Re: RE???[musl ] [math] I Found math library's bug in ceil, floor, round functions, Using arm toolchains

[Szabolcs Nagy <nsz@port70.net>]

* bobodog <8192542@qq.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