https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98212
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
As for f2, only the case of a == b when the jump is predicted unlikely looks
weird, e.g. take:
void foo (void);
void
bar (float a, float b)
{
if (__builtin_expect (a != b, 1))
foo ();
}
void
baz (float a, float b)
{
if (__builtin_expect (a == b, 1))
foo ();
}
void
qux (float a, float b)
{
if (__builtin_expect (a != b, 0))
foo ();
}
void
corge (float a, float b)
{
if (__builtin_expect (a == b, 0))
foo ();
}
bar: ucomiss %xmm1, %xmm0
jp .L4
je .L1
.L4: jmp foo
.L1: ret
baz: ucomiss %xmm1, %xmm0
jp .L6
jne .L6
jmp foo
.L6: ret
qux: ucomiss %xmm1, %xmm0
jp .L13
jne .L13
ret
.L13: jmp foo
corge: ucomiss %xmm1, %xmm0
jnp .L18
.L14: ret
.L18: jne .L14
jmp foo
I guess I need to debug the branch probabilities in that case, we should
certainly predict that operands are most of the time non-NaN and thus that PF
bit will not be set in 99% of cases or so.
