Date: Thu, 24 Feb 2000 17:38:09 +0100
From: Martin Ockajak <mandos AT hq DOT alert DOT sk>
To: pgcc AT delorie DOT com
Subject: Re: short add stuff
Message-ID: <20000224173809.A32390@hq.alert.sk>
References: <38B426AF DOT 280BF1C0 AT sgi DOT com>
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On Wed, Feb 23, 2000 at 10:27:59AM -0800, Linda Walsh wrote:
> Using pgcc from 'Mandrake(70)', it seems to default to "-mpentium".
> If I opt for 386, 486, or pentiumpro, it changes a
> mov	-14(%bp),%ax
> 
> into a:
> movzwl	-14(%bp),%eax
> 
> If I use the pgcc that comes with Suse(63), it defaults for 486, but
> only in the case of "-mpentiumpro" does it do the above substitution.

I don't know what (p)gcc versions these distros use but
use of movz insn family depends on the setting in gcc/config/i386.c
which in the case of the latest pgcc (2.95.3) says:

const int x86_movx = m_386 | m_PPRO | m_K6;

> So....I'm not familiar with the movzwl instruction.  What does it
> do and how does it's timing compare with the 'mov'.  It looks like
> a "move word and zero top 16 bits".

Exactly.
The problem is whether to use

xorl %reg0,%reg0
movw disp(%reg0),%reg1 

or single

movzwl disp(%reg0),%reg1 

>				       My guess is that this is the
> cause for the slowdown?

Surely not on Pentium.
On other CPUs, this is questionable.
AFAIK, on Athlon, probably on PPro and K6, movzxx are faster.

> Oddly, under SuSE, the 486 has the same alignment (.16) as the pentium
> option does on Mandrake.  Switching the two on the respective OS's, 
					  ^^^
> both result in a .4 alignmnent.  Of course this makes no sense.

If I understand you correctly, you mean -malign-xxxx=2.
For gcc on i386 architecture, alignment is set as power of 2,
so this is correct.

> RH seems to default to the 386 option.  Their 486 give a .16
> alignment, but -mpentium gives a .4 alignment, and the pentiumpro
> option gives .4 alignment but with the 'movzwl' instructions.
> 
> So exactly what *SHOULD* be the correct settings and should movzwl's
> be faster than movw's on any arch? 

Code and data aligning is often very non-trivial problem.
I'm afraid I can't give definite answers, if anybody.
For more info, see aligning related discussions in the gcc and pgcc
mailing lists archives.

> Oh -- also, the 386 opt was the only one that used the "leave" instruction
> to fixup the stack frame on exit.  All others use the 2 mov instructions.
> Did leave become slower on all subsequent x86's but it was faster on the 
> 386?

"leave" is faster on i386, K6 and Athlon, but slower on the rest.

> Thanks...
> -linda

-- 
Martin Ockajak a.k.a. Mandos  <mandos AT hq DOT alert DOT sk>  http://hq.alert.sk/~mandos
"The goal of Computer Science is to build something that will last at
least until we've finished building it."

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