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From: "sje at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/59371] New: Performance regression in GCC 4.8 and later versions.
Date: Mon, 02 Dec 2013 17:59:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59371

            Bug ID: 59371
           Summary: Performance regression in GCC 4.8 and later versions.
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sje at gcc dot gnu.org
            Target: mips*-*-*

If I compile this program with -O2 on MIPS:

int foo(int *p, unsigned short c)
{
    signed short i;
    int x = 0;
    for (i = 0; i < c; i++) {
        x = x + *p; p++;
    }
    return x;
}

With GCC 4.7.* or earlier I get loop code that looks like:

$L3:
    lw    $5,0($4)
    addiu    $3,$3,1
    seh    $3,$3
    addu    $2,$2,$5
    bne    $3,$6,$L3
    addiu    $4,$4,4

With GCC 4.8 and later I get:

$L3:
    lw    $7,0($4)
    addiu    $3,$3,1
    seh    $3,$3
    slt    $6,$3,$5
    addu    $2,$2,$7
    bne    $6,$0,$L3
    addiu    $4,$4,4

This loop has one more instruction in it and is slower.
A version of this bug appears in EEMBC 1.1.  If I change the loop
index to be unsigned then I get the better code but I can't change
the benchmark I am testing so I am trying to figure out what changed
in GCC and how to generate the faster code.