Re: [avr-gcc-list] [WinAVR 20090313] Bug in split-Pattern for swap

[Georg-Johann Lay]

Georg-Johann Lay schrieb:
> Hi guys, in pass .166r.split1, avr-gcc makes a wrong transformation for 
> the following testcase, compiled with -O:
>
> void g(unsigned char);
>
> void f1(unsigned x)
> {
>         unsigned char y;
>         y = (x >> 12) & 0x0F;
>         g(y);
> }
To be more specific, I attached the asm output for the even simpler test 
case

char f1 (unsigned x)
{
    return x >> 12;
}

compiled with -mmcu=atmega168 -O -fverbose-asm -dp -S.

The bug is also triggered for right shifts by 14 and 13, and also for 
WinAVR-20080512.

The bug is also present in a patched avr-gcc 4.3.3 distro at 
http://packages.debian.org/en/sid/gcc-avr. However, I cannot reproduce 
the bug with avr-gcc built from svn gcc_4_3_3_release and therefore I 
cannot say if the mentioned split patterns are the very problem or just 
reveal an other bug somewhere else.

Projects using the respective versions of avr-gcc can try 
-fno-split-wide-types as a fix.

Georg-Johann



        .file   "foo.c"
__SREG__ = 0x3f
__SP_H__ = 0x3e
__SP_L__ = 0x3d
__CCP__  = 0x34
__tmp_reg__ = 0
__zero_reg__ = 1
 ;  GNU C (WinAVR 20090313) version 4.3.2 (avr)
 ;      compiled by GNU C version 3.4.5 (mingw-vista special r3), GMP version 
4.2.3, MPFR version 2.4.0.
 ;  GGC heuristics: --param ggc-min-expand=47 --param ggc-min-heapsize=32702
 ;  options passed:  -imultilib avr5 -iprefix
 ;  e:\winavr-20090313\bin\../lib/gcc/avr/4.3.2/ foo.c -mmcu=atmega168 -O
 ;  -fverbose-asm
 ;  options enabled:  -falign-loops -fargument-alias -fauto-inc-dec
 ;  -fbranch-count-reg -fcommon -fcprop-registers -fdefer-pop
 ;  -fearly-inlining -feliminate-unused-debug-types -ffunction-cse
 ;  -fgcse-lm -fguess-branch-probability -fident -fif-conversion
 ;  -fif-conversion2 -finline-functions-called-once -fipa-pure-const
 ;  -fipa-reference -fivopts -fkeep-static-consts -fleading-underscore
 ;  -fmath-errno -fmerge-constants -fmerge-debug-strings
 ;  -fmove-loop-invariants -fomit-frame-pointer -fpeephole
 ;  -freg-struct-return -fsched-interblock -fsched-spec
 ;  -fsched-stalled-insns-dep -fsigned-zeros -fsplit-ivs-in-unroller
 ;  -fsplit-wide-types -ftoplevel-reorder -ftrapping-math -ftree-ccp
 ;  -ftree-ch -ftree-copy-prop -ftree-copyrename -ftree-dce
 ;  -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-loop-im
 ;  -ftree-loop-ivcanon -ftree-loop-optimize -ftree-parallelize-loops=
 ;  -ftree-reassoc -ftree-salias -ftree-scev-cprop -ftree-sink -ftree-sra
 ;  -ftree-ter -ftree-vect-loop-version -funit-at-a-time -fverbose-asm
 ;  -fzero-initialized-in-bss

 ;  Compiler executable checksum: abe89850c430a90419070abaa31bf632

        .text
.global f1
        .type   f1, @function
f1:
/* prologue: function */
/* frame size = 0 */
        swap r24         ;       ;  26  *swap   [length = 1]
        andi r24,lo8(15)         ; ,     ;  27  andqi3/2        [length = 1]
/* epilogue start */
        ret      ;  31  return  [length = 1]
        .size   f1, .-f1