[Bug tree-optimization/107520] New: Optimize std::lerp(d, d, 0.5)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107520

            Bug ID: 107520
           Summary: Optimize std::lerp(d, d, 0.5)
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

In some C++ code I have, it would be convenient if the compiler, possibly with
the help of the standard library, could make the following function cheap,
ideally just the identity. I'll probably end up wrapping lerp with a function
that first checks with __builtin_constant_p if the 2 bounds are equal, but I'll
post this in case people have ideas how to improve things.

#include <cmath>
double f(double d){
  return std::lerp(d, d, .5);
}

Currently, with -O3, we generate

        movapd  %xmm0, %xmm1
        pxor    %xmm0, %xmm0
        comisd  %xmm1, %xmm0
        jnb     .L7
        comisd  %xmm0, %xmm1
        jb      .L6
.L7:
        pxor    %xmm0, %xmm0
        ucomisd %xmm0, %xmm1
        jp      .L6
        je      .L11
.L6:
        movapd  %xmm1, %xmm0
        subsd   %xmm1, %xmm0
        mulsd   .LC1(%rip), %xmm0
        addsd   %xmm1, %xmm0
        maxsd   %xmm1, %xmm0
        ret
        .p2align 4,,10
        .p2align 3
.L11:
        mulsd   .LC1(%rip), %xmm1
        movapd  %xmm1, %xmm0
        addsd   %xmm1, %xmm0
        ret

(clang is better at avoiding the redundant comparison)

With -fno-trapping-math to help a bit, I see at the beginning

  if (d_2(D) == 0.0)
    goto <bb 3>; [34.00%]
  else
    goto <bb 4>; [66.00%]

  <bb 3> [local count: 475287355]:
  _7 = d_2(D) * 5.0e-1;
  _10 = _7 * 2.0e+0;

I think that even with the default -fsigned-zeros, simplifying to _10 = d_2(D)
is valid.

Adding -fno-signed-zeros

  <bb 2> [local count: 1073741824]:
  if (d_2(D) == 0.0)
    goto <bb 5>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 3> [local count: 598454470]:
  _13 = d_2(D) - d_2(D);
  _14 = _13 * 5.0e-1;
  __x_15 = d_2(D) + _14;
  if (d_2(D) u>= __x_15)
    goto <bb 5>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 4> [local count: 299227235]:

  <bb 5> [local count: 1073741825]:
  # _12 = PHI <d_2(D)(2), __x_15(4), d_2(D)(3)>
  return _12;

_13 is 0 or NaN, which doesn't change for _14, and __x_15 is just d_2, so we
always return d_2.