Formally Verified Argument Reduction with a Fused Multiply-Add

Sylvie Boldo (1) , Marc Daumas (2) , Ren-Cang Li (3)

1 PROVAL - Proof of Programs
2 ELIAUS - Electronique, Informatique, Automatique et Systèmes
3 University of Texas at Arlington [Arlington]

The Cody and Waite argument reduction technique works perfectly for reasonably large arguments, but as the input grows, there are no bits left to approximate the constant with enough accuracy. Under mild assumptions, we show that the result computed with a fused multiply-add provides a fully accurate result for many possible values of the input with a constant almost accurate to the full working precision. We also present an algorithm for a fully accurate second reduction step to reach full double accuracy (all the significand bits of two numbers are accurate) even in the worst cases of argument reduction. Our work recalls the common algorithms and presents proofs of correctness. All the proofs are formally verified using the Coq automatic proof checker.

Mots clés: Argument reduction; fma; formal proof; Coq

Domaines: Arithmétique des ordinateurs; Logiciel mathématique [cs.MS]; Performance et fiabilité [cs.PF]

Sylvie Boldo, Marc Daumas, Ren-Cang Li. Formally Verified Argument Reduction with a Fused Multiply-Add. IEEE Transactions on Computers, 2009, 58 (8), pp.1139-1145. ⟨10.1109/TC.2008.216⟩. ⟨hal-00168401v2⟩