poor optimisation case

[Tristan Wibberley <tristan at wibberley dot org>]

Hi

I've found a case which looks like it should be possible to optimise but
gcc (very recent trunk) isn't doing which could give improvements in
many cases - certainly in a case I've come across:

	#ifdef NEW
	unsigned int fn(unsigned int n, unsigned int dmax) throw()
	{
	  for (unsigned int d = 0; d < dmax; ++d) {
	    n += d?d:1;
	  }
	  return n;
	}
	#else
	unsigned int fn(unsigned int n, unsigned int dmax) throw()
	{
	  unsigned int add = 1;
	  for (unsigned int d = 0; d < dmax; add = ++d) {
	    n += add;
	  }
	  return n;
	}
	#endif

When compiled with -O3 -DOLD I get:

		.p2align 4,,15
	.globl _Z2fnjj
		.type	_Z2fnjj, @function
	_Z2fnjj:
	.LFB2:
		testl	%esi, %esi
		je	.L2
		movl	$1, %edx
		xorl	%eax, %eax
		.p2align 4,,10
		.p2align 3
	.L3:
		addl	$1, %eax
		addl	%edx, %edi
		cmpl	%esi, %eax
		movl	%eax, %edx
		jne	.L3
	.L2:
		movl	%edi, %eax
		ret
	.LFE2:
		.size	_Z2fnjj, .-_Z2fnjj
	
but with -DNEW I get:

		.p2align 4,,15
	.globl _Z2fnjj
		.type	_Z2fnjj, @function
	_Z2fnjj:
	.LFB2:
		testl	%esi, %esi
		je	.L2
		movl	$1, %edx
		xorl	%eax, %eax
		movl	$1, %ecx
		jmp	.L7
		.p2align 4,,10
		.p2align 3
	.L5:
		testl	%eax, %eax
		movl	%ecx, %edx
		cmovne	%eax, %edx
	.L7:
		addl	$1, %eax
		addl	%edx, %edi
		cmpl	%esi, %eax
		jne	.L5
	.L2:
		movl	%edi, %eax
		ret
	.LFE2:
		.size	_Z2fnjj, .-_Z2fnjj

The performance difference is about 50% with -DNEW taking 1.5 times as
long as -DOLD (that was with dmax == 1000000000).

The loop unfortunately can't always be written as in -DOLD as the
implementation of an iterator adapter might use ?: to special case the
first element of a sequence and when used in a generic algorithm which
just has the simple loop of -DNEW it ought to be optimised like -DOLD if
inlining occurs.

-- 
Tristan Wibberley

Any opinion expressed is mine (or else I'm playing devils advocate for
the sake of a good argument). My employer had nothing to do with this
communication.