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[PATCH] Fix powf overflow handling in non-nearest rounding mode
- From: Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>
- To: "newlib at sourceware dot org" <newlib at sourceware dot org>
- Cc: nd <nd at arm dot com>
- Date: Mon, 10 Dec 2018 15:04:37 +0000
- Subject: [PATCH] Fix powf overflow handling in non-nearest rounding mode
Follows the fix in https://github.com/ARM-software/optimized-routines
From 8ff59d2f4e0209f03c30462e9319baf4c6ba79f7 Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date: Mon, 10 Dec 2018 14:40:01 +0000
Subject: [PATCH] Fix powf overflow handling in non-nearest rounding mode
The threshold value at which powf overflows depends on the rounding mode
and the current check did not take this into account. So when the result
was rounded away from zero it could become infinity without setting
errno to ERANGE.
Example: pow(0x1.7ac7cp+5, 23) is 0x1.fffffep+127 + 0.1633ulp
If the result goes above 0x1.fffffep+127 + 0.5ulp then errno is set,
which is fine in nearest rounding mode, but
powf(0x1.7ac7cp+5, 23) is inf in upward rounding mode
powf(-0x1.7ac7cp+5, 23) is -inf in downward rounding mode
and the previous implementation did not set errno in these cases.
The fix tries to avoid affecting the common code path or calling a
function that may introduce a stack frame, so float arithmetics is used
to check the rounding mode and the threshold is selected accordingly.
---
newlib/libm/common/sf_pow.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/newlib/libm/common/sf_pow.c b/newlib/libm/common/sf_pow.c
index f7b22b612..2946c611b 100644
--- a/newlib/libm/common/sf_pow.c
+++ b/newlib/libm/common/sf_pow.c
@@ -220,7 +220,17 @@ powf (float x, float y)
{
/* |y*log(x)| >= 126. */
if (ylogx > 0x1.fffffffd1d571p+6 * POWF_SCALE)
+ /* |x^y| > 0x1.ffffffp127. */
return __math_oflowf (sign_bias);
+ if (WANT_ROUNDING && WANT_ERRNO
+ && ylogx > 0x1.fffffffa3aae2p+6 * POWF_SCALE)
+ /* |x^y| > 0x1.fffffep127, check if we round away from 0. */
+ if ((!sign_bias
+ && eval_as_float (1.0f + opt_barrier_float (0x1p-25f)) != 1.0f)
+ || (sign_bias
+ && eval_as_float (-1.0f - opt_barrier_float (0x1p-25f))
+ != -1.0f))
+ return __math_oflowf (sign_bias);
if (ylogx <= -150.0 * POWF_SCALE)
return __math_uflowf (sign_bias);
#if WANT_ERRNO_UFLOW
--
2.14.1