[llvm-bugs] [Bug 49909] New: std::clamp generates suboptimal assembly for primitive types

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49909

            Bug ID: 49909
           Summary: std::clamp generates suboptimal assembly for primitive
                    types
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: vittorio.romeo at outlook.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

`std::clamp` generates poor assembly compared to hand-written counterpart for
primitive types like `float`, even with `-Ofast -ffast-math`:

stdclamp(float, float, float):                         # @stdclamp(float,
float, float)
        movss   dword ptr [rsp - 4], xmm0
        movss   dword ptr [rsp - 8], xmm1
        movss   dword ptr [rsp - 12], xmm2
        ucomiss xmm2, xmm0
        lea     rax, [rsp - 12]
        lea     rcx, [rsp - 4]
        cmovb   rcx, rax
        ucomiss xmm0, xmm1
        lea     rax, [rsp - 8]
        cmovae  rax, rcx
        movss   xmm0, dword ptr [rax]           # xmm0 = mem[0],zero,zero,zero
        ret

myclamp(float, float, float):                          # @myclamp(float, float,
float)
        maxss   xmm0, xmm1
        minss   xmm0, xmm2
        ret

Live example:
https://godbolt.org/z/5oxvocevK

More information on:
https://secret.club/2021/04/09/std-clamp.html

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