Loop optimization

[bearophile]

Walter Bright:

> is not done because of roundoff error. Also,
>     0 * x => 0
> is also not done because it is not a correct replacement if x is a NaN.

I have done a little experiment, compiling this D1 code with LDC:


import tango.stdc.stdio: printf;
void main(char[][] args) {
    double x = cast(double)args.length;
    double y = 0 * x;
    printf("%f\n", y);
}


I think the asm generated by ldc shows what you say:


ldc -O3 -release -inline -output-s test
_Dmain:
	pushl	%ebp
	movl	%esp, %ebp
	andl	$-16, %esp
	subl	$32, %esp
	movsd	.LCPI1_0, %xmm0
	movd	8(%ebp), %xmm1
	orps	%xmm0, %xmm1
	subsd	%xmm0, %xmm1
	pxor	%xmm0, %xmm0
	mulsd	%xmm1, %xmm0
	movsd	%xmm0, 4(%esp)
	movl	$.str, (%esp)
	call	printf
	xorl	%eax, %eax
	movl	%ebp, %esp
	popl	%ebp
	ret	$8



So I have added an extra "unsafe floating point" optimization:

ldc -O3 -release -inline -enable-unsafe-fp-math -output-s test
_Dmain:
	subl	$12, %esp
	movl	$0, 8(%esp)
	movl	$0, 4(%esp)
	movl	$.str, (%esp)
	call	printf
	xorl	%eax, %eax
	addl	$12, %esp
	ret	$8


GCC has similar switches.

Bye,
bearophile