[Bug other/60486] [avr] missed optimization on detecting zero flag set

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60486

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |gjl at gcc dot gnu.org

--- Comment #1 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Created attachment 32326
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32326&action=edit
C test case

Here is a valid test case.

Compiled with avr-gcc 4.8.2

$ avr-gcc pr60486.c -S -Os -mmcu=atmega8 -dp

we get:

pr60486:
    movw r18,r24     ;  5    *movhi/1    [length = 1]
.L2:
    cp r22,r24     ;  21    *cmphi/3    [length = 2]
    cpc r23,r25
    brsh .L7     ;  22    branch    [length = 1]
    subi r18,1     ;  13    addhi3_clobber/2    [length = 2]
    sbc r19,__zero_reg__
    cp r18,__zero_reg__     ;  14    *cmphi/2    [length = 2]
    cpc r19,__zero_reg__
    breq .L5     ;  15    branch    [length = 1]
    adiw r24,1     ;  17    addhi3_clobber/1    [length = 1]
    rjmp .L2     ;  55    jump    [length = 1]
.L7:
    ldi r24,0     ;  7    *movhi/2    [length = 2]
    ldi r25,0
    ret     ;  49    return    [length = 1]
.L5:
    ldi r24,lo8(1)     ;  6    *movhi/5    [length = 2]
    ldi r25,0
    ret     ;  51    return    [length = 1]


The superfluous insn is #14 (*cmphi).
This worked with 4.7 with the output as follows:

pr60486:
    movw r18,r24     ;  7    *movhi/1    [length = 1]
    rjmp .L2     ;  48    jump    [length = 1]
.L4:
    subi r18,1     ;  15    addhi3_clobber/2    [length = 2]
    sbc r19,__zero_reg__
    breq .L5     ;  17    branch    [length = 1]
    adiw r24,1     ;  19    addhi3_clobber/1    [length = 1]
.L2:
    cp r22,r24     ;  23    *cmphi/3    [length = 2]
    cpc r23,r25
    brlo .L4     ;  24    branch    [length = 1]
    ldi r18,0     ;  9    *movhi/2    [length = 2]
    ldi r19,0
    rjmp .L3     ;  50    jump    [length = 1]
.L5:
    ldi r18,lo8(1)     ;  8    *movhi/5    [length = 2]
    ldi r19,0
.L3:
    movw r24,r18     ;  56    *movhi/1    [length = 1]
    ret     ;  55    return    [length = 1]