x86 linux oddity

[Michael Meissner <meissner at cygnus dot com>]

I was answering a question about optimization and noticed the
following oddity on the x86 egcs linux port.  Consider the following
program:

	int a, b, c, d, e, f, g, x, y;

	main(){
	  x=(b+c+d)*(e/f+g);
	  y=(b+c+d)/(e/f+g);
	  return 0;
	}

The current egcs linux port generates the following code using the
-fomit-frame-pointer -O2 switches on my pentium pro.  Note that it
stores and restores ebx, even though ebx is never used.

		.file	"foo.c"
		.version	"01.01"
	/ GNU C version egcs-2.90.14 971023 (gcc2-970802 experimental) (i686-pc-linux-gnulibc1) compiled by GNU C version 2.7.2.1.
	/ options passed:  -O2 -fomit-frame-pointer
	/ options enabled:  -fdefer-pop -fomit-frame-pointer -fcse-follow-jumps
	/ -fcse-skip-blocks -fexpensive-optimizations -fthread-jumps
	/ -fstrength-reduce -fpeephole -fforce-mem -ffunction-cse -finline
	/ -fkeep-static-consts -fcaller-saves -fpcc-struct-return
	/ -frerun-cse-after-loop -frerun-loop-opt -fschedule-insns2 -fcommon
	/ -fverbose-asm -fgnu-linker -fregmove -falias-check -fargument-alias
	/ -m80387 -mhard-float -mno-soft-float -mieee-fp -mfp-ret-in-387
	/ -mschedule-prologue -mcpu=pentiumpro -march=pentium

	gcc2_compiled.:
	.text
		.align 4
	.globl main
		.type	 main,@function
	main:
		pushl %esi
		movl e,%esi
		movl %esi,%eax
		cltd
		idivl f
		movl c,%ecx
		addl b,%ecx
		addl d,%ecx
		movl %eax,%esi
		addl g,%esi
		movl %ecx,%eax
		imull %esi,%eax
		movl %eax,x
		movl %ecx,%eax
		cltd
		idivl %esi
		movl %eax,%ecx
		movl %ecx,y
		pushl %ebx		<----- useless
		xorl %eax,%eax
		popl %ebx		<----- useless
		popl %esi
		ret
	.Lfe1:
		.size	 main,.Lfe1-main
		.comm	a,4,4
		.comm	b,4,4
		.comm	c,4,4
		.comm	d,4,4
		.comm	e,4,4
		.comm	f,4,4
		.comm	g,4,4
		.comm	x,4,4
		.comm	y,4,4
		.ident	"GCC: (GNU) egcs-2.90.14 971023 (gcc2-970802 experimental)"

-- 
Michael Meissner, Cygnus Solutions (Massachusetts office)
4th floor, 955 Massachusetts Avenue, Cambridge, MA 02139, USA
meissner@cygnus.com,	617-354-5416 (office),	617-354-7161 (fax)