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Date: Wed, 27 Feb 2019 20:14:07 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: FE Exception triggered by comparison
On Wed, 27 Feb 2019, Rich Felker wrote:
> Are there reasons we should perhaps use the __builtin versions of
> these when __GNUC__ indicates they're available? I like our bit test
> versions we have now, and I think they're sufficiently efficient, but
> I'm open to changes if there's a good reason.
Well, it really depends on what one considers 'sufficiently efficient'.
Instead of comparing a register with itself and testing flags (2 instructions)
you get (for 'int f(double x){return isnan(x);}'):
f:
movabsq $9223372036854775807, %rdx
movq %xmm0, %rax
andq %rdx, %rax
movabsq $9218868437227405312, %rdx
cmpq %rdx, %rax
seta %al
movzbl %al, %eax
ret
(note that movq %xmm0, %rax is going to be more costly than a normal
move as it crosses from fp to integer domain in the cpu)
I think musl bit test can be implemented more efficiently via right-shifting
the representation in %rax first, avoiding 64-bit immediates, but even then
I'd say the "native" version is preferable.
Alexander
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