Avoid exponent overflow as well as checking that we don't lose information with
opposing scaling. Use it in partsN(scalbn) and partsN(round_to_int_normal).
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
fpu/softfloat.c | 29 +++++++++++++++++++++++++++++
fpu/softfloat-parts.c.inc | 5 ++---
2 files changed, 31 insertions(+), 3 deletions(-)
diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index a762f4b43a..df94f299e1 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -461,6 +461,15 @@ typedef struct {
uint64_t frac_lo;
} FloatParts256;
+/*
+ * Minimum and maximum exponent for scalbn.
+ * These are chosen to be much larger than the true exponent for any input format,
+ * but also not at the bounds of INT32_{MIN,MAX} so that we can perform other
+ * arithmetic on the exponent without overflowing, particularly during uncanon.
+ */
+#define SCALBN_EXP_MAX 0x0fffffff
+#define SCALBN_EXP_MIN (-SCALBN_EXP_MAX)
+
/* These apply to the most significant word of each FloatPartsN. */
#define DECOMPOSED_BINARY_POINT 63
#define DECOMPOSED_IMPLICIT_BIT (1ull << DECOMPOSED_BINARY_POINT)
@@ -601,6 +610,26 @@ static float128 QEMU_FLATTEN float128_pack_raw(const FloatParts128 *p)
*----------------------------------------------------------------------------*/
#include "softfloat-specialize.c.inc"
+static int32_t exp_scalbn(int32_t exp, int32_t scale)
+{
+ /*
+ * Catch chains of scaling which lose information.
+ * In particular, if the exponent has been saturated,
+ * do not allow it to become unsaturated.
+ */
+ if (exp >= SCALBN_EXP_MAX) {
+ assert(scale >= 0);
+ } else if (exp <= SCALBN_EXP_MIN) {
+ assert(scale <= 0);
+ }
+ if (sadd32_overflow(exp, scale, &exp)) {
+ exp = scale < 0 ? SCALBN_EXP_MIN : SCALBN_EXP_MAX;
+ } else {
+ exp = MIN(MAX(exp, SCALBN_EXP_MIN), SCALBN_EXP_MAX);
+ }
+ return exp;
+}
+
/*
* Helper functions for softfloat-parts.c.inc, per-size operations.
*/
diff --git a/fpu/softfloat-parts.c.inc b/fpu/softfloat-parts.c.inc
index 45606f8402..4715187017 100644
--- a/fpu/softfloat-parts.c.inc
+++ b/fpu/softfloat-parts.c.inc
@@ -1098,8 +1098,7 @@ static bool partsN(round_to_int_normal)(FloatPartsN *a, FloatRoundMode rmode,
uint64_t frac_lsb, frac_lsbm1, rnd_even_mask, rnd_mask, inc;
int shift_adj;
- scale = MIN(MAX(scale, -0x10000), 0x10000);
- a->exp += scale;
+ a->exp = exp_scalbn(a->exp, scale);
if (a->exp < 0) {
bool one;
@@ -1623,7 +1622,7 @@ FloatPartsN partsN(scalbn)(const FloatPartsN *a, int n, float_status *s)
case float_class_normal:
{
FloatPartsN r = *a;
- r.exp += MIN(MAX(n, -0x10000), 0x10000);
+ r.exp = exp_scalbn(r.exp, n);
return r;
}
default:
--
2.43.0