Strange PIC code on ia32 since Aug. 29.

[Horst von Brand]

"H . J . Lu" <hjl@valinux.com> said:

[...]

> 2. Use gcc 2.95.2, egcs 1.1.2 or gcc 2.7.2.3.
> 
> Study psABI says about PIC and what the previous gcc generates. I am
> enlosing the correct asm output here. gcc shouldn't output some random
> asm code and call it PIC. Did you see the difference?
> 
> 
> H.J.
> ----
> 	.file	"p.c"
> 	.version	"01.01"
> gcc2_compiled.:
> .text
> 	.align 4
> .globl foo
> 	.type	 foo,@function
> foo:
> 	pushl	%ebp
> 	movl	%esp, %ebp
> 	pushl	%ebx
> 	pushl	%eax
> 	call	.L3
> .L3:
> 	popl	%ebx
> 	addl	$_GLOBAL_OFFSET_TABLE_+[.-.L3], %ebx
> 	call	bar@PLT
> 	movl	-4(%ebp), %ebx
> 	leave
> 	ret
> .Lfe1:
> 	.size	 foo,.Lfe1-foo
> 	.ident	"GCC: (GNU) 2.96 20000731 (experimental)"

AFAIU, @PLT doesn't use %ebx, so the original code you cited is right, gcc
optimized out the setting of %ebx (perhaps not 100% according to the specs,
but correct). OTOH, for:

int x;

foo ()
{
  bar (x);
}

I get (P3):

	.file	"tst.c"
	.version	"01.01"
gcc2_compiled.:
.text
	.align 16
.LPR0:
	movl	(%esp), %ebx
	ret
.globl foo
	.type	 foo,@function
foo:
	pushl	%ebp
	movl	%esp, %ebp
	pushl	%ebx
	subl	$16, %esp
	call	.LPR0
	addl	$_GLOBAL_OFFSET_TABLE_, %ebx
	movl	x@GOT(%ebx), %eax
	movl	(%eax), %eax
	pushl	%eax
	call	bar@PLT
	movl	-4(%ebp), %ebx
	movl	%ebp, %esp
	popl	%ebp
	ret
.Lfe1:
	.size	 foo,.Lfe1-foo
	.comm	x,4,4
	.ident	"GCC: (GNU) 2.96 20000901 (experimental)"