Re: [PATCH kbuild] kbuild: add -grecord-gcc-switches to clang build

From: Fangrui Song <maskray@google.com>
To: Yonghong Song <yhs@fb.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	arnaldo.melo@gmail.com, ast@kernel.org, bpf@vger.kernel.org,
	kernel-team@fb.com, linux-kbuild@vger.kernel.org,
	masahiroy@kernel.org, michal.lkml@markovi.net,
	clang-built-linux@googlegroups.com, sedat.dilek@gmail.com,
	morbo@google.com
Subject: Re: [PATCH kbuild] kbuild: add -grecord-gcc-switches to clang build
Date: Tue, 30 Mar 2021 17:25:07 -0700	[thread overview]
Message-ID: <20210331002507.xv4sxe27dqirmxih@google.com> (raw)
In-Reply-To: <0b8d17be-e015-83c3-88d8-7c218cd01536@fb.com>

On 2021-03-30, 'Yonghong Song' via Clang Built Linux wrote:
>
>
>On 3/29/21 3:52 PM, Nick Desaulniers wrote:
>>(replying to https://lore.kernel.org/bpf/20210328064121.2062927-1-yhs@fb.com/)
>>
>>Thanks for the patch!
>>
>>>+# gcc emits compilation flags in dwarf DW_AT_producer by default
>>>+# while clang needs explicit flag. Add this flag explicitly.
>>>+ifdef CONFIG_CC_IS_CLANG
>>>+DEBUG_CFLAGS	+= -grecord-gcc-switches
>>>+endif
>>>+

Yes, gcc defaults to -grecord-gcc-switches. Clang doesn't.

>>This adds ~5MB/1% to vmlinux of an x86_64 defconfig built with clang. Do we
>>want to add additional guards for CONFIG_DEBUG_INFO_BTF, so that we don't have
>>to pay that cost if that config is not set?
>
>Since this patch is mostly motivated to detect whether the kernel is
>built with clang lto or not. Let me add the flag only if lto is
>enabled. My measurement shows 0.5% increase to thinlto-vmlinux.
>The smaller percentage is due to larger .debug_info section
>(almost double) for thinlto vs. no lto.
>
> ifdef CONFIG_LTO_CLANG
> DEBUG_CFLAGS   += -grecord-gcc-switches
> endif
>
>This will make pahole with any clang built kernels, lto or non-lto.

I share the same concern about sizes. Can't pahole know it is clang LTO
via other means? If pahole just needs to know the one-bit information
(clang LTO vs not), having every compile option seems unnecessary....

>If the maintainer wants further restriction with CONFIG_DEBUG_INFO_BTF,
>I can do that in another revision.
>
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