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Date: Wed, 11 Dec 2019 10:53:41 +0100
From: "Stefan Kanthak" <stefan.kanthak@...go.de>
To: "Rich Felker" <dalias@...c.org>
Cc: <musl@...ts.openwall.com>
Subject: Re: More patches for math subtree
"Rich Felker" <dalias@...c.org> wrote:
> On Tue, Dec 10, 2019 at 10:32:26PM +0100, Stefan Kanthak wrote:
[ asm vs. C ]
>> Does any compiler emit branch-free instruction sequences like the
>> following for Intel CPUs without SSE4.1, i.e. without ROUNDSS/ROUNDSD?
>>
>> .code ; Intel syntax
>> ceil proc public
>> extern __real@...0000000000000:real8
>> movsd xmm1, __real@...0000000000000
>> extern __real@...0000000000000:real8
>> movsd xmm2, __real@...0000000000000
>> extern __real@...0000000000000:real8
>> movsd xmm3, __real@...0000000000000
>> movsd xmm4, xmm1
>> andnpd xmm1, xmm0
>> andpd xmm4, xmm0
>> cmpltsd xmm1, xmm3
>> andpd xmm1, xmm3
>> orpd xmm1, xmm4
>> movsd xmm3, xmm0
>> addsd xmm0, xmm1
>> subsd xmm0, xmm1
>> movsd xmm1, xmm0
>> cmpltsd xmm0, xmm3
>> andpd xmm0, xmm2
>> addsd xmm0, xmm1
>> orpd xmm0, xmm4
>> ret
>> ceil endp
>>
>> Or instruction sequences like
>>
>> .code ; Intel syntax
>> copysign proc public
>> movd rcx, xmm0
>> movd rdx, xmm1
>> shld rcx, rdx, 1
>> ror rcx, 1
>> movd xmm0, rcx
>> ret
>> copysign endp
>
> Not quite (but it might be possible to write the C in terms of shifts
> instead of masks such that it does), but I also don't think it's clear
> which version is better. Yours here is mildly smaller and might
> perform better, but when making changes that aren't clearly better
> there should be some evidence that it's actually an improvement --
> especially if it's not just improving existing arch optimizations but
> adding new ones where the C was formerly used.
Correct.
I expect the compiler to emit such properly optimised code instead of
calls to the library for standard functions like copysign(), fdim(),
etc. which can be written with just a few instructions ... what the
compiler but not (always) does.
JFTR: I don't know whether GCC or clang either provide intrinsics or
__builtin_* for such (or all those) small standard functions.
> Generally musl avoids asm and arch-specific files as much as possible,
> using them only for things that aren't representable in C or where
> the C is a lot larger or slower or both.
>
>> .code ; Intel syntax
>> fdim proc public
>> movsd xmm2, xmm0
>> cmpsd xmm0, xmm1, 6
>> subsd xmm2, xmm1
>> andpd xmm0, xmm2
>> ret
>> fdim endp
>
> Does this handle nans correctly?
Of course! It's equivalent to
double fdim(double a, double b)
{
uint64_t mask = (a <= b) ? 0ull : ~0ull;
union {double dbl; uint64_t ull;} u = {a - b};
u.ull &= mask;
return u.dbl;
}
[...]
> OK. I don't mind looking at these patches further as-is, and I'll try
> to continue offering constructive comments now, but it'll be after
> this release cycle (hopefully wrapping that up in the next week or so)
> before consideration for merging. musl 1.2.0 is already going to be a
> release with big changes (time64) and I don't want to risk subtle
> breakage with new changes that haven't been reviewed in detail yet or
> had time for users to test.
That's OK.
Stefan
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