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From:   Bill Wendling <morbo@google.com>
Date:   Sun, 13 Jun 2021 11:07:26 -0700
Message-ID: <CAGG=3QUrhVi37sYtybTUAZMpCDjB_pw+1OdkbZKvL6+UQrbMbQ@mail.gmail.com>
Subject: Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kees Cook <keescook@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Fangrui Song <maskray@google.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>
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On Sat, Jun 12, 2021 at 3:47 PM Bill Wendling <morbo@google.com> wrote:
>
> On Sat, Jun 12, 2021 at 1:56 PM Bill Wendling <morbo@google.com> wrote:
> > On Sat, Jun 12, 2021 at 1:25 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > > On Sat, Jun 12, 2021 at 12:10:03PM -0700, Bill Wendling wrote:
> > > Yes it is, but is that sufficient in this case? It very much isn't for
> > > KASAN, UBSAN, and a whole host of other instrumentation crud. They all
> > > needed their own 'bugger-off' attributes.
> > >
> > Now, for the "nointr" issue. I'll see if we need an additional change for that.
> >
> The GCOV implementation disables profiling in those directories where
> instrumentation would fail. We do the same. Both clang and gcc seem to
> treat the no_instrument_function attribute similarly.
>
An example:

$ cat n.c
int g(int);

int __attribute__((__no_instrument_function__))
__attribute__((no_instrument_function))
no_instr(int a) {
  int sum = 0;
  for (int i = 0; i < a; i++)
    sum += g(i);
  return sum;
}

int instr(int a) {
  int sum = 0;
  for (int i = 0; i < a; i++)
    sum += g(i);
  return sum;
}

$ gcc -S -o - n.c -fprofile-arcs -ftest-coverage -O2
        .globl  no_instr
        .type   no_instr, @function
no_instr:
.LFB0:
 ...
        addq    $1, __gcov0.no_instr(%rip)
        pushq   %rbp
 ...
.L3:
 ...
        addq    $1, 8+__gcov0.no_instr(%rip)
 ...
        addq    $1, 16+__gcov0.no_instr(%rip)
 ...
        addq    $1, 16+__gcov0.no_instr(%rip)
 ...
        ret
        .globl  instr
        .type   instr, @function
instr:
.LFB1:
 ...
        addq    $1, __gcov0.instr(%rip)
 ...
        addq    $1, 8+__gcov0.instr(%rip)
 ...
        addq    $1, 16+__gcov0.instr(%rip)
 ...
        addq    $1, 16+__gcov0.instr(%rip)
 ...
        ret

$ clang -S -o - n.c -fprofile-generate -O2
        .globl  no_instr                        # -- Begin function no_instr
        .p2align        4, 0x90
        .type   no_instr,@function
no_instr:                               # @no_instr
 ...
        addq    $1, .L__profc_no_instr+8(%rip)
 ...
        movq    .L__profc_no_instr(%rip), %rax
 ...
        movq    %rax, .L__profc_no_instr(%rip)
 ...
        retq
        .globl  instr                           # -- Begin function instr
        .p2align        4, 0x90
        .type   instr,@function
instr:                                  # @instr
 ...
        addq    $1, .L__profc_instr+8(%rip)
 ...
        movq    .L__profc_instr(%rip), %rax
 ...
        movq    %rax, .L__profc_instr(%rip)
 ...
        retq
.Lfunc_end1: