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From: "zero at smallinteger dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/95566] New: x86 instruction selection --- some REX
 prefixes unnecessary
Date: Sun, 07 Jun 2020 10:01:47 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95566

            Bug ID: 95566
           Summary: x86 instruction selection --- some REX prefixes
                    unnecessary
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zero at smallinteger dot com
  Target Milestone: ---

Created attachment 48696
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D48696&action=3Dedit
sample code

Consider the code attached, compiled with

gcc -O3 sample.c -o sample

Gcc produces unrolled loop code that follows the pattern below.

        movzx   ecx, WORD PTR [rsp-62]
        cmp     rdx, rcx

Here, rdx has the value of k >> 48.  The top 32 bits of rdx are zero after =
the
shift, so the entirety of k >> 48 is in edx.  Thus, the cmp instructions co=
uld
be

        cmp     edx, ecx

instead.  This difference avoids the REX prefix, and thus the instructions =
are
shorter.  After sufficient unrolling (or with e.g. more complex comparisons
that depend on k >> 48), shorter instructions without the REX prefix will be
better even accounting for the partial register dependency (or an instructi=
on
to break the dependency).  The Intel optimization manual says shorter
instructions are better.

The attachment is the entirety of sample.c.  I did not include other files
because this attachment appears to qualify for that exemption due to excuse
(ii): the attached test case is small and does not include any other file.

I originally found this behavior looking at the disassembly of gcc (Gentoo
9.2.0-r2 p3) 9.2.0.  I verified the same behavior with gcc 10.1 and gcc tru=
nk
at godbolt.=