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Fix overflow and underflow on i386.

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The return value of a 'float' function is in the x87 %st(0) register.
This is an 80bit 'long double' register.
If you multiply 0x1p100f by 0x1p100f the caller sees 0x1p200 - not the
  expected infinity.
So use a 'double' value which goes through a store-load sequence to generate
  the required exception and value.
This commit is contained in:
dsl 2014-03-16 22:30:43 +00:00
parent 18f26c5968
commit ec1660cff5
1 changed files with 26 additions and 5 deletions
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31
lib/libm/src/s_exp2f.c
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@ -25,7 +25,7 @@
*/
#include <sys/cdefs.h>
__RCSID("$NetBSD: s_exp2f.c,v 1.1 2010/01/11 16:28:39 christos Exp $");
__RCSID("$NetBSD: s_exp2f.c,v 1.2 2014/03/16 22:30:43 dsl Exp $");
#ifdef __FBSDID
__FBSDID("$FreeBSD: src/lib/msun/src/s_exp2f.c,v 1.9 2008/02/22 02:27:34 das Exp $");
#endif
@ -39,14 +39,35 @@ __FBSDID("$FreeBSD: src/lib/msun/src/s_exp2f.c,v 1.9 2008/02/22 02:27:34 das Exp
#define TBLSIZE (1 << TBLBITS)
static const float
huge = 0x1p100f,
redux = 0x1.8p23f / TBLSIZE,
P1 = 0x1.62e430p-1f,
P2 = 0x1.ebfbe0p-3f,
P3 = 0x1.c6b348p-5f,
P4 = 0x1.3b2c9cp-7f;
static volatile float twom100 = 0x1p-100f;
/*
* For out of range values we need to generate the appropriate
* underflow or overflow trap as well as generating infinity or zero.
* This means we have to get the fpu to execute an instruction that
* will generate the trap (and not have the compiler optimise it away).
* This is normally done by calculating 'huge * huge' or 'tiny * tiny'.
*
* i386 is particularly problematic.
* The 'float' result is returned on the x87 stack, so is 'long double'.
* If we just multiply two 'float' values the caller will see 0x1p+/-200
* (not 0 or infinity).
* If we use 'double' the compiler does a store-load which will convert the
* value and generate the required exception.
*/
#ifdef __i386__
static volatile double overflow = 0x1p+1000;
static volatile double underflow = 0x1p-1000;
#else
static volatile float huge = 0x1p+100;
static volatile float tiny = 0x1p-100;
#define overflow (huge * huge)
#define underflow (tiny * tiny)
#endif
static const double exp2ft[TBLSIZE] = {
0x1.6a09e667f3bcdp-1,
@ -112,9 +133,9 @@ exp2f(float x)
return (0.0); /* x is -Inf */
}
if(x >= 0x1.0p7f)
return (huge * huge); /* overflow */
return overflow; /* +infinity with overflow */
if(x <= -0x1.2cp7f)
return (twom100 * twom100); /* underflow */
return underflow; /* zero with underflow */
} else if (ix <= 0x33000000) { /* |x| <= 0x1p-25 */
return (1.0f + x);
}