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Fix x86/x86_64 expm1l spurious underflow exceptions (bug 16539)
- From: "Joseph S. Myers" <joseph at codesourcery dot com>
- To: <libc-alpha at sourceware dot org>
- Date: Tue, 24 Jun 2014 20:47:30 +0000
- Subject: Fix x86/x86_64 expm1l spurious underflow exceptions (bug 16539)
- Authentication-results: sourceware.org; auth=none
This patch fixes bug 16539, spurious underflow exceptions from x86 /
x86-64 expm1l. The problem is that the computation of a base-2
exponent with extra precision involves spurious underflows for
arguments that are small but not subnormal, so a check is added to
just return the argument in those cases. (If the argument *is*
subnormal, underflowing is correct and the existing code will always
underflow, so it suffices to keep using the existing code in that
case; some expm1 implementations have a bug (bug 16353) with missing
underflow exceptions, but I don't think there's such a bug in this
particular version.)
Tested x86_64 and x86; no ulps updates needed.
(auto-libm-test-out diffs omitted below.)
2014-06-24 Joseph Myers <joseph@codesourcery.com>
[BZ #16539]
* sysdeps/i386/fpu/e_expl.S (IEEE754_EXPL) [USE_AS_EXPM1L]: Just
return the argument for normal arguments with exponent below -64.
* sysdeps/x86_64/fpu/e_expl.S (IEEE754_EXPL) [USE_AS_EXPM1L]:
Likewise.
* math/auto-libm-test-in: Add another test of expm1.
* math/auto-libm-test-out: Regenerated.
diff --git a/math/auto-libm-test-in b/math/auto-libm-test-in
index 4eaa013..0826825 100644
--- a/math/auto-libm-test-in
+++ b/math/auto-libm-test-in
@@ -960,6 +960,8 @@ expm1 0x1p-64
expm1 -0x1p-64
expm1 0x1p-100
expm1 -0x1p-100
+# Bug 16353: underflow exception may be missing
+expm1 0x4.0000000000000028p-16384 missing-underflow
fma 1.0 2.0 3.0
fma 1.25 0.75 0.0625
diff --git a/sysdeps/i386/fpu/e_expl.S b/sysdeps/i386/fpu/e_expl.S
index eb4086b..c7e4373 100644
--- a/sysdeps/i386/fpu/e_expl.S
+++ b/sysdeps/i386/fpu/e_expl.S
@@ -108,6 +108,16 @@ ENTRY(IEEE754_EXPL)
andb %ah, %dh
cmpb $0x40, %dh
je 2f
+
+ /* Test for arguments that are small but not subnormal. */
+ movzwl 4+8(%esp), %eax
+ andl $0x7fff, %eax
+ cmpl $0x3fbf, %eax
+ jge 3f
+ /* Argument's exponent below -64; avoid spurious underflow if
+ normal. */
+ cmpl $0x0001, %eax
+ jge 2f
#else
movzwl 4+8(%esp), %eax
andl $0x7fff, %eax
diff --git a/sysdeps/x86_64/fpu/e_expl.S b/sysdeps/x86_64/fpu/e_expl.S
index 338527b..0ebe388 100644
--- a/sysdeps/x86_64/fpu/e_expl.S
+++ b/sysdeps/x86_64/fpu/e_expl.S
@@ -105,6 +105,16 @@ ENTRY(IEEE754_EXPL)
andb %ah, %dh
cmpb $0x40, %dh
je 2f
+
+ /* Test for arguments that are small but not subnormal. */
+ movzwl 8+8(%rsp), %eax
+ andl $0x7fff, %eax
+ cmpl $0x3fbf, %eax
+ jge 3f
+ /* Argument's exponent below -64; avoid spurious underflow if
+ normal. */
+ cmpl $0x0001, %eax
+ jge 2f
#else
movzwl 8+8(%rsp), %eax
andl $0x7fff, %eax
--
Joseph S. Myers
joseph@codesourcery.com