| Message ID | Pine.LNX.4.64.1406242046500.21652@digraph.polyomino.org.uk |
|---|---|
| State | New |
| Headers | show |
On 06/24/2014 10:47 PM, Joseph S. Myers wrote: > 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.) Thanks, Andreas
Hi Joseph, on S390 test-ldouble fails after this patch: Failure: expm1 (0x4.0000000000000028p-16384): Exception "Underflow" set Failure: expm1_downward (0x4.0000000000000028p-16384): Exception "Underflow" set Failure: expm1_towardzero (0x4.0000000000000028p-16384): Exception "Underflow" set Failure: expm1_upward (0x4.0000000000000028p-16384): Exception "Underflow" set But I haven´t debugged it yet. Bye On 06/24/2014 10:47 PM, Joseph S. Myers wrote: > 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 >
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