linux-kernel - Re: [PATCH] kbuild: add -fno-discard-value-names to cmd_cc_ll

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Date:   Thu, 13 Oct 2022 12:56:20 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Tom Rix <trix@...hat.com>, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] kbuild: add -fno-discard-value-names to cmd_cc_ll_c

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 <ndesaulniers@...gle.com>

Thanks for the patch!

Link: https://github.com/ClangBuiltLinux/linux/issues/1467
Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  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
>