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Re: [PATCH] Inline C99 math functions
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: Wilco Dijkstra <wdijkstr at arm dot com>
- Cc: 'Joseph Myers' <joseph at codesourcery dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Date: Wed, 17 Jun 2015 18:34:33 +0200
- Subject: Re: [PATCH] Inline C99 math functions
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On Wed, Jun 17, 2015 at 04:24:46PM +0100, Wilco Dijkstra wrote:
> > >
> > > > > __fpclassify_t: 8.76 7.04
> > > > > fpclassify_t: 4.91 5.17
> > > >
> > > > > __isnormal_inl_t: 8.77 7.16
> > > > > isnormal_t: 3.16 3.17
> > > >
> > > > Where did you get inline? I couldn't find it anywhere. Also such big
> > > > number for inline implementation is suspect
> > >
> > > It does (__fpclassify (x) == FP_NORMAL) like math.h which is obviously a bad
> > > idea and the reason for the low performance. Although the GCC isnormal builtin
> > > is not particularly fast, it still beats it by more than a factor of 2.
> > >
> > No, bad idea was not inlining fpclassify, that affects most of performance difference.
> > There is also problem that glibcdev/glibc/sysdeps/ieee754/dbl-64/s_fpclassify.c is bit slow as
> > it tests unlikely cases first but that is secondary.
>
> Even with the inlined fpclassify (inl2 below), isnormal is slower:
>
> __isnormal_inl2_t: 1.25 3.67
> __isnormal_inl_t: 4.59 2.89
> isnormal_t: 1 1
>
> So using a dedicated builtin for isnormal is important.
>
That makes result identical to one of isnan. That its slower is bug in
fpclassify which should first check for normal, then do unlikely checks.
> > > It's certainly correct, but obviously different microarchitectures will show
> > > different results. Note the GLIBC private inlines are not particularly good.
> > >
> > No, problem is that different benchmarks show different results on same
> > architecture. To speed things up run following to test all cases of
> > environment. Run attached tf script to get results on arm.
>
> I tried, but I don't think this is a good benchmark - you're not measuring
> the FP->int move for the branched version, and you're comparing the signed
> version of isinf vs the builtin which does isinf_ns.
>
Wilco that isinf is signed is again completely irrelevant. gcc is smart.
It get expanded into
if (foo ? (bar ? 1 : -1) : 0)
that gcc simplifies to
if (foo)
so it doesn't matter that checking sign would take 100 cycles as its
deleted code.
Also as it could make branched version only slower when it beats builtin
then also nonsigned one would beat builtin.
And do you have assembly to show it doesn't measure move or its just
your guess? On x64 it definitely measures move and I could add that gcc
messes that bit by moving several times. objdump -d
on gcc -DT2 -DBRANCHED -DI1="__attribute((always_inline))" -DI2="__attribute__((always_inline))" ft.c -c
clearly shows that conversion is done several times.
24c: 48 8b 45 f0 mov -0x10(%rbp),%rax
250: 48 01 d0 add %rdx,%rax
253: f2 0f 10 00 movsd (%rax),%xmm0
257: f2 0f 11 45 e8 movsd %xmm0,-0x18(%rbp)
25c: 48 8d 45 c8 lea -0x38(%rbp),%rax
260: 48 89 45 e0 mov %rax,-0x20(%rbp)
264: f2 0f 10 45 e8 movsd -0x18(%rbp),%xmm0
269: f2 0f 11 45 d8 movsd %xmm0,-0x28(%rbp)
26e: f2 0f 10 45 d8 movsd -0x28(%rbp),%xmm0
273: f2 0f 11 45 c0 movsd %xmm0,-0x40(%rbp)
278: 48 8b 45 c0 mov -0x40(%rbp),%rax
27c: 48 89 45 d0 mov %rax,-0x30(%rbp)
280: 48 8b 45 d0 mov -0x30(%rbp),%rax
284: 48 8d 14 00 lea (%rax,%rax,1),%rdx
288: 48 b8 00 00 00 00 00 movabs $0xffe0000000000000,%rax
28f: 00 e0 ff
292: 48 39 c2 cmp %rax,%rdx
295: 75 1e jne 2b5 <main+0xe3>
297: 48 b8 00 00 00 00 00 movabs $0x7ff0000000000000,%rax
29e: 00 f0 7f
2a1: 48 39 45 d0 cmp %rax,-0x30(%rbp)
2a5: 75 07 jne 2ae <main+0xdc>
2a7: b8 01 00 00 00 mov $0x1,%eax
2ac: eb 0c jmp 2ba <main+0xe8>
2ae: b8 ff ff ff ff mov $0xffffffff,%eax
2b3: eb 05 jmp 2ba <main+0xe8>
2b5: b8 00 00 00 00 mov $0x0,%eax
2ba: 85 c0 test %eax,%eax
2bc: 74 05 je 2c3 <main+0xf1>
So will you publish results or not as it would show your builtins in
unfavorable ligth?
> > Which doesn't matter. As gcc optimized unneded checks away you won't do
> > unneeded checks. As using:
> >
> > __builtin_fpclassify (FP_NAN, FP_INFINITE, \
> > FP_NORMAL, FP_SUBNORMAL, FP_ZERO, x),0);
> > return result == FP_INFINITE || result == FP_NAN;
> >
> > is slower than:
> >
> > return __builtin_isinf (x) || __builtin_isnan (x);
> >
> > Your claim is false, run attached tf2 script to test.
>
> That's not what I am seeing, using two explicit isinf/isnan calls (test2) is faster
> than inlined fpclassify (test1):
>
> __fpclassify_test2_t: 1 4.41
> __fpclassify_test1_t: 1.23 4.66
>
You need to run my benchmark. You get different results if thats inside
if or not so I want to also know my results.