[committed] libstdc++: Support VAX floats in std::strong_order

[committed] libstdc++: Support VAX floats in std::strong_order

[Jonathan Wakely]

Tested x86_64-linux, and basic soundness check on vax-dec-netbsdelf.

Pushed to trunk.

-- >8 --

The VAX float and double format does not support NaN, so the
std::partial_ordering returned by <=> will never be 'unordered'. We can
just use the partial_ordering value as the strong_ordering.

libstdc++-v3/ChangeLog:

	* libsupc++/compare (_Strong_ordering::_S_fp_cmp) [__vax__]: Use
	<=> comparison.
---
 libstdc++-v3/libsupc++/compare | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
index 050cf7ed20d..3c22d9addf1 100644
--- a/libstdc++-v3/libsupc++/compare
+++ b/libstdc++-v3/libsupc++/compare
@@ -843,6 +843,11 @@ namespace std
 	static constexpr strong_ordering
 	_S_fp_cmp(_Tp __x, _Tp __y) noexcept
 	{
+#ifdef __vax__
+	  // VAX format has no NaN, only "excess" for Inf, so totally ordered.
+	  return __builtin_bit_cast(strong_ordering, __x <=> __y);
+#endif
+
 	  auto __ix = _S_fp_bits(__x);
 	  auto __iy = _S_fp_bits(__y);
 
-- 
2.34.1

Re: [committed] libstdc++: Support VAX floats in std::strong_order

[Jonathan Wakely]

On Thu, 10 Mar 2022 at 11:53, Jonathan Wakely via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Tested x86_64-linux, and basic soundness check on vax-dec-netbsdelf.

But apparently not enough of a soundness check, because
isnan(__builtin_nan("")) is true for VAX, so GCC seems to have a NaN
pattern, despite what I read online about the format.

Fix on the way ...

Re: [committed] libstdc++: Support VAX floats in std::strong_order

[Jonathan Wakely]

[-- Attachment #1: Type: text/plain, Size: 650 bytes --]

On Thu, 10 Mar 2022 at 12:16, Jonathan Wakely wrote:
>
> On Thu, 10 Mar 2022 at 11:53, Jonathan Wakely via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Tested x86_64-linux, and basic soundness check on vax-dec-netbsdelf.
>
> But apparently not enough of a soundness check, because
> isnan(__builtin_nan("")) is true for VAX, so GCC seems to have a NaN
> pattern, despite what I read online about the format.
>
> Fix on the way ...

Here's the fix that adds support for VAX NaN (and works around
PR104865 which I discovered while trying to make this work).

Tested x86_64-linux, and slightly tested on vax-dec-netbsdelf again.

Pushed to trunk.

[-- Attachment #2: patch.txt --]
[-- Type: text/plain, Size: 1737 bytes --]

commit 73f3b8a53e6664c079731c2a183c16621481d039
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 10 14:17:03 2022

    libstdc++: Fix std::strong_order to handle NaN on VAX
    
    I mistakenly believed that VAX floats do not support NaN, but with GCC
    __builtin_isnan(__builtin_nan("")) is true. That means my previous
    change to <compare> is wrong, because it fails to handle NaN.
    
    When std::numeric_limits<floating-point-type>::is_iec559 is false, as on
    VAX, the standard only requires an ordering that is consistent with the
    ordering observed by comparison operators. With this change the ordering
    is -NaN < numbers < +NaN, and there is no support for different NaN bit
    patterns (as I'm not even sure if GCC supports any for VAX).
    
    libstdc++-v3/ChangeLog:
    
            * libsupc++/compare (_Strong_order::_S_fp_cmp) [__vax__]:
            Handle NaN.

diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare
index 3c22d9addf1..6e1ed53eeed 100644
--- a/libstdc++-v3/libsupc++/compare
+++ b/libstdc++-v3/libsupc++/compare
@@ -844,8 +844,16 @@ namespace std
 	_S_fp_cmp(_Tp __x, _Tp __y) noexcept
 	{
 #ifdef __vax__
-	  // VAX format has no NaN, only "excess" for Inf, so totally ordered.
-	  return __builtin_bit_cast(strong_ordering, __x <=> __y);
+	  if (__builtin_isnan(__x) || __builtin_isnan(__y))
+	    {
+	      int __ix = (bool) __builtin_isnan(__x);
+	      int __iy = (bool) __builtin_isnan(__y);
+	      __ix *= __builtin_signbit(__x) ? -1 : 1;
+	      __iy *= __builtin_signbit(__y) ? -1 : 1;
+	      return __ix <=> __iy;
+	    }
+	  else
+	    return __builtin_bit_cast(strong_ordering, __x <=> __y);
 #endif
 
 	  auto __ix = _S_fp_bits(__x);

Re: [committed] libstdc++: Support VAX floats in std::strong_order

[Koning, Paul]

> On Mar 10, 2022, at 9:27 AM, Jonathan Wakely via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
> On Thu, 10 Mar 2022 at 12:16, Jonathan Wakely wrote:
>> 
>> On Thu, 10 Mar 2022 at 11:53, Jonathan Wakely via Libstdc++
>> <libstdc++@gcc.gnu.org> wrote:
>>> 
>>> Tested x86_64-linux, and basic soundness check on vax-dec-netbsdelf.
>> 
>> But apparently not enough of a soundness check, because
>> isnan(__builtin_nan("")) is true for VAX, so GCC seems to have a NaN
>> pattern, despite what I read online about the format.

VAX float has signalling NaN, but not a non-signalling NaN nor an Inf.  See the VAX architecture manual.  Signalling NaN (called "reserved operand") is encoded as sign=1 and exponent=0.

	paul