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From: "kspalaiologos at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/105429] Unnecessary moves generated with _mm_crc32_u64
Date: Fri, 29 Apr 2022 12:38:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105429

--- Comment #2 from Palaiologos <kspalaiologos at gmail dot com> ---
I have observed the same behaviour with and without `mov eax, eax`. CRC32 i=
s a
32-bit checksum, so I'd presume that the high bits aren't considered by the
instruction.

To support my claim, Vol. 2A 3-257 of Intel Software Development Manual giv=
es
the following operation for 2 REX.W 0F 38 F1 /r:

>>>
TEMP1[63-0] :=3D BIT_REFLECT64 (SRC[63-0])
TEMP2[31-0] :=3D BIT_REFLECT32 (DEST[31-0])
TEMP3[95-0] :=3D TEMP1[63-0] =C2=AB 32
TEMP4[95-0] :=3D TEMP2[31-0] =C2=AB 64
TEMP5[95-0] :=3D TEMP3[95-0] XOR TEMP4[95-0]
TEMP6[31-0] :=3D TEMP5[95-0] MOD2 11EDC6F41H
DEST[31-0] :=3D BIT_REFLECT (TEMP6[31-0])
DEST[63-32] :=3D 00000000H
<<<=