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Re: 2.95, x86: severe performance problems with short arithmetic
- To: Zack Weinberg <zack@bitmover.com>
- Subject: Re: 2.95, x86: severe performance problems with short arithmetic
- From: Jeffrey A Law <law@cygnus.com>
- Date: Tue, 10 Aug 1999 23:02:56 -0600
- cc: John Wehle <john@feith.com>, gcc@gcc.gnu.org
- Reply-To: law@cygnus.com
> Does this include something like this?
>
> movzbw %dl, %ax
> addl %eax, %esi
>
> It certainly sounds like it, and would explain why such a tiny
> difference turns into a 2x performance loss.
I believe so, yes. However, if the destination of such an insn is not inside
a strict_low_part, we can probably turn the first instruction into a
movzbl. That would avoid the partial register stall between the movzb/add,
but could potentially introduce a stall with an earlier insn.
jeff