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

From: Yonghong Song <yhs@fb.com>
To: Fangrui Song <maskray@google.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 18:47:51 -0700	[thread overview]
Message-ID: <79f231f2-2d14-0900-332e-cba42f770d9e@fb.com> (raw)
In-Reply-To: <20210331002507.xv4sxe27dqirmxih@google.com>

On 3/30/21 5:25 PM, Fangrui Song wrote:
> 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.

Could you know why? dwarf size concern?

> 
>>> 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....

This is v2 of the patch
   https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@fb.com/
The flag will be guarded with CONFIG_LTO_CLANG.

As mentioned in commit message of v2, the alternative is
to go through every cu to find out whether DW_FORM_ref_addr is used
or not. In other words, check every possible cross-cu references
to find whether cross-cu reference actually happens or not. This
is quite heavy for pahole...

What we really want to know is whether cross-cu reference happens
or not? If there is an easy way to get it, that will be great.

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