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Message-ID: <20141014080634.GC4874@port70.net>
Date: Tue, 14 Oct 2014 10:06:35 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: RE???[musl ] [math] I Found math library's bug in ceil,
floor, round functions, Using arm toolchains
* bobodog <8192542@...com> [2014-10-14 14:26:01 +0800]:
> diff --git a/src/math/ceil.c b/src/math/ceil.c
> index 22dc224..8634145 100644
> --- a/src/math/ceil.c
> +++ b/src/math/ceil.c
> @@ -4,15 +4,16 @@ double ceil(double x)
> {
> union {double f; uint64_t i;} u = {x};
> int e = u.i >> 52 & 0x7ff;
> + uint64_t n = 0x1p52;
> double_t y;
>
> if (e >= 0x3ff+52 || x == 0)
> return x;
> /* y = int(x) - x, where int(x) is an integer neighbor of x */
> if (u.i >> 63)
> - y = (double)(x - 0x1p52) + 0x1p52 - x;
> + y = (double)(x - n) + n - x;
> else
> - y = (double)(x + 0x1p52) - 0x1p52 - x;
> + y = (double)(x + n) - n - x;
> /* special case because of non-nearest rounding modes */
> if (e <= 0x3ff-1) {
> FORCE_EVAL(y);???
>
> To solve this problem, must define a variate: uint64_t n = 0x1p52;
> using n to instead of 0x1p52, then all result are correctly.???
>
this is not necessary (and possibly makes things slower)
in c99 the difference must not be observable, your compiler is broken if it is
>
> The compile flags:
> MCFLAGS := -mcpu=cortex-a8 -mtune=cortex-a8 -march=armv7-a -mfpu=neon -ftree-vectorize -ffast-math -mfloat-abi=softfp???
>
never ever -ffast-math
if you use that flag all bets are off
>
> I belive, when using this flags, the bug will happen. any one tell me how to solve it. we need optimizing flags to compile musl libc.???
>
when compiling libc dont use that flag
otherwise you can use it but i strongly suggest not to,
unless you are fully aware of the consequences
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