fine. Will try 4.4.0 now.
4.4.1 seems OK (Ubuntu).
This is annoying. But I'll withdraw the patches; if there's another reason
that 4.4.0 is bad, we can ban it and reintroduce this. I don't think that
breaking this hack is enough to declare 4.4.0 verboten.
Thanks,
Rusty.
Hi Rusty,
On Thu, 5 Nov 2009 16:58:36 +1030 Rusty Russell <[email protected]> wrote:
>
> Huh? virtio_has_feature does:
>
> if (__builtin_constant_p(fbit))
> BUILD_BUG_ON(fbit >= 32);
> else
> BUG_ON(fbit >= 32);
In Linus' tree (and linux-next) it looks like this:
static inline bool virtio_has_feature(const struct virtio_device *vdev,
unsigned int fbit)
{
/* Did you forget to fix assumptions on max features? */
MAYBE_BUILD_BUG_ON(fbit >= 32);
if (fbit < VIRTIO_TRANSPORT_F_START)
virtio_check_driver_offered_feature(vdev, fbit);
return test_bit(fbit, vdev->features);
}
> So, if it's not a constant, gcc should throw away that first branch. If it
> is, it should optimize it away (and no, these are not real bugs AFAICT).
Your version above may well fix the problem. Alternatively marking it
__always_inline way work as well.
> I did a 4.3.3 allmodconfig with this patch on 64-bit a few days back and all
> was fine. Will try 4.4.0 now.
4.4 is more problematic.
--
Cheers,
Stephen Rothwell [email protected]
http://www.canb.auug.org.au/~sfr/
On Thu, 5 Nov 2009 05:08:42 pm Stephen Rothwell wrote:
> Hi Rusty,
>
> On Thu, 5 Nov 2009 16:58:36 +1030 Rusty Russell <[email protected]> wrote:
> >
> > Huh? virtio_has_feature does:
> >
> > if (__builtin_constant_p(fbit))
> > BUILD_BUG_ON(fbit >= 32);
> > else
> > BUG_ON(fbit >= 32);
>
> In Linus' tree (and linux-next) it looks like this:
Ah. My patch series fixes that as part of removing MAYBE_BUILD_BUG_ON.
I've put both in for linux-next.
Cheers,
Rusty.