[Bug rtl-optimization/27856] New: With -Os, loading a constant to a register can use another register

["etienne_lorrain at yahoo dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

$ cat tmp.c
unsigned athird (unsigned val)
  {
  return val / 3;
  }
$ /home/etienne/projet/toolchain/bin/gcc -S -Os -o tmp.s -fomit-frame-pointer
-fverbose-asm tmp.c
$ cat tmp.s
        .file   "tmp.c"
# GNU C version 4.1.1 (i686-pc-linux-gnu)
#       compiled by GNU C version 4.1.1.
# GGC heuristics: --param ggc-min-expand=62 --param ggc-min-heapsize=60570
# options passed:  -mtune=pentiumpro -auxbase-strip -Os
# -fomit-frame-pointer -fverbose-asm
# options enabled:  -falign-loops -fargument-alias -fbranch-count-reg
# -fcaller-saves -fcommon -fcprop-registers -fcrossjumping
# -fcse-follow-jumps -fcse-skip-blocks -fdefer-pop
# -fdelete-null-pointer-checks -fearly-inlining
# -feliminate-unused-debug-types -fexpensive-optimizations -ffunction-cse
# -fgcse -fgcse-lm -fguess-branch-probability -fident -fif-conversion
# -fif-conversion2 -finline-functions -finline-functions-called-once
# -fipa-pure-const -fipa-reference -fipa-type-escape -fivopts
# -fkeep-static-consts -fleading-underscore -floop-optimize
# -floop-optimize2 -fmath-errno -fmerge-constants -fomit-frame-pointer
# -foptimize-register-move -foptimize-sibling-calls -fpcc-struct-return
# -fpeephole -fpeephole2 -fregmove -freorder-functions
# -frerun-cse-after-loop -frerun-loop-opt -fsched-interblock -fsched-spec
# -fsched-stalled-insns-dep -fschedule-insns2 -fshow-column
# -fsplit-ivs-in-unroller -fstrength-reduce -fstrict-aliasing
# -fthread-jumps -ftrapping-math -ftree-ccp -ftree-copy-prop
# -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-fre
# -ftree-loop-im -ftree-loop-ivcanon -ftree-loop-optimize -ftree-lrs
# -ftree-salias -ftree-sink -ftree-sra -ftree-store-ccp
# -ftree-store-copy-prop -ftree-ter -ftree-vect-loop-version -ftree-vrp
# -funit-at-a-time -fverbose-asm -fzero-initialized-in-bss -m32 -m80387
# -m96bit-long-double -malign-stringops -mfancy-math-387 -mfp-ret-in-387
# -mieee-fp -mno-red-zone -mpush-args -mtls-direct-seg-refs

# Compiler executable checksum: acc0f3237f8807740daa75cf2b5b2d98

        .text
.globl athird
        .type   athird, @function
athird:
        movl    4(%esp), %eax   # val, val
        movl    $3, %edx        #, tmp63
        movl    %edx, %ecx      # tmp63,
        xorl    %edx, %edx      # tmp62
        divl    %ecx    #
        ret
        .size   athird, .-athird
        .ident  "GCC: (GNU) 4.1.1"
        .section        .note.GNU-stack,"",@progbits

  Here tmp63 is not needed and the two lines:
        movl    $3, %edx        #, tmp63
        movl    %edx, %ecx      # tmp63,
  Should be replaced by:
        movl    $3, %ecx


-- 
           Summary: With -Os, loading a constant to a register can use
                    another register
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: etienne_lorrain at yahoo dot fr
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27856