Re: Fix 61441

[Joseph Myers <joseph at codesourcery dot com>]

On Mon, 14 Sep 2015, Richard Biener wrote:

> I'll leave the correctness part of the patch to Joseph who knows FP
> arithmetic better than me,
> implementation-wise this is ok if you fix the REAL_CST sharing issue.

Changing fold_abs_const is wrong - abs of sNaN is sNaN, no exceptions 
raised.  Changing real_arithmetic is wrong for the NEGATE_EXPR and 
ABS_EXPR cases, both of which should just affect the sign bit without 
quieting sNaNs.

All the comments in the patch should end with ".  " (full stop, two 
spaces).

If -fsignaling-nans, then folding of expressions involving sNaNs should be 
disabled, outside of static initializers - such expressions should not get 
folded to return an sNaN (it's incorrect to fold sNaN + 1 to sNaN, for 
example).  I think existing code may ensure that (the HONOR_SNANS check in 
const_binop, for example).

Inside static initializers, expressions involving sNaNs still need to be 
folded (so sNaN + 1 becomes qNaN inside such an initializer, for example, 
with the translation-time exception being discarded).  Again, existing 
code should handle this: START_FOLD_INIT / END_FOLD_INIT already handle 
clearing and restoring flag_signaling_nans.

My understanding of the design of the existing code is that real.c will do 
the arithmetic regardless of whether it might raise an exception or have 
rounding-mode-dependent results, with fold-const.c being responsible for 
deciding whether the result can be used to fold the expression in 
question.  That is, you shouldn't need any flag_signaling_nans conditions 
in real.c; rather, if IEEE semantics mean an sNaN is quieted, real.c 
should do so unconditionally.  It should be the callers in fold-const.c 
that check HONOR_SNANS and disallow folding when it would lose exceptions.

-- 
Joseph S. Myers
joseph@codesourcery.com