Re: SP-GiST micro-optimizations

On 28.08.2012 20:30, Tom Lane wrote:
> Heikki Linnakangas<heikki.linnakangas@enterprisedb.com>  writes:
>> Drilling into the profile, I came up with three little optimizations:
>
>> 1. Within spgdoinsert, a significant portion of the CPU time is spent on
>> line 2033 in spgdoinsert.c:
>
>> memset(&out, 0, sizeof(out));
>
>> That zeroes out a small struct allocated in the stack. Replacing that
>> with MemSet() makes it faster, reducing the time spent on zeroing that
>> struct from 10% to 1.5% of the time spent in spgdoinsert(). That's not
>> very much in the big scheme of things, but it's a trivial change so
>> seems worth it.
>
> Fascinating.  I'd been of the opinion that modern compilers would inline
> memset() for themselves and MemSet was probably not better than what the
> compiler could do these days.  What platform are you testing on?

x64, gcc 4.7.1, running Debian.

The assembly generated for the MemSet is:

	.loc 1 2033 0 discriminator 3
	movq	$0, -432(%rbp)
.LVL166:
	movq	$0, -424(%rbp)
.LVL167:
	movq	$0, -416(%rbp)
.LVL168:
	movq	$0, -408(%rbp)
.LVL169:
	movq	$0, -400(%rbp)
.LVL170:
	movq	$0, -392(%rbp)

while the corresponding memset code is:

	.loc 1 2040 0 discriminator 6
	xorl	%eax, %eax
	.loc 1 2042 0 discriminator 6
	cmpb	$0, -669(%rbp)
	.loc 1 2040 0 discriminator 6
	movq	-584(%rbp), %rdi
	movl	$6, %ecx
	rep stosq

In fact, with -mstringop=unrolled_loop, I can coerce gcc to produce code 
similar to the MemSet version:

	movq	%rax, -440(%rbp)
	.loc 1 2040 0 discriminator 6
	xorl	%eax, %eax
.L254:
	movl	%eax, %edx
	addl	$32, %eax
	cmpl	$32, %eax
	movq	$0, -432(%rbp,%rdx)
	movq	$0, -424(%rbp,%rdx)
	movq	$0, -416(%rbp,%rdx)
	movq	$0, -408(%rbp,%rdx)
	jb	.L254
	leaq	-432(%rbp), %r9
	addq	%r9, %rax
	.loc 1 2042 0 discriminator 6
	cmpb	$0, -665(%rbp)
	.loc 1 2040 0 discriminator 6
	movq	$0, (%rax)
	movq	$0, 8(%rax)

I'm not sure why gcc doesn't choose that by default. Perhaps it's CPU 
specific which variant is faster - I was quite surprised that MemSet was 
such a clear win on my laptop. Or maybe it's a speed-space tradeoff, and 
gcc chooses the more compact version, although using -O3 instead of -O2 
made no difference.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com