<cmath>: Use Clang builtins for classification/comparison functions

[StephanTLavavej]

Fixes #4609.

This is limited to functions that the UCRT has templated, where our non-template overloads will be preferred. For example, this PR omits fpclassify() because the UCRT overloads it for float, double, and long double (to call _fdtest(), _dtest(), and _ldtest(), respectively), so even though Clang provides a constexpr-compatible __builtin_fpclassify, the STL can't provide overloads without introducing ambiguity.

Note that we define our overloads in the global namespace and then drag them into namespace std. Therefore, as long as users include <cmath>, their global namespace calls will benefit from this improvement.

Also note an additional limitation: calls to the binary comparison functions with mixed argument types will select the UCRT templates as being better matches.

In the medium future, it looks like we're going to need a major UCRT overhaul of <math.h> to implement all of the constexpr demanded by C++23/26 - at that time we can also untangle the additional overloads that the STL is providing (for both integral and floating-point), and teach the UCRT to use MSVC and Clang builtins when possible.

The MSVC compiler back-end should probably provide similar builtins - I am a lazy kitty 🐈 💤 so I haven't asked them, but again the upcoming constexpr overhaul work will likely result in such machinery becoming available.

C:\Temp>type meow.cpp

#include <cmath>

bool is_this_finite(double dbl) {
    return std::isfinite(dbl);
}

C:\Temp>clang-cl /EHsc /nologo /W4 /MT /O2 /c /FAs meow.cpp

Click to expand codegen before:

C:\Temp>type meow.asm
        .text
        .def    @feat.00;
        .scl    3;
        .type   0;
        .endef
        .globl  @feat.00
.set @feat.00, 0
        .intel_syntax noprefix
        .file   "meow.cpp"
        .def    "?is_this_finite@@YA_NN@Z";
        .scl    2;
        .type   32;
        .endef
        .section        .text,"xr",one_only,"?is_this_finite@@YA_NN@Z"
        .globl  "?is_this_finite@@YA_NN@Z"      # -- Begin function ?is_this_finite@@YA_NN@Z
        .p2align        4, 0x90
"?is_this_finite@@YA_NN@Z":             # @"?is_this_finite@@YA_NN@Z"
.seh_proc "?is_this_finite@@YA_NN@Z"
# %bb.0:
        push    rbx
        .seh_pushreg rbx
        sub     rsp, 48
        .seh_stackalloc 48
        .seh_endprologue
        mov     rax, qword ptr [rip + __security_cookie]
        xor     rax, rsp
        mov     qword ptr [rsp + 40], rax
        movsd   qword ptr [rsp + 32], xmm0
        lea     rcx, [rsp + 32]
        call    _dtest
        test    ax, ax
        setle   bl
        mov     rcx, qword ptr [rsp + 40]
        xor     rcx, rsp
        call    __security_check_cookie
        mov     eax, ebx
        add     rsp, 48
        pop     rbx
        ret
        .seh_endproc
                                        # -- End function
        .section        .drectve,"yni"
        .ascii  " /DEFAULTLIB:libcmt.lib"
        .ascii  " /DEFAULTLIB:oldnames.lib"
        .ascii  " /FAILIFMISMATCH:\"_MSC_VER=1900\""
        .ascii  " /FAILIFMISMATCH:\"_ITERATOR_DEBUG_LEVEL=0\""
        .ascii  " /FAILIFMISMATCH:\"RuntimeLibrary=MT_StaticRelease\""
        .ascii  " /DEFAULTLIB:libcpmt.lib"
        .addrsig
        .globl  _fltused

Click to expand codegen after:

C:\Temp>type meow.asm
        .text
        .def    @feat.00;
        .scl    3;
        .type   0;
        .endef
        .globl  @feat.00
.set @feat.00, 0
        .intel_syntax noprefix
        .file   "meow.cpp"
        .def    "?is_this_finite@@YA_NN@Z";
        .scl    2;
        .type   32;
        .endef
        .section        .text,"xr",one_only,"?is_this_finite@@YA_NN@Z"
        .globl  "?is_this_finite@@YA_NN@Z"      # -- Begin function ?is_this_finite@@YA_NN@Z
        .p2align        4, 0x90
"?is_this_finite@@YA_NN@Z":             # @"?is_this_finite@@YA_NN@Z"
# %bb.0:
        movq    rax, xmm0
        movabs  rcx, 9223372036854775807
        and     rcx, rax
        movabs  rax, 9218868437227405312
        cmp     rcx, rax
        setl    al
        ret
                                        # -- End function
        .section        .drectve,"yni"
        .ascii  " /DEFAULTLIB:libcmt.lib"
        .ascii  " /DEFAULTLIB:oldnames.lib"
        .ascii  " /FAILIFMISMATCH:\"_MSC_VER=1900\""
        .ascii  " /FAILIFMISMATCH:\"_ITERATOR_DEBUG_LEVEL=0\""
        .ascii  " /FAILIFMISMATCH:\"RuntimeLibrary=MT_StaticRelease\""
        .ascii  " /DEFAULTLIB:libcpmt.lib"
        .addrsig
        .globl  _fltused

[frederick-vs-ja]

Also note an additional limitation: calls to the binary comparison functions with mixed argument types will select the UCRT templates as being better matches.

I'm planning to create a follow-up PR addressing this. The changes would be somehow boilerplate though.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.