Re: [PATCH] Use fabsl instead of fabs on long double operand in floatscan.c

[Szabolcs Nagy <nsz@port70.net>]

* Dan Gohman <sunfish@mozilla.com> [2019-10-18 07:02:11 -0700]:
> This fixes a compiler warning with clang:
> 
> floatscan.c:304:13: warning: absolute value function 'fabs' given an
> argument of type 'long double' but has parameter of type 'double' which may
> cause truncation of value [-Wabsolute-value].
> 
> This does change the behavior of the expression because the value is no
> longer rounded to double, however from my reading of the code, the rounding
> doesn't seem intended. However, if it is, I suggest introducing an explicit
> cast, to document the intent.
> 
> Dan

> From 1fecc521dc43b25366cd4a3062964ff3abc7506e Mon Sep 17 00:00:00 2001
> From: Dan Gohman <sunfish@mozilla.com>
> Date: Fri, 18 Oct 2019 06:22:49 -0700
> Subject: [PATCH] Use `fabsl` instead of `fabs` on long double in floatscan.c
> 
> This fixes a compiler warning:
> 
> floatscan.c:304:13: warning: absolute value function 'fabs' given an argument
> of type 'long double' but has parameter of type 'double' which may cause
> truncation of value [-Wabsolute-value]

the bug can cause spurious errno=ERANGE setting when
converting decimal string to long double on targets
where LDBL_MANT_DIG > DBL_MANT_DIG.

i believe there is no other side-effect.

example reproducer (should print 0 errno, current musl
prints 34 on aarch64):

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <float.h>
int main()
{
	char buf[100];
	sprintf(buf, "%.42Le", LDBL_MAX);
	errno = 0;
	if (strtold(buf, 0) != LDBL_MAX) return -1;
	printf("%s %d\n", buf, errno);
}


> ---
>  src/internal/floatscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/internal/floatscan.c b/src/internal/floatscan.c
> index 278bf250..99a1ec29 100644
> --- a/src/internal/floatscan.c
> +++ b/src/internal/floatscan.c
> @@ -301,7 +301,7 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
>  	y -= bias;
>  
>  	if ((e2+LDBL_MANT_DIG & INT_MAX) > emax-5) {
> -		if (fabs(y) >= CONCAT(0x1p, LDBL_MANT_DIG)) {
> +		if (fabsl(y) >= CONCAT(CONCAT(0x1p, LDBL_MANT_DIG), l)) {
>  			if (denormal && bits==LDBL_MANT_DIG+e2-emin)
>  				denormal = 0;
>  			y *= 0.5;
> -- 
> 2.17.1
>