On 12/7/22 13:10, Jakub Jelinek wrote:
Hi!
As mentioned in PR107967, ibm-ldouble-format documents that
+- has 1ulp accuracy, * 2ulps and / 3ulps.
So, even if the result is exact, we need to widen the range a little bit.
The following patch does that. I just wonder what it means for reverse
division (the op1_range case), which we implement through multiplication,
when division has 3ulps error and multiplication just 2ulps. In any case,
this format is a mess and for non-default rounding modes can't be trusted
at all, instead of +inf or something close to it it happily computes -inf.
2022-12-07 Jakub Jelinek <ja...@redhat.com>
* range-op-float.cc (frange_arithmetic): For mode_composite,
on top of rounding in the right direction accept extra 1ulp
error for PLUS/MINUS_EXPR, extra 2ulps error for MULT_EXPR
and extra 3ulps error for RDIV_EXPR.
--- gcc/range-op-float.cc.jj 2022-12-07 12:46:01.536123757 +0100
+++ gcc/range-op-float.cc 2022-12-07 12:50:40.812085139 +0100
@@ -344,22 +344,70 @@ frange_arithmetic (enum tree_code code,
}
}
}
- if (round && (inexact || !real_identical (&result, &value)))
+ if (!inexact && !real_identical (&result, &value))
+ inexact = true;
+ if (round && (inexact || mode_composite))
{
if (mode_composite)
{
- if (real_isdenormal (&result, mode)
- || real_iszero (&result))
+ if (real_isdenormal (&result, mode) || real_iszero (&result))
{
// IBM extended denormals only have DFmode precision.
REAL_VALUE_TYPE tmp;
real_convert (&tmp, DFmode, &value);
- frange_nextafter (DFmode, tmp, inf);
+ if (inexact)
+ frange_nextafter (DFmode, tmp, inf);
+ switch (code)
+ {
+ case PLUS_EXPR:
+ case MINUS_EXPR:
+ // ibm-ldouble-format documents 1ulp for + and -.
+ frange_nextafter (DFmode, tmp, inf);
+ break;
+ case MULT_EXPR:
+ // ibm-ldouble-format documents 2ulps for *.
+ frange_nextafter (DFmode, tmp, inf);
+ frange_nextafter (DFmode, tmp, inf);
+ break;
+ case RDIV_EXPR:
+ // ibm-ldouble-format documents 3ulps for /.
+ frange_nextafter (DFmode, tmp, inf);
+ frange_nextafter (DFmode, tmp, inf);
+ frange_nextafter (DFmode, tmp, inf);
+ break;
+ default:
+ if (!inexact)
+ return;
+ break;
It looks like this chunk...
+ }
real_convert (&result, mode, &tmp);
return;
}
}
- frange_nextafter (mode, result, inf);
+ if (inexact)
+ frange_nextafter (mode, result, inf);
+ if (mode_composite)
+ switch (code)
+ {
+ case PLUS_EXPR:
+ case MINUS_EXPR:
+ // ibm-ldouble-format documents 1ulp for + and -.
+ frange_nextafter (mode, result, inf);
+ break;
+ case MULT_EXPR:
+ // ibm-ldouble-format documents 2ulps for *.
+ frange_nextafter (mode, result, inf);
+ frange_nextafter (mode, result, inf);
+ break;
+ case RDIV_EXPR:
+ // ibm-ldouble-format documents 3ulps for /.
+ frange_nextafter (mode, result, inf);
+ frange_nextafter (mode, result, inf);
+ frange_nextafter (mode, result, inf);
+ break;
+ default:
+ break;
+ }
...is the same as this chunk. Plus, all this mode composite stuff is
polluting what was a rather clean function. Would it be possible to
abstract this into an inline function, and then we could do:
if (mode_composite)
frange_composite_nextafter (...);
else
frange_nextafter (...);
or perhaps abstract the whole nextafter in frange_arithmetic into:
frange_arithmetic_nextafter () {
if (mode_composite) { do ugly stuff }
else frange_nextafter (...)
}
I'm most worried about maintainability, not correctness here, cause you
obviously know what you're doing ;-).
Aldy
}
}
Jakub
