**diff options**

author | Rich Felker <dalias@aerifal.cx> | 2012-04-11 14:51:08 -0400 |
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committer | Rich Felker <dalias@aerifal.cx> | 2012-04-11 14:51:08 -0400 |

commit | 4054da9ba062c694dc4fde5c577fcb6da7743bc9 (patch) | |

tree | 9f0e3246c2ac8a76d0e0e568b543de385a138c15 | |

parent | 5837a0bb6b5cf516f79527e837368af0b494d51a (diff) | |

download | musl-4054da9ba062c694dc4fde5c577fcb6da7743bc9.tar.gz |

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.

-rw-r--r-- | src/internal/floatscan.c | 5 |

1 files 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<LD_B1B_DIG+3; i++) { uint32_t tmp = x[k] & (1<<sh)-1; x[k] = (x[k]>>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); |