* [Bug libstdc++/96958] Long Double in Hash Table policy forces soft-float calculations
2020-09-07 16:12 [Bug libstdc++/96958] New: Long Double in Hash Table policy forces soft-float calculations jgreenhalgh at gcc dot gnu.org
@ 2020-09-07 16:24 ` jgreenhalgh at gcc dot gnu.org
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From: jgreenhalgh at gcc dot gnu.org @ 2020-09-07 16:24 UTC (permalink / raw)
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--- Comment #1 from James Greenhalgh <jgreenhalgh at gcc dot gnu.org> ---
Asleep at the wheel today, I had intended to link to the
https://gcc.gnu.org/pipermail/libstdc++/2011-September/036420.html original
discussion rather than leave it as a tedious exercise for the reader.
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2020-09-07 16:12 [Bug libstdc++/96958] New: Long Double in Hash Table policy forces soft-float calculations jgreenhalgh at gcc dot gnu.org
2020-09-07 16:24 ` [Bug libstdc++/96958] " jgreenhalgh at gcc dot gnu.org
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Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2020-09-07
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
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2020-09-07 16:12 [Bug libstdc++/96958] New: Long Double in Hash Table policy forces soft-float calculations jgreenhalgh at gcc dot gnu.org
2020-09-07 16:24 ` [Bug libstdc++/96958] " jgreenhalgh at gcc dot gnu.org
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From: rguenth at gcc dot gnu.org @ 2020-09-08 6:35 UTC (permalink / raw)
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Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
IMHO _any_ FP calculation in that spot is unwanted (but _M_max_load_factor is a
FP value?)
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--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, the public API for the load factor is defined in terms of float. We use a
higher precision type internally though.
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From: houdek.ryan@fex-emu.org @ 2020-09-16 17:17 UTC (permalink / raw)
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Ryan Houdek <houdek.ryan@fex-emu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |houdek.ryan@fex-emu.org
--- Comment #4 from Ryan Houdek <houdek.ryan@fex-emu.org> ---
Hello. Original creator of this fork here.
It would be nice to at least remove the long double requirement here, I
personally don't have much stock in the internal implementation details other
than that.
I believe other STL compliant hashable implementations end up doing something
using integers for their bucket calculations. So I guess it isn't strictly
necessary that it needs to even be double there.
In my particular case the long double implementation falls down a particularly
slow soft float path for me, so this has real performance implications. More so
than just the regular AArch64 soft float path.
Looking forward to this getting rectified.
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--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:
https://gcc.gnu.org/g:a1343e5c74093124d7fbce6052d838f47a8eeb20
commit r11-4581-ga1343e5c74093124d7fbce6052d838f47a8eeb20
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Fri Oct 30 15:14:33 2020 +0000
libstdc++: Use double for unordered container load factors [PR 96958]
These calculations were changed to use long double nearly ten years ago
in order to get more precision than float:
https://gcc.gnu.org/pipermail/libstdc++/2011-September/036420.html
However, double should be sufficient, whlie being potentially faster
than long double, and not requiring soft FP calculations for targets
without native long double support.
libstdc++-v3/ChangeLog:
PR libstdc++/96958
* include/bits/hashtable_policy.h (_Prime_rehash_policy)
(_Power2_rehash_policy): Use double instead of long double.
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2020-09-07 16:12 [Bug libstdc++/96958] New: Long Double in Hash Table policy forces soft-float calculations jgreenhalgh at gcc dot gnu.org
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Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |11.0
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I've just realised there's a second place I need to replace long double ...
which explains why the commit above didn't fix the test failures I was seeing
on POWER9 when mixing ibm128 and ieee128 long double formats.
I probably should have looked at this commit sooner:
(In reply to James Greenhalgh from comment #0)
> It was pointed out that some forks of GCC (
> https://github.com/FEX-Emu/gcc/commit/
> 8a2b7389f50a50a4e26ec98101d47fb1fc1c1bcd ) reduce the hashtable policy
> implementation from a long double to a double. Doing this reduces it from a
> soft-float calculation to hardware floating-point.
N.B. the commit msg in that fork talks about Oracle making this change. I don't
think it has anything to do with Oracle at all, except that one of the main
libstdc++ contributors happens to work for them. But he didn't change it
because of anything Oracle was doing or wanted, it was just normal libstdc++
maintenance.
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--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:
https://gcc.gnu.org/g:943cc2a1b70f2d755b4fed97b1c4b49234d92899
commit r11-4585-g943cc2a1b70f2d755b4fed97b1c4b49234d92899
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Sat Oct 31 00:52:57 2020 +0000
libstdc++: Use double for unordered container load factors [PR 96958]
My previous commit for this PR changed the types from long double to
double, but didn't change the uses of __builtin_ceill and
__builtin_floorl. It also failed to change the non-inline functions in
src/c++11/hashtable_c++0x.cc. This should fix it properly now.
libstdc++-v3/ChangeLog:
PR libstdc++/96958
* include/bits/hashtable_policy.h (_Prime_rehash_policy)
(_Power2_rehash_policy): Use ceil and floor instead of ceill and
floorl.
* src/c++11/hashtable_c++0x.cc (_Prime_rehash_policy): Likewise.
Use double instead of long double.
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2020-09-07 16:12 [Bug libstdc++/96958] New: Long Double in Hash Table policy forces soft-float calculations jgreenhalgh at gcc dot gnu.org
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Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |FIXED
Status|NEW |RESOLVED
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Should be properly fixed now.
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2020-09-07 16:12 [Bug libstdc++/96958] New: Long Double in Hash Table policy forces soft-float calculations jgreenhalgh at gcc dot gnu.org
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--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
N.B. the calls to __builtin_ceill and __builtin_floorl also need to be changed
to avoid implicit conversions to long double.
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2020-09-07 16:12 [Bug libstdc++/96958] New: Long Double in Hash Table policy forces soft-float calculations jgreenhalgh at gcc dot gnu.org
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--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There are a few other places doing unnecessary long double arithmetic, e.g.
r11-4588-60d9f254876a00260992b2f37639ef4d82d9db8f
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