On Mon, Nov 14, 2011 at 9:01 AM, Jiangning Liu <jiangning....@arm.com> wrote:
> Hi,
>
> For this test case,
>
> int x;
> extern void f(void);
>
> void g(int *a)
> {
> a[x] = 1;
> if (x == 100)
> f();
> a[x] = 2;
> }
>
> For trunk, the x86 assembly code is like below,
>
> movl x, %eax
> movl 16(%esp), %ebx
> movl $1, (%ebx,%eax,4)
> movl x, %eax // Is this a redundant one?
> cmpl $100, %eax
> je .L4
> movl $2, (%ebx,%eax,4)
> addl $8, %esp
> .cfi_remember_state
> .cfi_def_cfa_offset 8
> popl %ebx
> .cfi_restore 3
> .cfi_def_cfa_offset 4
> ret
> .p2align 4,,7
> .p2align 3
> .L4:
> .cfi_restore_state
> call f
> movl x, %eax
> movl $2, (%ebx,%eax,4)
> addl $8, %esp
> .cfi_def_cfa_offset 8
> popl %ebx
> .cfi_restore 3
> .cfi_def_cfa_offset 4
> Ret
>
> Is the 2nd "movl x, %eax" is a redundant one for single thread programming
> model? If yes, can this be optimized away?
f() may change the value of x, so you cannot optimize away the load on that
execution path.
> Thanks,
> -Jiangning
>
>
>
>
