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author | Kumar Gala <galak@kernel.crashing.org> | 2008-10-22 07:19:00 +0200 |
---|---|---|
committer | David S. Miller <davem@davemloft.net> | 2008-10-23 06:53:53 +0200 |
commit | 930cc144a043ff95e56b6888fa51c618b33f89e7 (patch) | |
tree | 95a446505d2e15ea2d1f4464c2a7a014402a4b95 /include/math-emu | |
parent | sparc: Add checkstack support (diff) | |
download | linux-930cc144a043ff95e56b6888fa51c618b33f89e7.tar.xz linux-930cc144a043ff95e56b6888fa51c618b33f89e7.zip |
math-emu: Fix signalling of underflow and inexact while packing result.
I'm trying to move the powerpc math-emu code to use the include/math-emu bits.
In doing so I've been using TestFloat to see how good or bad we are
doing. For the most part the current math-emu code that PPC uses has
a number of issues that the code in include/math-emu seems to solve
(plus bugs we've had for ever that no one every realized).
Anyways, I've come across a case that we are flagging underflow and
inexact because we think we have a denormalized result from a double
precision divide:
000.FFFFFFFFFFFFF / 3FE.FFFFFFFFFFFFE
soft: 001.0000000000000 ..... syst: 001.0000000000000 ...ux
What it looks like is the results out of FP_DIV_D are:
D:
sign: 0
mantissa: 01000000 00000000
exp: -1023 (0)
The problem seems like we aren't normalizing the result and bumping the exp.
Now that I'm digging into this a bit I'm thinking my issue has to do with
the fix DaveM put in place from back in Aug 2007 (commit
405849610fd96b4f34cd1875c4c033228fea6c0f):
[MATH-EMU]: Fix underflow exception reporting.
2) we ended up rounding back up to normal (this is the case where
we set the exponent to 1 and set the fraction to zero), this
should set inexact too
...
Another example, "0x0.0000000000001p-1022 / 16.0", should signal both
inexact and underflow. The cpu implementations and ieee1754
literature is very clear about this. This is case #2 above.
Here is the distilled glibc test case from Jakub Jelinek which prompted that
commit:
--------------------
#include <float.h>
#include <fenv.h>
#include <stdio.h>
volatile double d = DBL_MIN;
volatile double e = 0x0.0000000000001p-1022;
volatile double f = 16.0;
int
main (void)
{
printf ("%x\n", fetestexcept (FE_UNDERFLOW));
d /= f;
printf ("%x\n", fetestexcept (FE_UNDERFLOW));
e /= f;
printf ("%x\n", fetestexcept (FE_UNDERFLOW));
return 0;
}
--------------------
It looks like the case I have we are exact before rounding, but think it
looks like the rounding case since it appears as if "overflow is set".
000.FFFFFFFFFFFFF / 3FE.FFFFFFFFFFFFE = 001.0000000000000
I think the following adds the check for my case and still works for the
issue your commit was trying to resolve.
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include/math-emu')
-rw-r--r-- | include/math-emu/op-common.h | 17 |
1 files changed, 13 insertions, 4 deletions
diff --git a/include/math-emu/op-common.h b/include/math-emu/op-common.h index cc1ec396f8d6..bc50aa0d6d73 100644 --- a/include/math-emu/op-common.h +++ b/include/math-emu/op-common.h @@ -139,18 +139,27 @@ do { \ if (X##_e <= _FP_WFRACBITS_##fs) \ { \ _FP_FRAC_SRS_##wc(X, X##_e, _FP_WFRACBITS_##fs); \ - _FP_ROUND(wc, X); \ if (_FP_FRAC_HIGH_##fs(X) \ & (_FP_OVERFLOW_##fs >> 1)) \ { \ X##_e = 1; \ _FP_FRAC_SET_##wc(X, _FP_ZEROFRAC_##wc); \ - FP_SET_EXCEPTION(FP_EX_INEXACT); \ } \ else \ { \ - X##_e = 0; \ - _FP_FRAC_SRL_##wc(X, _FP_WORKBITS); \ + _FP_ROUND(wc, X); \ + if (_FP_FRAC_HIGH_##fs(X) \ + & (_FP_OVERFLOW_##fs >> 1)) \ + { \ + X##_e = 1; \ + _FP_FRAC_SET_##wc(X, _FP_ZEROFRAC_##wc); \ + FP_SET_EXCEPTION(FP_EX_INEXACT); \ + } \ + else \ + { \ + X##_e = 0; \ + _FP_FRAC_SRL_##wc(X, _FP_WORKBITS); \ + } \ } \ if ((FP_CUR_EXCEPTIONS & FP_EX_INEXACT) || \ (FP_TRAPPING_EXCEPTIONS & FP_EX_UNDERFLOW)) \ |