Message-ID: <20200214180527.z44b4bmzn336mff2@google.com>
Date:   Fri, 14 Feb 2020 10:05:27 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>, jpoimboe@...hat.com,
        peterz@...radead.org, clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool: ignore .L prefixed local symbols

I know little about objtool, but if it may be used by other
architectures, hope the following explanations don't appear to be too
off-topic:)

On 2020-02-14, Arvind Sankar wrote:
>On Thu, Feb 13, 2020 at 11:20:55AM -0800, Fangrui Song wrote:
>> On 2020-02-13, Nick Desaulniers wrote:
>> >Top of tree LLVM has optimizations related to
>> >-fno-semantic-interposition to avoid emitting PLT relocations for
>> >references to symbols located in the same translation unit, where it
>> >will emit "local symbol" references.
>> >
>> >Clang builds fall back on GNU as for assembling, currently. It appears a
>> >bug in GNU as introduced around 2.31 is keeping around local labels in
>> >the symbol table, despite the documentation saying:
>> >
>> >"Local symbols are defined and used within the assembler, but they are
>> >normally not saved in object files."
>>
>> If you can reword the paragraph above mentioning the fact below without being
>> more verbose, please do that.
>>
>> If the reference is within the same section which defines the .L symbol,
>> there is no outstanding relocation. If the reference is outside the
>> section, there will be an R_X86_64_PLT32 referencing .L
>>
>
>Can you describe what case the clang change is supposed to optimize?
>AFAICT, it kicks in when the symbol is known by the compiler to be local
>to the DSO and defined in the same translation unit.
>
>But then there are two cases:
>(a) we have call foo, where foo is defined in the same section as the
>call instruction. In this case the assembler should be able to fully
>resolve foo and not generate any relocation, regardless of whether foo
>is global or local.

If foo is STB_GLOBAL or STB_WEAK, the assembler cannot fully resolve a
reference to foo in the same section, unless the assembler can assume
(the codegen tells it) the call to foo cannot be interposed by another
foo definition at runtime.

>(b) we have call foo, where foo is defined in a different section from
>the call instruction. In this case the assembler must generate a
>relocation regardless of whether foo is global or local, and the linker
>should eliminate it.
>In what case does does replacing call foo with call .Lfoo$local help?

For -fPIC -fno-semantic-interposition, the assembly emitter can perform
the following optimization:

   void foo() {}
   void bar() { foo(); }

   .globl foo, bar
   foo:
   .Lfoo$local:
     ret
   bar:
     call foo  --> call .Lfoo$local
     ret

call foo generates an R_X86_64_PLT32. In a -shared link, it creates an
unneeded PLT entry for foo.

call .Lfoo$local generates an R_X86_64_PLT32. In a -shared link, .Lfoo$local is
non-preemptible => no PLT entry is created.

For -fno-PIC and -fPIE, the final link is expected to be -no-pie or
-pie. This optimization does not save anything, because PLT entries will
not be generated. With clang's integrated assembler, it may increase the
number of STT_SECTION symbols (because .Lfoo$local will be turned to a
STT_SECTION relative relocation), but the size increase is very small.


I want to teach clang -fPIC to use -fno-semantic-interposition by
default. (It is currently an LLVM optimization, not realized in clang.)
clang traditionally makes various -fno-semantic-interposition
assumptions and can perform interprocedural optimizations even if the
strict ELF rule disallows them.

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