Date: Mon, 10 May 2004 16:02:46 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: David Schultz <das@freebsd.org> Cc: freebsd-standards@freebsd.org Subject: Re: Fixing ilogb() Message-ID: <20040510155514.L1063@gamplex.bde.org> In-Reply-To: <20040509201148.P8241@gamplex.bde.org> References: <20040508194748.GN29712@wombat.fafoe.narf.at> <20040509201148.P8241@gamplex.bde.org>
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On Sun, 9 May 2004, I wrote: > ... > % @@ -43,11 +43,27 @@ > % subl $4,%esp > % > % fldl 8(%ebp) > % + fxam > % + fnstsw %ax > % + sahf > > This is the main runtime overhead. I think it can mostly be avoided by > checking the return value. ilogb() can only be INT_MIN after overflow > or other conversion errors (check this). There 3 cases: > - logb(0) = -Inf; fistpl(-Inf) = INT_MIN + IE > - logb(Inf) = Inf; fistpl(-Inf) = INT_MIN + IE > - logb(NaN) = same NaN; fistpl(NaN) = INT_MIN + IE > After finding one of these rare cases, the exact case still needs to be > determined by looking at the original value or the result of fxtract. > Then fucom with 0 should be a faster way to do the classification. A > full classification is not needed sice denormals are not special here > and unsupported formats are unsupported here too. Another thing to decide is whether the exception flags should be set (or not) to indicate overflow. I think they should be. Checking after doing the operation sets them; checking before does not. The standard is not clear. It says that the result is equivalent to (int)logb() for the non-overflowing cases but has special rules with unusual wording for the overflowing cases. It says "If x is zero they [the ilogb functions] compute the value FP_ILOGB0. ..." Computation a constant value is different from returning it, so this can be interpreted as saying that the exception flags may be set. I think there's a meta-rule that math functions set execption flags where appropriate. Brucehelp
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