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Fix strtod rounding of half the least subnormal (bug 16151)


This patch fixes bug 16151, incorrect strtod handling of inputs that
are exactly plus or minus half the least subnormal, for
round-to-nearest.  In this case, the caller of round_and_return
computes a MANT_DIG-bit mantissa, all of which needs shifting out, and
the computation of whether any bits were set in that mantissa wrongly
tests the high-order limb (this limb gets used as round_limb, so the
relevant subset of bits in it get handled later, so this loop can
completely ignore it when setting more_bits).  This patch makes the
required adjustment to the loop; the other shift case with such a loop
is already correct.  Tested x86_64 and x86.

2013-11-11  Joseph Myers  <joseph@codesourcery.com>

	[BZ #16151]
	* stdlib/strtod_l.c (round_and_return): Do not consider
	retval[RETURN_LIBM_SIZE - 1] when determining more_bits for an
	exponent one less than half the least subnormal exponent.
	* stdlib/test-strtod-round-data: Add more tests.
	* stdlib/tst-strtod-round.c (tests): Regenerated.

diff --git a/stdlib/strtod_l.c b/stdlib/strtod_l.c
index 90541cd..c1c5c0d 100644
--- a/stdlib/strtod_l.c
+++ b/stdlib/strtod_l.c
@@ -228,7 +228,7 @@ round_and_return (mp_limb_t *retval, intmax_t exponent, int negative,
 
 	  round_limb = retval[RETURN_LIMB_SIZE - 1];
 	  round_bit = (MANT_DIG - 1) % BITS_PER_MP_LIMB;
-	  for (i = 0; i < RETURN_LIMB_SIZE; ++i)
+	  for (i = 0; i < RETURN_LIMB_SIZE - 1; ++i)
 	    more_bits |= retval[i] != 0;
 	  MPN_ZERO (retval, RETURN_LIMB_SIZE);
 	}
diff --git a/stdlib/tst-strtod-round-data b/stdlib/tst-strtod-round-data
index 86d460e..a6c3767 100644
--- a/stdlib/tst-strtod-round-data
+++ b/stdlib/tst-strtod-round-data
@@ -109,3 +109,11 @@
 -0x0.7p-1074
 -0x0.7p-16445
 -0x0.7p-16494
+0x1p-150
+0x1p-1075
+0x1p-16446
+0x1p-16495
+-0x1p-150
+-0x1p-1075
+-0x1p-16446
+-0x1p-16495
diff --git a/stdlib/tst-strtod-round.c b/stdlib/tst-strtod-round.c
index 9a44026..e7aaed1 100644
--- a/stdlib/tst-strtod-round.c
+++ b/stdlib/tst-strtod-round.c
@@ -7535,6 +7535,294 @@ static const struct test tests[] = {
 	-0x0p+0L,
 	-0x0p+0L,
 	-0x0p+0L),
+  TEST ("0x1p-150",
+	false,
+	0x0p+0f,
+	0x0p+0f,
+	0x0p+0f,
+	0x8p-152f,
+	true,
+	0x4p-152,
+	0x4p-152,
+	0x4p-152,
+	0x4p-152,
+	true,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	true,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	true,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	true,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	true,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L,
+	0x4p-152L),
+  TEST ("0x1p-1075",
+	false,
+	0x0p+0f,
+	0x0p+0f,
+	0x0p+0f,
+	0x8p-152f,
+	false,
+	0x0p+0,
+	0x0p+0,
+	0x0p+0,
+	0x4p-1076,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x4p-1076L,
+	true,
+	0x2p-1076L,
+	0x2p-1076L,
+	0x2p-1076L,
+	0x2p-1076L,
+	true,
+	0x2p-1076L,
+	0x2p-1076L,
+	0x2p-1076L,
+	0x2p-1076L,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x4p-1076L,
+	true,
+	0x2p-1076L,
+	0x2p-1076L,
+	0x2p-1076L,
+	0x2p-1076L),
+  TEST ("0x1p-16446",
+	false,
+	0x0p+0f,
+	0x0p+0f,
+	0x0p+0f,
+	0x8p-152f,
+	false,
+	0x0p+0,
+	0x0p+0,
+	0x0p+0,
+	0x4p-1076,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x4p-1076L,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x8p-16448L,
+	true,
+	0x4p-16448L,
+	0x4p-16448L,
+	0x4p-16448L,
+	0x4p-16448L,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x4p-1076L,
+	true,
+	0x4p-16448L,
+	0x4p-16448L,
+	0x4p-16448L,
+	0x4p-16448L),
+  TEST ("0x1p-16495",
+	false,
+	0x0p+0f,
+	0x0p+0f,
+	0x0p+0f,
+	0x8p-152f,
+	false,
+	0x0p+0,
+	0x0p+0,
+	0x0p+0,
+	0x4p-1076,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x4p-1076L,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x8p-16448L,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x4p-16448L,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x4p-1076L,
+	false,
+	0x0p+0L,
+	0x0p+0L,
+	0x0p+0L,
+	0x4p-16496L),
+  TEST ("-0x1p-150",
+	false,
+	-0x8p-152f,
+	-0x0p+0f,
+	-0x0p+0f,
+	-0x0p+0f,
+	true,
+	-0x4p-152,
+	-0x4p-152,
+	-0x4p-152,
+	-0x4p-152,
+	true,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	true,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	true,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	true,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	true,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L,
+	-0x4p-152L),
+  TEST ("-0x1p-1075",
+	false,
+	-0x8p-152f,
+	-0x0p+0f,
+	-0x0p+0f,
+	-0x0p+0f,
+	false,
+	-0x4p-1076,
+	-0x0p+0,
+	-0x0p+0,
+	-0x0p+0,
+	false,
+	-0x4p-1076L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	true,
+	-0x2p-1076L,
+	-0x2p-1076L,
+	-0x2p-1076L,
+	-0x2p-1076L,
+	true,
+	-0x2p-1076L,
+	-0x2p-1076L,
+	-0x2p-1076L,
+	-0x2p-1076L,
+	false,
+	-0x4p-1076L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	true,
+	-0x2p-1076L,
+	-0x2p-1076L,
+	-0x2p-1076L,
+	-0x2p-1076L),
+  TEST ("-0x1p-16446",
+	false,
+	-0x8p-152f,
+	-0x0p+0f,
+	-0x0p+0f,
+	-0x0p+0f,
+	false,
+	-0x4p-1076,
+	-0x0p+0,
+	-0x0p+0,
+	-0x0p+0,
+	false,
+	-0x4p-1076L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	false,
+	-0x8p-16448L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	true,
+	-0x4p-16448L,
+	-0x4p-16448L,
+	-0x4p-16448L,
+	-0x4p-16448L,
+	false,
+	-0x4p-1076L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	true,
+	-0x4p-16448L,
+	-0x4p-16448L,
+	-0x4p-16448L,
+	-0x4p-16448L),
+  TEST ("-0x1p-16495",
+	false,
+	-0x8p-152f,
+	-0x0p+0f,
+	-0x0p+0f,
+	-0x0p+0f,
+	false,
+	-0x4p-1076,
+	-0x0p+0,
+	-0x0p+0,
+	-0x0p+0,
+	false,
+	-0x4p-1076L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	false,
+	-0x8p-16448L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	false,
+	-0x4p-16448L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	false,
+	-0x4p-1076L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L,
+	false,
+	-0x4p-16496L,
+	-0x0p+0L,
+	-0x0p+0L,
+	-0x0p+0L),
 };
 
 static int

-- 
Joseph S. Myers
joseph@codesourcery.com


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