https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95566
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=48696&action=edit
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 could
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 instruction
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 trunk
at godbolt.
