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Date: Thu, 20 Mar 2014 23:59:05 -0500
From: Andrew M <>
Subject: Re: [PHC] Supporting AVX2/SSE2 or not with a single binary

CPU feature detection is only part the battle! How you then implement the
accelerated versions is the real headache. Do you

a) Use intrinsics only. Lets you do write-once-run-everywhere, but unless
the code is trivially optimized, different compilers can produce wildly
varying performance profiles. Non-Visual Studio compilers will also have to
compile each version with the appropriate extension flags set.

b) Drop to inline asm for the important stuff. This requires separate
32bit/64bit versions, but with .intel_syntax and some macros you can use a
single 32 bit version across all compilers (Crypto++ does this).
Unfortunately, Visual Studio doesn't allow 64 bit inline asm, so you have
to fall back to intrinsics on it. Also, clang switched to their own
assembler, which doesn't/didn't yet support .intel_syntax, requiring a
compiler flag to switch back to the system assembler.

c) Use external asm. It's possible to support both windows and *nix with
only gcc, using MinGW/MinGW64, but you do need some kind of stub on Win64
because it uses a different calling convention than System V, so register
saving/parameter translation has to be done.

d) Use external asm with a separate assembler (Nasm/Tasm/Fasm). Complicates
the build process, but frees you from gcc, so alternate compilers (Visual
Studio) are now possible. Yasm has an AT&T syntax parser, and I've been
toying with an abomination (supported with a few macros) which allows a
file to be assembled with either gcc or Yasm, avoiding the need for a
separate assembler when gcc is available, while still letting Visual Studio
use the same file.

Further complications:

Figuring out what CPU extensions the compiler supports if using intrinsics,
or what the assembler supports if using assembler (they aren't the same!).
Yasm/Nasm simplify this as you just require the latest version, or have
macro checks based on the version. I don't know how to do this with gas
outside of just seeing which instructions compile.

Bundling extension support in the OS/executable format is potentially not a
useful idea. It may be desirable to have access to all underlying
implementations and manually select one in cases such as old AMDs where
SSE2 can be slower than straight x86 depending, or

"Future releases will need to run a self-test to gather data about which
revisions have the problem and then we can selectively disable the
fast-path code on those devices."

I have a headache now.

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