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Fix __ieee754_logl (-LDBL_MAX) in FE_DOWNWARD mode (bug 17022)
- From: "Joseph S. Myers" <joseph at codesourcery dot com>
- To: <libc-alpha at sourceware dot org>
- Date: Wed, 4 Jun 2014 17:35:42 +0000
- Subject: Fix __ieee754_logl (-LDBL_MAX) in FE_DOWNWARD mode (bug 17022)
- Authentication-results: sourceware.org; auth=none
This patch fixes __ieee754_logl (-LDBL_MAX) on x86_64 and x86 not to
subtract 1 from its argument and so cause spurious overflow in
FE_DOWNWARD mode. (For any argument strictly less than -1, it doesn't
matter whether or not 1 is subtracted before computing log1p, as long
as the result doesn't overflow to -Inf.)
Tested x86_64 and x86. (This particular case lacks test coverage,
since the testsuite doesn't cover -lieee, but it will be covered by
tests after the following patch to test pow in all rounding modes,
which was the context in which this bug was found.)
2014-06-04 Joseph Myers <joseph@codesourcery.com>
[BZ #17022]
* sysdeps/i386/fpu/e_logl.S (__ieee754_logl): Do not subtract 1
from arguments -2 or below.
* sysdeps/i386/i686/fpu/e_logl.S (__ieee754_logl): Likewise.
* sysdeps/x86_64/fpu/e_logl.S (__ieee754_logl): Likewise.
diff --git a/sysdeps/i386/fpu/e_logl.S b/sysdeps/i386/fpu/e_logl.S
index edae1d7..828e98a 100644
--- a/sysdeps/i386/fpu/e_logl.S
+++ b/sysdeps/i386/fpu/e_logl.S
@@ -40,8 +40,11 @@ ENTRY(__ieee754_logl)
fld %st // x : x : log(2)
sahf
jc 3f // in case x is NaN or +-Inf
+ movzwl 4+8(%esp), %eax
+ cmpl $0xc000, %eax
+ jae 6f // x <= -2, avoid overflow from -LDBL_MAX - 1.
4: fsubl MO(one) // x-1 : x : log(2)
- fld %st // x-1 : x-1 : x : log(2)
+6: fld %st // x-1 : x-1 : x : log(2)
fabs // |x-1| : x-1 : x : log(2)
fcompl MO(limit) // x-1 : x : log(2)
fnstsw // x-1 : x : log(2)
diff --git a/sysdeps/i386/i686/fpu/e_logl.S b/sysdeps/i386/i686/fpu/e_logl.S
index a0d1107..0ccc8fc 100644
--- a/sysdeps/i386/i686/fpu/e_logl.S
+++ b/sysdeps/i386/i686/fpu/e_logl.S
@@ -39,8 +39,11 @@ ENTRY(__ieee754_logl)
LOAD_PIC_REG (dx)
#endif
fld %st // x : x : log(2)
+ movzwl 4+8(%esp), %eax
+ cmpl $0xc000, %eax
+ jae 5f // x <= -2, avoid overflow from -LDBL_MAX - 1.
fsubl MO(one) // x-1 : x : log(2)
- fld %st // x-1 : x-1 : x : log(2)
+5: fld %st // x-1 : x-1 : x : log(2)
fabs // |x-1| : x-1 : x : log(2)
fld MO(limit) // 0.29 : |x-1| : x-1 : x : log(2)
fcomip %st(1) // |x-1| : x-1 : x : log(2)
diff --git a/sysdeps/x86_64/fpu/e_logl.S b/sysdeps/x86_64/fpu/e_logl.S
index 315afc0..047b8db 100644
--- a/sysdeps/x86_64/fpu/e_logl.S
+++ b/sysdeps/x86_64/fpu/e_logl.S
@@ -38,8 +38,11 @@ ENTRY(__ieee754_logl)
fld %st // x : x : log(2)
testb $1, %ah
jnz 3f // in case x is NaN or +-Inf
+ movzwl 8+8(%rsp), %eax
+ cmpl $0xc000, %eax
+ jae 6f // x <= -2, avoid overflow from -LDBL_MAX - 1.
4: fsubl MO(one) // x-1 : x : log(2)
- fld %st // x-1 : x-1 : x : log(2)
+6: fld %st // x-1 : x-1 : x : log(2)
fabs // |x-1| : x-1 : x : log(2)
fcompl MO(limit) // x-1 : x : log(2)
fnstsw // x-1 : x : log(2)
--
Joseph S. Myers
joseph@codesourcery.com