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Subject: [Bug other/54815] New: [avr] missed optimization with operations with constant operands
Date: Thu, 04 Oct 2012 18:20:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54815

             Bug #: 54815
           Summary: [avr] missed optimization with operations with
                    constant operands
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: other
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gjl@gcc.gnu.org
                CC: eric.weddington@atmel.com
            Target: avr


Created attachment 28359
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28359
mov.c: C test case

== C code ==

void f (int, int);

void f_and (int x)
{
    f (x, x & 0xff23);
}

void f_or (int x)
{
    f (x, x | 42);
}

void f_xor (int x)
{
    f (x, x ^ 0x80);
}



== Compile with ==

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

The result is 

f_or:
    ldi r22,lo8(42)     ;  15    *movhi/5    [length = 2]
    ldi r23,0
    or r22,r24     ;  6    iorhi3/1    [length = 2]
    or r23,r25
    rjmp f

but the code could MOVW first to the destination and the IOR instead of loading
the target with the const and then IOR against the source:

f_or:
    movw r22,r24   ; *movhi    [length = 1]
    ori r22,lo8(42)     ;  iorhi3    [length = 1]
    rjmp f

Similar for the other test functions.