Re: [PATCH v3] LoongArch:Implement 128-bit floating point functions in gcc.

[chenxiaolong <chenxiaolong@loongson.cn>]

在 2023-08-17四的 15:08 +0000，Joseph Myers写道：
> On Thu, 17 Aug 2023, Xi Ruoyao via Gcc-patches wrote:
> 
> > So I guess we just need
> > 
> > builtin_define ("__builtin_fabsq=__builtin_fabsf128");
> > builtin_define ("__builtin_nanq=__builtin_nanf128");
> > 
> > etc. to map the "q" builtins to "f128" builtins if we really need
> > the
> > "q" builtins.
> > 
> > Joseph: the problem here is many customers of LoongArch CPUs wish
> > to
> > compile their old code with minimal change.  Is it acceptable to
> > add
> > these builtin_define's like rs6000-c.cc?  Note "a new architecture"
> > does
> > not mean we'll only compile post-C2x-era programs onto it.
> 
> The powerpc support for __float128 started in GCC 6, predating the
> support 
> for _FloatN type names, built-in functions etc. in GCC 7 - that's
> why 
> there's such backwards compatibility support there.  That name only
> exists 
> on a few architectures.
> 
> If people really want to compile code using the old __float128 names
> for 
> LoongArch I suppose you could have such #defines, but it would be
> better 
> for people to make their code use the standard names (as supported
> from 
> GCC 7 onwards, though only from GCC 13 in C++) and then put
> backwards 
> compatibility in their code for using the __float128 names if they
> want to 
> support the type with older GCC (GCC 6 or before for C; GCC 12 or
> before 
> for C++) on x86_64 / i386 / powerpc / ia64.  Such backwards
> compatibility 
> in user code is more likely to be relevant for C++ than for C, given
> how 
> the C++ support was added to GCC much more recently.  (Note: I
> haven't 
> checked when other compilers added support for the _Float128 name or 
> associated built-in functions, whether for C or for C++, which might
> also 
> affect when user code wants such compatibility.)
> 
Thank you for your valuable comments. On the LoongArch architecture,
the "__float128" type is associated with float128_type_node and the "q"
suffix function is mapped to the "f128" function. This allows
compatibility with both "__float128" and "_Float128" types in the GCC
compiler. The new code is modified as follows:
  Add the following to the loongarch-builtins.c file:
+lang_hooks.types.register_builtin_type (float128_type_node,
"__float128");
  Add the following to the loongarch-c.c file:
+builtin_define ("__builtin_fabsq=__builtin_fabsf128");
+builtin_define ("__builtin_copysignq=__builtin_copysignf128");
+builtin_define ("__builtin_nanq=__builtin_nanf128");
+builtin_define ("__builtin_nansq=__builtin_nansf128");
+builtin_define ("__builtin_infq=__builtin_inff128");
+builtin_define ("__builtin_huge_valq=__builtin_huge_valf128");

 The regression tests of the six functions were added without problems.
However, the implementation of the __builtin_nansq() function does not
get the result we want. The questions are as follows:
 x86_64:
    _Float128 ret=__builtin_nansf128("NAN");

    compiled to (with gcc test.c -O2 ):
 		.cfi_offset 1, -8
	bl	%plt(__builtin_nansf128)
        ..
 LoongArch:
    _Float128 ret=__builtin_nansf128("NAN");
      compiled to (with gcc test.c -O2 ):
	.cfi_offset 1, -8
	bl	%plt(__builtin_nansf128)
        ..
   Obviously, there may have been legacy issues with the implementation
when "_Float128 __builtin_nansf128()" was first supported.
Architectures including LoongArch, x86_64, arm, etc. are no longer
supported, and some of the remaining architectures are unproven.
   I will continue to follow up the implementation of the builtin
function and complete the function.