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authorKumar Gala <galak@kernel.crashing.org>2008-10-21 22:19:00 -0700
committerDavid S. Miller <davem@davemloft.net>2008-10-22 21:53:53 -0700
commit930cc144a043ff95e56b6888fa51c618b33f89e7 (patch)
tree95a446505d2e15ea2d1f4464c2a7a014402a4b95 /include/math-emu/op-common.h
parentd41e2d7317cd55cc5135356a05c289537b0f6d70 (diff)
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/op-common.h')
-rw-r--r--include/math-emu/op-common.h17
1 files changed, 13 insertions, 4 deletions
diff --git a/include/math-emu/op-common.h b/include/math-emu/op-common.h
index cc1ec396f8d..bc50aa0d6d7 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)) \