https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=298260
Bug ID: 298260
Summary: [libm] fmaf() mishandled some subnormal input
Product: Base System
Version: CURRENT
Hardware: Any
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: bin
Assignee: [email protected]
Reporter: [email protected]
/*
* Bug pointed out by kib at
*
https://lists.freebsd.org/archives/freebsd-numerics/2026-September/000135.html
*
* fmaf does not handle some subnormal cases correctly. The current
* software implementation for fmaf() yields
*
* x*y+z=0x1.0002p-133
*
* which can be shown to be wrong via a call to the x86_64 fma instruction.
*
* The correct output to the following program is
*
* x*y+z=0x1.0001p-133
*/
#include <math.h>
#include <stdio.h>
int
main(void)
{
float x = -0x1.001p-81f, y = 0x1.ffe002p-70f, z = 0x1.0002p-133f;
float res;
res = fmaf(x, y, z);
printf("x*y+z=%a\n", res);
return (0);
}
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