On Monday 18 April 2005 20:53, Nicholas Nethercote wrote:
> Hi,
>
> I've been looking at GCC's use of sign-extensions when dealing with
> integers smaller than a machine word size. It looks like there is room
> for improvement.
>
> Consider this C function:
>
> short g(short x)
> {
> short i;
> for (i = 0; i < 10; i++) {
> x += i;
> }
> return x;
> }
>
> On x86, using a GCC 4.0.0 20050130, with -O2 I get this code:
>
> g:
> pushl %ebp
> xorl %edx, %edx
> movl %esp, %ebp
> movswl 8(%ebp),%ecx
> .p2align 4,,15
> .L2:
> leal (%ecx,%edx), %eax
> movswl %ax,%ecx # 1
> leal 1(%edx), %eax
> movzwl %ax, %eax # 2
> cmpw $10, %ax
> movswl %ax,%edx # 3
> jne .L2
>
> popl %ebp
> movl %ecx, %eax
> ret
> .size g, .-g
> .p2align 4,,15
>
> The three extensions (#1, #2, #3) here are unnecessarily conservative.
> This would be better:
>
> g:
> pushl %ebp
> xorl %edx, %edx
> movl %esp, %ebp
> movswl 8(%ebp),%ecx
> .p2align 4,,15
> .L2:
> leal (%ecx,%edx), %ecx # x += i
> leal 1(%edx), %edx # i++
> cmpw $10, %dx # i < 10 ?
> jne .L2
>
> popl %ebp
> movswl %cx, %eax
> ret
Here is what I get on amd64 with -m32:
.file "t.c"
.text
.p2align 4,,15
.globl g
.type g, @function
g:
pushl %ebp
xorl %edx, %edx
movl %esp, %ebp
movswl 8(%ebp),%eax
.p2align 4,,15
.L2:
addl %edx, %eax
incl %edx
cmpl $10, %edx
cwtl
jne .L2
popl %ebp
ret
.size g, .-g
.ident "GCC: (GNU) 4.1.0 20050412 (experimental)"
.section .note.GNU-stack,"",@progbits
Looks a bit more like your optimal code ;-)
Gr.
Steven
