When debugging LLVM IR, it can be handy for clang to not discard value
names used for local variables and parameters. Compare the generated IR.
Implicit Default (-fdiscard-value-names):
define i32 @core_sys_select(i32 %0, ptr %1, ptr %2, ptr %3, ptr %4) {
%6 = alloca i64
%7 = alloca %struct.poll_wqueues
%8 = alloca [64 x i32]
Explicit -fno-discard-value-names:
define i32 @core_sys_select(i32 %n, ptr %inp, ptr %outp, ptr %exp,
ptr %end_time) {
%expire.i = alloca i64
%table.i = alloca %struct.poll_wqueues
%stack_fds = alloca [64 x i32]
The rule for generating human readable LLVM IR (.ll) is only useful as a
debugging feature:
$ make LLVM=1 fs/select.ll
Clang defaults to -fdiscard-value-names to save memory when generating
LLVM IR. For debugging purposes, the improvement in readability at a
cost of more verbose IR is a cost we're happy to pay.
Signed-off-by: Nick Desaulniers <[email protected]>
---
scripts/Makefile.build | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 27be77c0d6d8..d0e4f476dfee 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -140,7 +140,7 @@ $(obj)/%.symtypes : $(src)/%.c FORCE
# LLVM assembly
# Generate .ll files from .c
quiet_cmd_cc_ll_c = CC $(quiet_modtag) $@
- cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -o $@ $<
+ cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<
$(obj)/%.ll: $(src)/%.c FORCE
$(call if_changed_dep,cc_ll_c)
base-commit: 93ed07a23fd08b8613f64cf0a15d7fbdaca010fd
--
2.38.0.rc2.412.g84df46c1b4-goog
On Fri, Oct 07, 2022 at 01:32:36PM -0700, Nick Desaulniers wrote:
> When debugging LLVM IR, it can be handy for clang to not discard value
> names used for local variables and parameters. Compare the generated IR.
>
> Implicit Default (-fdiscard-value-names):
> define i32 @core_sys_select(i32 %0, ptr %1, ptr %2, ptr %3, ptr %4) {
> %6 = alloca i64
> %7 = alloca %struct.poll_wqueues
> %8 = alloca [64 x i32]
>
> Explicit -fno-discard-value-names:
> define i32 @core_sys_select(i32 %n, ptr %inp, ptr %outp, ptr %exp,
> ptr %end_time) {
> %expire.i = alloca i64
> %table.i = alloca %struct.poll_wqueues
> %stack_fds = alloca [64 x i32]
>
> The rule for generating human readable LLVM IR (.ll) is only useful as a
> debugging feature:
>
> $ make LLVM=1 fs/select.ll
>
> Clang defaults to -fdiscard-value-names to save memory when generating
> LLVM IR. For debugging purposes, the improvement in readability at a
> cost of more verbose IR is a cost we're happy to pay.
>
> Signed-off-by: Nick Desaulniers <[email protected]>
Thanks for the patch!
Link: https://github.com/ClangBuiltLinux/linux/issues/1467
Reviewed-by: Nathan Chancellor <[email protected]>
> ---
> scripts/Makefile.build | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 27be77c0d6d8..d0e4f476dfee 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -140,7 +140,7 @@ $(obj)/%.symtypes : $(src)/%.c FORCE
> # LLVM assembly
> # Generate .ll files from .c
> quiet_cmd_cc_ll_c = CC $(quiet_modtag) $@
> - cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -o $@ $<
> + cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<
>
> $(obj)/%.ll: $(src)/%.c FORCE
> $(call if_changed_dep,cc_ll_c)
>
> base-commit: 93ed07a23fd08b8613f64cf0a15d7fbdaca010fd
> --
> 2.38.0.rc2.412.g84df46c1b4-goog
>
On 2022-10-13, Nathan Chancellor wrote:
>On Fri, Oct 07, 2022 at 01:32:36PM -0700, Nick Desaulniers wrote:
>> When debugging LLVM IR, it can be handy for clang to not discard value
>> names used for local variables and parameters. Compare the generated IR.
>>
>> Implicit Default (-fdiscard-value-names):
>> define i32 @core_sys_select(i32 %0, ptr %1, ptr %2, ptr %3, ptr %4) {
>> %6 = alloca i64
>> %7 = alloca %struct.poll_wqueues
>> %8 = alloca [64 x i32]
>>
>> Explicit -fno-discard-value-names:
>> define i32 @core_sys_select(i32 %n, ptr %inp, ptr %outp, ptr %exp,
>> ptr %end_time) {
>> %expire.i = alloca i64
>> %table.i = alloca %struct.poll_wqueues
>> %stack_fds = alloca [64 x i32]
>>
>> The rule for generating human readable LLVM IR (.ll) is only useful as a
>> debugging feature:
>>
>> $ make LLVM=1 fs/select.ll
>>
>> Clang defaults to -fdiscard-value-names to save memory when generating
>> LLVM IR. For debugging purposes, the improvement in readability at a
>> cost of more verbose IR is a cost we're happy to pay.
A LLVM_ENABLE_ASSEERTIONS=off build of Clang defaults to -fdiscard-value-names.
A LLVM_ENABLE_ASSEERTIONS=on build of Clang defaults to -fno-discard-value-names.
See clang/lib/Driver/ToolChains/Clang.cpp:5030
>> Signed-off-by: Nick Desaulniers <[email protected]>
>
>Thanks for the patch!
>
>Link: https://github.com/ClangBuiltLinux/linux/issues/1467
>Reviewed-by: Nathan Chancellor <[email protected]>
Reviewed-by: Fangrui Song <[email protected]>
>> ---
>> scripts/Makefile.build | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
>> index 27be77c0d6d8..d0e4f476dfee 100644
>> --- a/scripts/Makefile.build
>> +++ b/scripts/Makefile.build
>> @@ -140,7 +140,7 @@ $(obj)/%.symtypes : $(src)/%.c FORCE
>> # LLVM assembly
>> # Generate .ll files from .c
>> quiet_cmd_cc_ll_c = CC $(quiet_modtag) $@
>> - cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -o $@ $<
>> + cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<
>>
>> $(obj)/%.ll: $(src)/%.c FORCE
>> $(call if_changed_dep,cc_ll_c)
>>
>> base-commit: 93ed07a23fd08b8613f64cf0a15d7fbdaca010fd
>> --
>> 2.38.0.rc2.412.g84df46c1b4-goog
>>
>
On Thu, Oct 13, 2022 at 1:41 PM Fangrui Song <[email protected]> wrote:
>
> On 2022-10-13, Nathan Chancellor wrote:
> >On Fri, Oct 07, 2022 at 01:32:36PM -0700, Nick Desaulniers wrote:
> >> When debugging LLVM IR, it can be handy for clang to not discard value
> >> names used for local variables and parameters. Compare the generated IR.
> >>
> >> Implicit Default (-fdiscard-value-names):
> >> define i32 @core_sys_select(i32 %0, ptr %1, ptr %2, ptr %3, ptr %4) {
> >> %6 = alloca i64
> >> %7 = alloca %struct.poll_wqueues
> >> %8 = alloca [64 x i32]
> >>
> >> Explicit -fno-discard-value-names:
> >> define i32 @core_sys_select(i32 %n, ptr %inp, ptr %outp, ptr %exp,
> >> ptr %end_time) {
> >> %expire.i = alloca i64
> >> %table.i = alloca %struct.poll_wqueues
> >> %stack_fds = alloca [64 x i32]
> >>
> >> The rule for generating human readable LLVM IR (.ll) is only useful as a
> >> debugging feature:
> >>
> >> $ make LLVM=1 fs/select.ll
> >>
> >> Clang defaults to -fdiscard-value-names to save memory when generating
> >> LLVM IR. For debugging purposes, the improvement in readability at a
> >> cost of more verbose IR is a cost we're happy to pay.
>
> A LLVM_ENABLE_ASSEERTIONS=off build of Clang defaults to -fdiscard-value-names.
>
> A LLVM_ENABLE_ASSEERTIONS=on build of Clang defaults to -fno-discard-value-names.
>
> See clang/lib/Driver/ToolChains/Clang.cpp:5030
Thanks!
I thought I recalled this; I was literally just rereading through
https://github.com/ClangBuiltLinux/linux/issues/1467#issuecomment-1046071624
again to understand.
That would be useful to include in the commit message, and is more
precise than "Clang defaults to -fdiscard-value-names to save memory
when generating LLVM IR." Masahiro, would you mind including or
replacing that, or shall I send a v2?
I was surprised to find later in the day that I was running an
assertions-disabled (ie. release) build of clang, which I almost never
do! Non-llvm-developers are more likely to be using release builds
than assertions-enabled builds of llvm, so this patch still very much
has merit.
>
> >> Signed-off-by: Nick Desaulniers <[email protected]>
> >
> >Thanks for the patch!
> >
> >Link: https://github.com/ClangBuiltLinux/linux/issues/1467
> >Reviewed-by: Nathan Chancellor <[email protected]>
>
> Reviewed-by: Fangrui Song <[email protected]>
>
> >> ---
> >> scripts/Makefile.build | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> >> index 27be77c0d6d8..d0e4f476dfee 100644
> >> --- a/scripts/Makefile.build
> >> +++ b/scripts/Makefile.build
> >> @@ -140,7 +140,7 @@ $(obj)/%.symtypes : $(src)/%.c FORCE
> >> # LLVM assembly
> >> # Generate .ll files from .c
> >> quiet_cmd_cc_ll_c = CC $(quiet_modtag) $@
> >> - cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -o $@ $<
> >> + cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<
> >>
> >> $(obj)/%.ll: $(src)/%.c FORCE
> >> $(call if_changed_dep,cc_ll_c)
> >>
> >> base-commit: 93ed07a23fd08b8613f64cf0a15d7fbdaca010fd
> >> --
> >> 2.38.0.rc2.412.g84df46c1b4-goog
> >>
> >
--
Thanks,
~Nick Desaulniers
On Fri, Oct 14, 2022 at 5:54 AM Nick Desaulniers
<[email protected]> wrote:
>
> On Thu, Oct 13, 2022 at 1:41 PM Fangrui Song <[email protected]> wrote:
> >
> > On 2022-10-13, Nathan Chancellor wrote:
> > >On Fri, Oct 07, 2022 at 01:32:36PM -0700, Nick Desaulniers wrote:
> > >> When debugging LLVM IR, it can be handy for clang to not discard value
> > >> names used for local variables and parameters. Compare the generated IR.
> > >>
> > >> Implicit Default (-fdiscard-value-names):
> > >> define i32 @core_sys_select(i32 %0, ptr %1, ptr %2, ptr %3, ptr %4) {
> > >> %6 = alloca i64
> > >> %7 = alloca %struct.poll_wqueues
> > >> %8 = alloca [64 x i32]
> > >>
> > >> Explicit -fno-discard-value-names:
> > >> define i32 @core_sys_select(i32 %n, ptr %inp, ptr %outp, ptr %exp,
> > >> ptr %end_time) {
> > >> %expire.i = alloca i64
> > >> %table.i = alloca %struct.poll_wqueues
> > >> %stack_fds = alloca [64 x i32]
> > >>
> > >> The rule for generating human readable LLVM IR (.ll) is only useful as a
> > >> debugging feature:
> > >>
> > >> $ make LLVM=1 fs/select.ll
> > >>
> > >> Clang defaults to -fdiscard-value-names to save memory when generating
> > >> LLVM IR. For debugging purposes, the improvement in readability at a
> > >> cost of more verbose IR is a cost we're happy to pay.
> >
> > A LLVM_ENABLE_ASSEERTIONS=off build of Clang defaults to -fdiscard-value-names.
> >
> > A LLVM_ENABLE_ASSEERTIONS=on build of Clang defaults to -fno-discard-value-names.
> >
> > See clang/lib/Driver/ToolChains/Clang.cpp:5030
>
> Thanks!
>
> I thought I recalled this; I was literally just rereading through
> https://github.com/ClangBuiltLinux/linux/issues/1467#issuecomment-1046071624
> again to understand.
