[Bug tree-optimization/91645] Missed optimization with sqrt(x*x)

[lisyarus at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91645

--- Comment #5 from Nikita Lisitsa <lisyarus at gmail dot com> ---
Oh, thank you a lot! Indeed, this version compiles to just mulss & sqrtss

float test (float x)
{
    float y = x*x;
    if (std::isless(y, 0.f))
        __builtin_unreachable();
    return std::sqrt(y);
}

Yet, I still don't quite understand what is happening here. Is it because the
standard '<' operator is still subject to FE_* ?

Concerning pragmas, the code

#pragma GCC optimize ("no-math-errno")
float test (float x)
{
    return std::sqrt(x*x);
}

produces the following assembly

std::sqrt(float):
        pxor    xmm2, xmm2
        movaps  xmm1, xmm0
        ucomiss xmm2, xmm0
        sqrtss  xmm1, xmm1
        ja      .L8
        movaps  xmm0, xmm1
        ret
.L8:
        sub     rsp, 24
        movss   DWORD PTR [rsp+12], xmm1
        call    sqrtf
        movss   xmm1, DWORD PTR [rsp+12]
        add     rsp, 24
        movaps  xmm0, xmm1
        ret
test(float):
        mulss   xmm0, xmm0
        jmp     std::sqrt(float)

So, the only notable difference is that now 'std::sqrt(float)' is not inlined,
but is tail-called instead. Thus, the pragma seems not to work?