Was just looking again at the assembly file generated by GCC, and noted
this pattern I have already seen - maybe beginning with 4.0.
In short:
[EMAIL PROTECTED]:~/projet/gujin$ /home/etienne/projet/toolchain/bin/gcc -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../configure --prefix=/home/etienne/projet/toolchain
--enable-languages=c
Thread model: posix
gcc version 4.1.1 20060517 (prerelease)
[EMAIL PROTECTED]:~/projet/gujin$ cat tmp.c
unsigned short nbinch (unsigned short Hsize_cm)
{
return (3 * Hsize_cm / 4 + 2) / 3;
}
[EMAIL PROTECTED]:~/projet/gujin$ /home/etienne/projet/toolchain/bin/gcc tmp.c
-S -Os -o
tmp.s -fomit-frame-pointer -fverbose-asm -march=i386
[EMAIL PROTECTED]:~/projet/gujin$ cat tmp.s
.file "tmp.c"
# GNU C version 4.1.1 20060517 (prerelease) (i686-pc-linux-gnu)
# compiled by GNU C version 4.1.1 20060517 (prerelease).
# GGC heuristics: --param ggc-min-expand=62 --param ggc-min-heapsize=60570
# options passed: -iprefix -march=i386 -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 -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: b39e0195e7cdfc211bb5a7d4c9f1eb60
.text
.globl nbinch
.type nbinch, @function
nbinch:
movzwl 4(%esp), %eax # Hsize_cm, Hsize_cm
leal (%eax,%eax,2), %eax #, tmp63
movl $4, %edx #, tmp67
movl %edx, %ecx # tmp67,
cltd
idivl %ecx #
addl $2, %eax #, tmp68
movl $3, %edx #, tmp72
movl %edx, %ecx # tmp72,
cltd
idivl %ecx #
movzwl %ax, %eax # tmp70, tmp61
ret
.size nbinch, .-nbinch
.ident "GCC: (GNU) 4.1.1 20060517 (prerelease)"
.section .note.GNU-stack,"",@progbits
The extra options "-fomit-frame-pointer -fverbose-asm -march=i386" are just
there to
simplify the output; the temporary variables tmp67 and tmp72 are created even
without,
and the %edx register is only used once to initialise the %ecx register.
The %edx register is cleared by "cltd" instruction because "%eax" is positive.
Is there any reason why:
movl $4, %edx #, tmp67
movl %edx, %ecx # tmp67,
is not replaced by:
movl $4, %ecx
and:
movl $3, %edx #, tmp72
movl %edx, %ecx # tmp72,
is not replaced by:
movl $3, %ecx
Etienne.
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