gcc-2.7 creates faster code than pgcc-1.1.1

[Zack Weinberg]

On Thu, 4 Mar 1999 22:20:18 +0100, Jamie Lokier wrote:
>> After several day of search I finally find out offending
>> instruction that slow down gzip compiled with egcs-1.1.1/pgcc-1.1.1
>> on PentiumPro 180MHz (132MB RAM) but the result seems crazy to me.
>> 
>> This instruction is:
>> andl $255, %eax
>> in flush_window (util.c) function body (it is inlined from updcrc)
>> 
>> if you manually replace it with
>> movzbl %al, $eax
>> this will boost decompression by 20%.
>
>In the past I have written hand-optimised assembly language, tuned for
>the different x86 families, and I found movzbl to be a very effective
>instruction on the Pentium Pro.  So what you describe sounds correct.
>
>Another is to do xorl %eax,%eax just before loading something into %al.
>That is fast on the PPro too.

A related issue:  I see us generate a lot of code like this for loops over
strings:

loop:
	xorl %eax, %eax
	movb (%esi), %al
	incl %esi
	movb %al, (%edi)
	incl %edi
	testl %eax
	jne loop

After the first iteration, the xorl is unnecessary.  We ought to be able to
hoist it out of the loop.

zw