2022-07-25 05:08:08

by Li kunyu

Subject: [PATCH] x86/boot/arch/variable: I don't think (void *) Pointers need to be cast

I first observe (void *) type coercion and non coercion through assembly
language. It seems that there is no difference.
Then I output the assigned information through the print function and
found that the pointer that is not coerced is directly assigned when
executing the print function (opcode a1), while the coerced pointer
needs to execute the assembly instruction xlat (opcode d7), which seems
to be more efficient without coerced conversion.
At present, I just started to try to analyze this part of knowledge
(machine code), please forgive me if the analysis is wrong.

Signed-off-by: Li kunyu <[email protected]>
---
arch/x86/boot/bitops.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/bitops.h b/arch/x86/boot/bitops.h
index 02e1dea11d94..8518ae214c9b 100644
--- a/arch/x86/boot/bitops.h
+++ b/arch/x86/boot/bitops.h
@@ -19,13 +19,13 @@

static inline bool constant_test_bit(int nr, const void *addr)
{
- const u32 *p = (const u32 *)addr;
+ const u32 *p = addr;
return ((1UL << (nr & 31)) & (p[nr >> 5])) != 0;
}
static inline bool variable_test_bit(int nr, const void *addr)
{
bool v;
- const u32 *p = (const u32 *)addr;
+ const u32 *p = addr;

asm("btl %2,%1" CC_SET(c) : CC_OUT(c) (v) : "m" (*p), "Ir" (nr));
return v;
--
2.18.2

2022-08-03 09:44:34

by Ingo Molnar

Subject: Re: [PATCH] x86/boot/arch/variable: I don't think (void *) Pointers need to be cast

* Li kunyu <[email protected]> wrote:

> I first observe (void *) type coercion and non coercion through assembly
> language. It seems that there is no difference.
> Then I output the assigned information through the print function and
> found that the pointer that is not coerced is directly assigned when
> executing the print function (opcode a1), while the coerced pointer
> needs to execute the assembly instruction xlat (opcode d7), which seems
> to be more efficient without coerced conversion.
> At present, I just started to try to analyze this part of knowledge
> (machine code), please forgive me if the analysis is wrong.
>
> Signed-off-by: Li kunyu <[email protected]>
> ---
> arch/x86/boot/bitops.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/bitops.h b/arch/x86/boot/bitops.h
> index 02e1dea11d94..8518ae214c9b 100644
> --- a/arch/x86/boot/bitops.h
> +++ b/arch/x86/boot/bitops.h
> @@ -19,13 +19,13 @@
>
> static inline bool constant_test_bit(int nr, const void *addr)
> {
> - const u32 *p = (const u32 *)addr;
> + const u32 *p = addr;
> return ((1UL << (nr & 31)) & (p[nr >> 5])) != 0;
> }
> static inline bool variable_test_bit(int nr, const void *addr)
> {
> bool v;
> - const u32 *p = (const u32 *)addr;
> + const u32 *p = addr;
>
> asm("btl %2,%1" CC_SET(c) : CC_OUT(c) (v) : "m" (*p), "Ir" (nr));
> return v;

It's true that the forced-type casting of 'addr' is unnecessary in the
cases above, I'm not sure how the kernel would end up with an XLAT
instruction being generated in that sequence.

But your patch is a good cleanup in its own right - I've applied the patch
below to tip:x86/cleanups, with a different changelog.

Thanks,

Ingo

===================>
From: Li kunyu <[email protected]>
Date: Mon, 25 Jul 2022 12:23:58 +0800
Subject: [PATCH] x86/boot: Remove superfluous type casting from arch/x86/boot/bitops.h

'const void *' will auto-type-convert to just about any other const pointer type,
no need to force it.

