From 4054da9ba062c694dc4fde5c577fcb6da7743bc9 Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Wed, 11 Apr 2012 14:51:08 -0400 Subject: [PATCH] optimize floatscan downscaler to skip results that won't be needed when upscaling, even the very last digit is needed in cases where the input is exact; no digits can be discarded. but when downscaling, any digits less significant than the mantissa bits are destined for the great bitbucket; the only influence they can have is their presence (being nonzero). thus, we simply throw them away early. the result is nearly a 4x performance improvement for processing huge values. the particular threshold LD_B1B_DIG+3 is not chosen sharply; it's simply a "safe" distance past the significant bits. it would be nice to replace it with a sharp bound, but i suspect performance will be comparable (within a few percent) anyway. --- src/internal/floatscan.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/src/internal/floatscan.c b/src/internal/floatscan.c index 6390d46a..b2313293 100644 --- a/src/internal/floatscan.c +++ b/src/internal/floatscan.c @@ -200,16 +200,17 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po /* FIXME: find a way to compute optimal sh */ if (rp > 9+9*LD_B1B_DIG) sh = 9; e2 += sh; - for (k=a; k!=z; k=(k+1 & MASK)) { + for (i=0; (k=(a+i & MASK))!=z && i>sh) + carry; carry = (1000000000>>sh) * tmp; if (k==a && !x[k]) { a = (a+1 & MASK); + i--; rp -= 9; } } - if (carry) { + if (carry && k==z) { if ((z+1 & MASK) != a) { x[z] = carry; z = (z+1 & MASK); -- 2.25.1