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From: "kazeemanuar at googlemail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/112102] New: Inefficient Integer multiplication on MIPS
 processors
Date: Thu, 26 Oct 2023 14:46:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112102

            Bug ID: 112102
           Summary: Inefficient Integer multiplication on MIPS processors
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kazeemanuar at googlemail dot com
  Target Milestone: ---

Running integer multiplication with the -Os flag enabled can generate 2
unnecessary NOP instructions. This increases the cost of integer multiplica=
tion
from 7 to 9 cycles in most cases.


Example code:
int test(int a, int b, int c, int d)
{
     return 788*a + 789 * b + 187 + c + d;
}

output:
        li      $2,788
        mult    $4,$2
        li      $2,789  <--- could be moved down into one of the NOPs
        mflo    $4
        nop
        nop
        mult    $5,$2
        mflo    $5
        addu    $4,$4,$5
        addiu   $4,$4,187 <--- could be moved up into one of the NOPs
        addu    $4,$4,$6  <--- could be moved up into one of the NOPs
        jr      $31
        addu    $2,$4,$7

This happens on all GCC versions as far as I can tell.
Compiler explorer link: https://godbolt.org/z/M3x3s3KhM=