[Bug c/56219] New: avr-gcc-4.7.2 missing __uint24 loop optimisation

[Bug c/56219] New: avr-gcc-4.7.2 missing __uint24 loop optimisation

[demiurg_spb at freemail dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56219

             Bug #: 56219
           Summary: avr-gcc-4.7.2 missing __uint24 loop optimisation
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: demiurg_spb@freemail.ru


int main(void)
{
//    static const unsigned long __flash br[8] = {0,}; // case1: 194 bytes
    static const __uint24      __flash br[8] = {0,}; // case2: 190 bytes
(should be case1 minus 8 bytes)

    unsigned char i=0;

    for (;;i++)
    {
        *((volatile unsigned char*)100) = br[i%8];
    }

    return (0);
}

//case1: avr-gcc-4.7.2 -S -mmcu=atmega8 -Os main.c
    ldi r24,0
.L2:
    mov r30,r24
    andi r30,lo8(7)
    ldi r25,lo8(4)
    mul r30,r25
    movw r30,r0
    clr __zero_reg__
    subi r30,lo8(-(br.1322))
    sbci r31,hi8(-(br.1322))
    lpm r25,Z
    sts 100,r25
    subi r24,lo8(-(1))
    rjmp .L2

//case2: avr-gcc-4.7.2 -S -mmcu=atmega8 -Os main.c
    ldi r18,0
.L2:
    mov r24,r18
    andi r24,lo8(7)
    ldi r25,0
    movw r30,r24
    lsl r30
    rol r31
    add r30,r24
    adc r31,r25
    subi r30,lo8(-(br.1322))
    sbci r31,hi8(-(br.1322))
    lpm r24,Z
    sts 100,r24
    subi r18,lo8(-(1))
    rjmp .L2

[Bug target/56219] avr-gcc-4.7.2 missing __uint24 loop optimisation

[demiurg_spb at freemail dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56219

--- Comment #1 from demiurg_spb at freemail dot ru 2013-02-06 12:34:53 UTC ---
It would be optimal move outside loop all extra instructions, and use temp
register r0 more active.

    ldi r24,0
    ldi r25,lo8(4)                     // outside loop
.L2:
    mov r30,r24
    andi r30,lo8(7)
    mul r30,r25
    movw r30,r0
    subi r30,lo8(-(br.1322))
    sbci r31,hi8(-(br.1322))
    lpm r0,Z                           // use r0 as temp
    sts 100,r0
    subi r24,lo8(-(1))
    rjmp .L2
    clr __zero_reg__                   // outside loop

[Bug target/56219] avr-gcc-4.7.2 missing __uint24 loop optimisation

[demiurg_spb at freemail dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56219

--- Comment #2 from demiurg_spb at freemail dot ru 2013-02-06 13:06:01 UTC ---
oops r0 - is call-clobbered:(

[Bug target/56219] avr-gcc-4.7.2 missing __uint24 loop optimisation

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56219

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |WONTFIX

--- Comment #3 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2013-04-12 20:05:11 UTC ---
Closed as WON'T FIX

It might be very well the case that code using __[u]int24 (PSImode) variables
is not compiled as efficient as could be.

Maybe some generic optimizations work also for PSImode so that you'll see
better code for PSI operations as a byproduct of some generic optimizations in
the future.  But sorry, no PSI-specific optimization hacks in the avr backend
for now.

Remember that R0 is special and does not survive an insn, i.e. it's used like a
scratch register.