[Bug rtl-optimization/70432] New: Move operation across function call to reduce save / restore

["glisse at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70432

            Bug ID: 70432
           Summary: Move operation across function call to reduce save /
                    restore
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

I have no idea how one would do such a transformation, and there is likely a
dup of this somewhere... The compiler generates much smaller code for f2 than
f1.

void g(void);
int f1(int a,int b,int c){
  g();
  return a+b+c;
}
int f2(int a,int b,int c){
  int r=a+b+c;
  g();
  return r;
}

f1:
.LFB0:
        .cfi_startproc
        pushq   %r12
        .cfi_def_cfa_offset 16
        .cfi_offset 12, -16
        pushq   %rbp
        .cfi_def_cfa_offset 24
        .cfi_offset 6, -24
        movl    %esi, %r12d
        pushq   %rbx
        .cfi_def_cfa_offset 32
        .cfi_offset 3, -32
        movl    %edi, %ebx
        movl    %edx, %ebp
        call    g
        leal    (%rbx,%r12), %eax
        popq    %rbx
        .cfi_def_cfa_offset 24
        addl    %ebp, %eax
        popq    %rbp
        .cfi_def_cfa_offset 16
        popq    %r12
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc
.LFE0:
        .size   f1, .-f1
        .p2align 4,,15
        .globl  f2
        .type   f2, @function
f2:
.LFB1:
        .cfi_startproc
        addl    %esi, %edi
        pushq   %rbx
        .cfi_def_cfa_offset 16
        .cfi_offset 3, -16
        leal    (%rdi,%rdx), %ebx
        call    g
        movl    %ebx, %eax
        popq    %rbx
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc