* [committed] libstdc++: Support VAX floats in std::strong_order
@ 2022-03-10 11:51 Jonathan Wakely
2022-03-10 12:16 ` Jonathan Wakely
0 siblings, 1 reply; 4+ messages in thread
From: Jonathan Wakely @ 2022-03-10 11:51 UTC (permalink / raw)
To: libstdc++, gcc-patches
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
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [committed] libstdc++: Support VAX floats in std::strong_order
2022-03-10 11:51 [committed] libstdc++: Support VAX floats in std::strong_order Jonathan Wakely
@ 2022-03-10 12:16 ` Jonathan Wakely
2022-03-10 14:27 ` Jonathan Wakely
0 siblings, 1 reply; 4+ messages in thread
From: Jonathan Wakely @ 2022-03-10 12:16 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: libstdc++, gcc Patches
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 ...
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [committed] libstdc++: Support VAX floats in std::strong_order
2022-03-10 12:16 ` Jonathan Wakely
@ 2022-03-10 14:27 ` Jonathan Wakely
2022-03-10 17:46 ` Koning, Paul
0 siblings, 1 reply; 4+ messages in thread
From: Jonathan Wakely @ 2022-03-10 14:27 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: libstdc++, gcc Patches
[-- 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);
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [committed] libstdc++: Support VAX floats in std::strong_order
2022-03-10 14:27 ` Jonathan Wakely
@ 2022-03-10 17:46 ` Koning, Paul
0 siblings, 0 replies; 4+ messages in thread
From: Koning, Paul @ 2022-03-10 17:46 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: libstdc++, GCC Patches
> 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
^ permalink raw reply [flat|nested] 4+ messages in thread
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2022-03-10 11:51 [committed] libstdc++: Support VAX floats in std::strong_order Jonathan Wakely
2022-03-10 12:16 ` Jonathan Wakely
2022-03-10 14:27 ` Jonathan Wakely
2022-03-10 17:46 ` Koning, Paul
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