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From: "unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/107167] It looks like GCC wastes registers on
 trivial computations when result can be cached
Date: Thu, 06 Oct 2022 06:03:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107167

--- Comment #3 from cqwrteur <unlvsur at live dot com> ---
(In reply to Andrew Pinski from comment #1)
> This is a reassociation, scheduling issue and register allocation issue.
>=20
> Plus your example might be slower due to dependencies.
>=20
> Without a full example of where gcc ra goes wrong, gcc actually produces
> much better code for this example due to register renaming in hw.
> Note many x86_64 also does register renaming for the stack too

https://github.com/openssl/openssl/blob/a8572674f12ceb39f7e66ccbaa8918b922c=
76739/crypto/sha/asm/sha512-x86_64.pl#L16

They mentioned that before. 40% improvement over compiler-generated code.
"I really wonder why gcc # [being armed with inline assembler] fails to
generate as fast code."


# sha256/512_block procedure for x86_64.
#
# 40% improvement over compiler-generated code on Opteron. On EM64T
# sha256 was observed to run >80% faster and sha512 - >40%. No magical
# tricks, just straight implementation... I really wonder why gcc
# [being armed with inline assembler] fails to generate as fast code.
# The only thing which is cool about this module is that it's very
# same instruction sequence used for both SHA-256 and SHA-512. In
# former case the instructions operate on 32-bit operands, while in
# latter - on 64-bit ones. All I had to do is to get one flavor right,
# the other one passed the test right away:-)=