[Bug tree-optimization/99946] New: fail to exchange if conditions in terms of likely/unlikely probability

[jiangning.liu at amperecomputing dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99946

            Bug ID: 99946
           Summary: fail to exchange if conditions in terms of
                    likely/unlikely probability
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jiangning.liu at amperecomputing dot com
  Target Milestone: ---

For this simple case,

$ cat test_cond.c 
#define likely(x)       __builtin_expect((x),1)
#define unlikely(x)     __builtin_expect((x),0)

extern void g(void);

int a, b;
void f(void)
{
  if (likely(a>0))
    if (unlikely(b>0))
      g();
}

We expect gcc compiler can exchange the if conditions to be like below,

  if (unlikely(b>0))
    if (likely(a>0))
      g();

This way, performance can be improved due to saving the comparison for a>0.

At the moment, gcc generate code as below,

.LFB0:
        .cfi_startproc
        movl    a(%rip), %edx
        testl   %edx, %edx
        jle     .L1
        movl    b(%rip), %eax
        testl   %eax, %eax
        jg      .L4
.L1:
        ret