>
> That would be useful to include in the commit message, and is more
> precise than "Clang defaults to -fdiscard-value-names to save memory
> when generating LLVM IR." Masahiro, would you mind including or
> replacing that, or shall I send a v2?
Can you send v2, or provide the entire new commit description?
Thank you.
>
> I was surprised to find later in the day that I was running an
> assertions-disabled (ie. release) build of clang, which I almost never
> do! Non-llvm-developers are more likely to be using release builds
> than assertions-enabled builds of llvm, so this patch still very much
> has merit.
>
> >
> > >> Signed-off-by: Nick Desaulniers <[email protected]>
> > >
> > >Thanks for the patch!
> > >
> > >Link: https://github.com/ClangBuiltLinux/linux/issues/1467
> > >Reviewed-by: Nathan Chancellor <[email protected]>
> >
> > Reviewed-by: Fangrui Song <[email protected]>
> >
> > >> ---
> > >> scripts/Makefile.build | 2 +-
> > >> 1 file changed, 1 insertion(+), 1 deletion(-)
> > >>
> > >> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > >> index 27be77c0d6d8..d0e4f476dfee 100644
> > >> --- a/scripts/Makefile.build
> > >> +++ b/scripts/Makefile.build
> > >> @@ -140,7 +140,7 @@ $(obj)/%.symtypes : $(src)/%.c FORCE
> > >> # LLVM assembly
> > >> # Generate .ll files from .c
> > >> quiet_cmd_cc_ll_c = CC $(quiet_modtag) $@
> > >> - cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -o $@ $<
> > >> + cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<
> > >>
> > >> $(obj)/%.ll: $(src)/%.c FORCE
> > >> $(call if_changed_dep,cc_ll_c)
> > >>
> > >> base-commit: 93ed07a23fd08b8613f64cf0a15d7fbdaca010fd
> > >> --
> > >> 2.38.0.rc2.412.g84df46c1b4-goog
> > >>
> > >
>
>
>
> --
> Thanks,
> ~Nick Desaulniers
--
Best Regards
Masahiro Yamada
When debugging LLVM IR, it can be handy for clang to not discard value
names used for local variables and parameters. Compare the generated IR.
-fdiscard-value-names:
define i32 @core_sys_select(i32 %0, ptr %1, ptr %2, ptr %3, ptr %4) {
%6 = alloca i64
%7 = alloca %struct.poll_wqueues
%8 = alloca [64 x i32]
-fno-discard-value-names:
define i32 @core_sys_select(i32 %n, ptr %inp, ptr %outp, ptr %exp,
ptr %end_time) {
%expire.i = alloca i64
%table.i = alloca %struct.poll_wqueues
%stack_fds = alloca [64 x i32]
The rule for generating human readable LLVM IR (.ll) is only useful as a
debugging feature:
$ make LLVM=1 fs/select.ll
As Fangrui notes:
A LLVM_ENABLE_ASSERTIONS=off build of Clang defaults to
-fdiscard-value-names.
A LLVM_ENABLE_ASSERTIONS=on build of Clang defaults to
-fno-discard-value-names.
Explicitly enable -fdiscard-value-names so that the IR always contains
value names regardless of whether assertions were enabled or not.
Assertions generally are not enabled in releases of clang packaged by
distributions.
Link: https://github.com/ClangBuiltLinux/linux/issues/1467
Reviewed-by: Nathan Chancellor <[email protected]>
Reviewed-by: Fangrui Song <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
---
Changes v1 -> v2:
* Only update commit message:
* Pick up Nathan & Fangrui's RB tags.
* Add link (I forgot I filed a bug about this a while ago!).
* Add note from Fangrui.
* Add final paragraph.
scripts/Makefile.build | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 22adbf89cb31..41f3602fc8de 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -140,7 +140,7 @@ $(obj)/%.symtypes : $(src)/%.c FORCE
# LLVM assembly
# Generate .ll files from .c
quiet_cmd_cc_ll_c = CC $(quiet_modtag) $@
- cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -o $@ $<
+ cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<
$(obj)/%.ll: $(src)/%.c FORCE
$(call if_changed_dep,cc_ll_c)
--
2.38.0.413.g74048e4d9e-goog