| From b688ae989a7c56026c9a0d9857f07812daab4be3 Mon Sep 17 00:00:00 2001 |
| From: Kumar Gala <galak@kernel.crashing.org> |
| Date: Tue, 21 Oct 2008 22:19:00 -0700 |
| Subject: math-emu: Fix signalling of underflow and inexact while packing result. |
| |
| |
| From: Kumar Gala <galak@kernel.crashing.org> |
| |
| [ Upstream commit 930cc144a043ff95e56b6888fa51c618b33f89e7 ] |
| |
| 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> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de> |
| |
| --- |
| include/math-emu/op-common.h | 17 +++++++++++++---- |
| 1 file changed, 13 insertions(+), 4 deletions(-) |
| |
| --- 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)) \ |