Thanks for your pull request and interest in making D better, @liushuyu! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Testing this PR locally

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dub run digger -- build "master + dmd#20826"

please use aliases instead of pragma(mangle, ...), like:

pragma(scanf)
     int __isoc99_scanf(scope const char* format, scope ...);
     ///
     alias scanf = __isoc99_scanf;
``

I don't know if I understand you correctly, but

extern (C) void a();

version (PPC64) {
    static if (real.mant_dig == 113) {
        extern (C) void a__ieee128();
        alias a = a__ieee128;
    }
}

void b() {
    a();
}

This does not work. b will still call a instead of a__ieee128 even if the conditions are met.
LDC and GDC also do not issue any warnings or errors in this case.

Assuming we can't do anything about the alias problem in the compiler, an alternative may be doing this (also taking the suggestions from the LDC pull request):

version (PPC64)
    enum PPCUseIEEE128 = real.mant_dig == 113;
else
    enum PPCUseIEEE128 = false;

static if (PPCUseIEEE128)
{
    real __strtoieee128(scope inout(char)* nptr, scope inout(char)** endptr);
    alias strtold = __strtoieee128;
}
else
{
    real strtold(scope inout(char)* nptr, scope inout(char)** endptr);
}

... which is a bit verbose for math.d as there are a lot of functions to be aliased.

You only need to have one static if and one else branch, you can put multiple declarations for the different function is there.

You only need to have one static if and one else branch, you can put multiple declarations for the different function is there.

I know, but my concern is that declaring each function twice may increase maintenance burden (both versions have the same D function signature), as opposed to using mangle only needs one function definition

All CI tests have passed now.

Uh oh, wrong button pressed.