Re: [PATCH] kbuild: add -fno-discard-value-names to cmd_cc_ll_c

[Fangrui Song]

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 <ndesaulniers@xxxxxxxxxx>
>
> Thanks for the patch!
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1467
> Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>

> > ---
> >  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