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From: "arjan at linux dot intel.com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
Date: Tue, 24 May 2022 18:55:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101891

--- Comment #9 from Arjan van de Ven <arjan at linux dot intel.com> ---
I don't have recent measurements since we did this work quite some time ago.

basically on the CPU level (speaking for Intel style cpus at least), a CPU =
can
eliminate (meaning: no execution resources used) 1 to 3 (depending on
generation) register to register per clock cycle.. There's ALSO a path in t=
he
hardware for optimizing XOR <reg><reg> sequences to avoid execution
resources... when we did both we maximized the total number of these
eliminations...
while only XOR you can get bottlenecked on execution if you have too many.
(all the mov's should have no other instructions depending on them, so even
though they depend on the XOR, they're still fully 'orphan' for the out of
order engine)=