Re: optimizations

[<tm_gccmail@mail.kloo.net>]

On Wed, 15 Jan 2003, Reza Roboubi wrote:

> Bonzini wrote:
> 
> CHANGE:
> -------
> .L2:
> movl -4(%ebp), %eax <== still does the load
> cmpl $16, %eax
> je .L7
> incl %eax
> movl %eax, -4(%ebp) <== and store
> jmp .L2
> .L7:
> 
> TO:
> -------
> movl -4(%ebp), %eax
> .L2:
> cmpl $16, %eax
> je .L7
> incl %eax
> jmp .L2
> .L7:
> movl %eax, -4(%ebp)

The optimization you are suggesting is called "load hoist/store sink" if I
remember correctly.

Here is the story as I remember it:

When egcs-1.0 or 1.1  was released, people noticed a large performance
drop from gcc-2.7.2. I did a little investigation, and verified a large
performanc drop on Whetstone. I did a comprehensive analysis of it, and
isolated a case similar to yours where a variable in a critical loop was
entirely contained in registers in 2.7.2 but was loaded/save from/to
memory in gcc-2.95.

I mentioned this on the gcc-bugs mailing list, and Mark Mitchell
contributed a fairly simple load hoisting improvement to the loop
optmiizer which restored performance on Whetstone.

If you look at the gcc-bugs archives for 1998, you may be able to find
this message thread.

This load-hoisting optimization seems to be responsible for the hoisted
load in your testcase.  However, the corresponding store sink portion of
the optimizer has never been written, and I believe that is why the store
is not sunk out of the loop on your testcase.

Toshi