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ca7b4bd236
cc Aldy Hernandez <aldyh@redhat.com> and Andrew MacLeod <amacleod@redhat.com>,
they are author of new VRP analysis for GCC, just to make sure I didn't
mis-understanding or mis-interpreting anything on GCC site.
GCC 11 have better value range analysis, that give GCC more confidence
to perform more aggressive optimization, but it cause scalbn/scalbnf get
wrong result.
Using scalbn to demostrate what happened on GCC 11, see comments with VRP
prefix:
```c
double scalbn (double x, int n)
{
/* VRP RESULT: n = [-INF, +INF] */
__int32_t k,hx,lx;
...
k = (hx&0x7ff00000)>>20;
/* VRP RESULT: k = [0, 2047] */
if (k==0) {
/* VRP RESULT: k = 0 */
...
k = ((hx&0x7ff00000)>>20) - 54;
if (n< -50000) return tiny*x; /*underflow*/
/* VRP RESULT: k = -54 */
}
/* VRP RESULT: k = [-54, 2047] */
if (k==0x7ff) return x+x; /* NaN or Inf */
/* VRP RESULT: k = [-54, 2046] */
k = k+n;
if (k > 0x7fe) return huge*copysign(huge,x); /* overflow */
/* VRP RESULT: k = [-INF, 2046] */
/* VRP RESULT: n = [-INF, 2100],
because k + n <= 0x7fe is false, so:
1. -INF < [-54, 2046] + n <= 0x7fe(2046) < INF
2. -INF < [-54, 2046] + n <= 2046 < INF
3. -INF < n <= 2046 - [-54, 2046] < INF
4. -INF < n <= [0, 2100] < INF
5. n = [-INF, 2100] */
if (k > 0) /* normal result */
{SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20)); return x;}
if (k <= -54) {
/* VRP OPT: Evaluate n > 50000 as true...*/
if (n > 50000) /* in case integer overflow in n+k */
return huge*copysign(huge,x); /*overflow*/
else return tiny*copysign(tiny,x); /*underflow*/
}
k += 54; /* subnormal result */
SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
return x*twom54;
}
```
However give the input n = INT32_MAX, k = k+n will overflow, and then we
expect got `huge*copysign(huge,x)`, but new VRP optimization think
`n > 50000` is never be true, so optimize that into `tiny*copysign(tiny,x)`.
so the solution here is to moving the overflow handle logic before `k = k + n`.
86 lines
2.1 KiB
C
86 lines
2.1 KiB
C
/* sf_scalbn.c -- float version of s_scalbn.c.
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* Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
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*/
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunPro, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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#include "fdlibm.h"
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#include <limits.h>
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#include <float.h>
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#if INT_MAX > 50000
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#define OVERFLOW_INT 50000
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#else
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#define OVERFLOW_INT 30000
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#endif
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#ifdef __STDC__
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static const float
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#else
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static float
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#endif
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two25 = 3.355443200e+07, /* 0x4c000000 */
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twom25 = 2.9802322388e-08, /* 0x33000000 */
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huge = 1.0e+30,
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tiny = 1.0e-30;
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#ifdef __STDC__
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float scalbnf (float x, int n)
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#else
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float scalbnf (x,n)
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float x; int n;
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#endif
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{
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__int32_t k,ix;
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__uint32_t hx;
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GET_FLOAT_WORD(ix,x);
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hx = ix&0x7fffffff;
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k = hx>>23; /* extract exponent */
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if (FLT_UWORD_IS_ZERO(hx))
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return x;
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if (!FLT_UWORD_IS_FINITE(hx))
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return x+x; /* NaN or Inf */
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if (FLT_UWORD_IS_SUBNORMAL(hx)) {
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x *= two25;
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GET_FLOAT_WORD(ix,x);
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k = ((ix&0x7f800000)>>23) - 25;
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if (n< -50000) return tiny*x; /*underflow*/
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}
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if (n > OVERFLOW_INT) /* in case integer overflow in n+k */
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return huge*copysignf(huge,x); /*overflow*/
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k = k+n;
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if (k > FLT_LARGEST_EXP) return huge*copysignf(huge,x); /* overflow */
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if (k > 0) /* normal result */
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{SET_FLOAT_WORD(x,(ix&0x807fffff)|(k<<23)); return x;}
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if (k < FLT_SMALLEST_EXP)
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return tiny*copysignf(tiny,x); /*underflow*/
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k += 25; /* subnormal result */
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SET_FLOAT_WORD(x,(ix&0x807fffff)|(k<<23));
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return x*twom25;
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}
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#ifdef _DOUBLE_IS_32BITS
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#ifdef __STDC__
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double scalbn(double x, int n)
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#else
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double scalbn(x,n)
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double x;
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int n;
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#endif
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{
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return (double) scalbnf((float) x, n);
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}
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#endif /* defined(_DOUBLE_IS_32BITS) */
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