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From: "gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/49939] New: [avr] Skip 2-word instructions if applicable
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49939

           Summary: [avr] Skip 2-word instructions if applicable
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gjl@gcc.gnu.org
                CC: eric.weddington@atmel.com
            Target: avr


Curent avr-gcc from trunk compiles this C code

char c;

void foo (char a, char b)
{
    if (a)
        c = b;
}

with -Os to

foo:
    tst r24     ;  7    *cmpqi/1    [length = 1]
    breq .L1     ;  8    branch    [length = 1]
    sts c,r22     ;  10    *movqi/3    [length = 2]
.L1:
    ret     ;  19    return    [length = 1]

which could be smarter using a CPSE skip instruction instead like so:

foo:
    cpse r24, __zero_reg__
    sts c,r22
.L1:
    ret

At current, avr-gcc just emits skips over 1-word instructions.

As of http://gcc.gnu.org/viewcvs?view=revision&revision=177049
avr-gcc now knows if it is also legal to skip 2-word instructions.