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Subject: [Bug target/54816] New: [avr] shift is better than widening mul
Date: Thu, 04 Oct 2012 18:28:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54816

             Bug #: 54816
           Summary: [avr] shift is better than widening mul
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gjl@gcc.gnu.org
                CC: eric.weddington@atmel.com
            Target: avr


The following C test case 

int wmul (char a, char b)
{
    return a * (char) (b << 3);
}

$ avr-gcc wmul.c -S -Os -mmcu=atmega8 -dp

produces with current avr-gcc:


wmul:
    ldi r25,lo8(8)     ;  25    movqi_insn/2    [length = 1]
    muls r22,r25     ;  26    mulqihi3    [length = 3]
    movw r22,r0
    clr __zero_reg__
    muls r24,r22     ;  17    mulqihi3    [length = 3]
    movw r24,r0
    clr __zero_reg__
    ret     ;  29    return    [length = 1]
    .ident    "GCC: (GNU) 4.8.0 20121004 (experimental)"


avr-gcc-4.7 was smarter with its code:

wmul:
    lsl r22     ;  10    *ashlqi3/5    [length = 3]
    lsl r22
    lsl r22
    muls r24,r22     ;  12    mulqihi3    [length = 3]
    movw r22,r0
    clr __zero_reg__
    movw r24,r22     ;  31    *movhi/1    [length = 1]
    ret     ;  30    return    [length = 1]
    .ident    "GCC: (GNU) 4.7.2"


The 4.7 code is faster, smaller and has smaller register pressure.