Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

Bill Wendling
2021-06-13T18:07:56Z

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: