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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/101908] [12 regression] cray regression with -O2
 -ftree-slp-vectorize compared to -O2
Date: Tue, 01 Mar 2022 09:33:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101908

--- Comment #29 from Hongtao.liu <crazylht at gmail dot com> ---
>>From Agner Fog's excellent optimization
manuals(https://www.agner.org/optimize/microarchitecture.pdf).

For ICX/TGL:

An aligned write of 128 bits or more followed by a read of one or both of t=
he
two halves or
the four quarters, etc., has little or no penalty. A partial read that does=
 not
fit into the halves
or quarters fails to forward. The write-to-read latency is 19-20 clock cycl=
es
when forwarding
fails.
A read that is bigger than the write, or a read that covers both written and
unwritten bytes,
fails to forward. The write-to-read latency is 19-20 clock cycles.

And for Intel software optimization guide:

There are several cases in which data is passed through memory, and the sto=
re
may need to be sepa-
rated from the load:
=E2=80=A2 Spills, save and restore registers in a stack frame.
=E2=80=A2 Parameter passing.
=E2=80=A2 Global and volatile variables.
=E2=80=A2 Type conversion between integer and floating-point.
=E2=80=A2 When compilers do not analyze code that is inlined, forcing varia=
bles that
are involved in the interface with inlined code to be in memory, creating m=
ore
memory variables and preventing the elimination of
redundant loads.=