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Re: Miscompilation of remainder expressions

From: Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>
To: Michael Veksler <mveksler at tx dot technion dot ac dot il>
Cc: gcc at gcc dot gnu dot org
Date: Tue, 16 Jan 2007 13:19:19 +0100
Subject: Re: Miscompilation of remainder expressions
References: <45AB8770.5040402@cs.unipr.it> <45ABE54F.2010505@tx.technion.ac.il> <20070116045327.GB24654@iram.es> <45AC905A.2060602@tx.technion.ac.il>

Compared to the slow idivl, abs could be negligible, right. However, abs
does introduce new data dependence which might add a noticeable cost.

Is there an abs instruction in the i386 instruction set?

No, the closest thing (input in eax, output in edx) is

cltq
addl %edx, %eax
xorl %eax, %edx

But the register allocations constraints are pretty heavy considering that idivl already puts strain on the RA.

Paolo

Follow-Ups:
- Re: Miscompilation of remainder expressions
  - From: Lucas (a.k.a T-Bird or bsdfan3)

References:
- Miscompilation of remainder expressions
  - From: Roberto Bagnara
- Re: Miscompilation of remainder expressions
  - From: Michael Veksler
- Re: Miscompilation of remainder expressions
  - From: Gabriel Paubert
- Re: Miscompilation of remainder expressions
  - From: Michael Veksler

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