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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Fri, 14 Oct 2022 22:30:33 +0900
Message-ID: <CAK7LNAT7yY3VL=so0+h=fsefT2QXsKHn4KjCFJ5VtGGkvSRjLg@mail.gmail.com>
Subject: Re: [PATCH] kbuild: add -fno-discard-value-names to cmd_cc_ll_c
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Fangrui Song <maskray@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Tom Rix <trix@redhat.com>, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev
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On Fri, Oct 14, 2022 at 5:54 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Thu, Oct 13, 2022 at 1:41 PM Fangrui Song <maskray@google.com> 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 <ndesaulniers@google.com>
> > >
> > >Thanks for the patch!
> > >
> > >Link: https://github.com/ClangBuiltLinux/linux/issues/1467
> > >Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> >
> > Reviewed-by: Fangrui Song <maskray@google.com>
> >
> > >> ---
> > >>  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