[ mingo: Rewrote the changelog. ]

Signed-off-by: Li kunyu <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
arch/x86/boot/bitops.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/bitops.h b/arch/x86/boot/bitops.h
index 02e1dea11d94..8518ae214c9b 100644
--- a/arch/x86/boot/bitops.h
+++ b/arch/x86/boot/bitops.h
@@ -19,13 +19,13 @@

static inline bool constant_test_bit(int nr, const void *addr)
{
- const u32 *p = (const u32 *)addr;
+ const u32 *p = addr;
return ((1UL << (nr & 31)) & (p[nr >> 5])) != 0;
}
static inline bool variable_test_bit(int nr, const void *addr)
{
bool v;
- const u32 *p = (const u32 *)addr;
+ const u32 *p = addr;

asm("btl %2,%1" CC_SET(c) : CC_OUT(c) (v) : "m" (*p), "Ir" (nr));
return v;

2022-08-03 16:08:58

by tip-bot2 for Jacob Pan

Subject: [tip: x86/cleanups] x86/boot: Remove superfluous type casting from arch/x86/boot/bitops.h

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID: a50be38f617b11b451c0a432c574859419e69471
Gitweb: https://git.kernel.org/tip/a50be38f617b11b451c0a432c574859419e69471
Author: Li kunyu <[email protected]>
AuthorDate: Mon, 25 Jul 2022 12:23:58 +08:00
Committer: Ingo Molnar <[email protected]>
CommitterDate: Wed, 03 Aug 2022 11:32:29 +02:00

x86/boot: Remove superfluous type casting from arch/x86/boot/bitops.h

'const void *' will auto-type-convert to just about any other const pointer type,
no need to force it.

[ mingo: Rewrote the changelog. ]

Signed-off-by: Li kunyu <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
arch/x86/boot/bitops.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/bitops.h b/arch/x86/boot/bitops.h
index 02e1dea..8518ae2 100644
--- a/arch/x86/boot/bitops.h
+++ b/arch/x86/boot/bitops.h
@@ -19,13 +19,13 @@

static inline bool constant_test_bit(int nr, const void *addr)
{
- const u32 *p = (const u32 *)addr;
+ const u32 *p = addr;
return ((1UL << (nr & 31)) & (p[nr >> 5])) != 0;
}
static inline bool variable_test_bit(int nr, const void *addr)
{
bool v;
- const u32 *p = (const u32 *)addr;
+ const u32 *p = addr;

asm("btl %2,%1" CC_SET(c) : CC_OUT(c) (v) : "m" (*p), "Ir" (nr));
return v;

2022-08-15 23:20:30

by tip-bot2 for Jacob Pan

Subject: [tip: x86/cleanups] x86/boot: Remove superfluous type casting from arch/x86/boot/bitops.h

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID: 039f0e054a29d06970892240d70143150d2aaec2
Gitweb: https://git.kernel.org/tip/039f0e054a29d06970892240d70143150d2aaec2
Author: Li kunyu <[email protected]>
AuthorDate: Mon, 25 Jul 2022 12:23:58 +08:00
Committer: Borislav Petkov <[email protected]>
CommitterDate: Mon, 15 Aug 2022 19:17:43 +02:00

x86/boot: Remove superfluous type casting from arch/x86/boot/bitops.h

'const void *' will auto-type-convert to just about any other const
pointer type, no need to force it.

[ mingo: Rewrote the changelog. ]

Signed-off-by: Li kunyu <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
arch/x86/boot/bitops.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/bitops.h b/arch/x86/boot/bitops.h
index 02e1dea..8518ae2 100644
--- a/arch/x86/boot/bitops.h
+++ b/arch/x86/boot/bitops.h
@@ -19,13 +19,13 @@

static inline bool constant_test_bit(int nr, const void *addr)
{
- const u32 *p = (const u32 *)addr;
+ const u32 *p = addr;
return ((1UL << (nr & 31)) & (p[nr >> 5])) != 0;
}
static inline bool variable_test_bit(int nr, const void *addr)
{
bool v;
- const u32 *p = (const u32 *)addr;
+ const u32 *p = addr;

asm("btl %2,%1" CC_SET(c) : CC_OUT(c) (v) : "m" (*p), "Ir" (nr));
return v;