Re: Fix 61441

[Joseph Myers <joseph@codesourcery.com>]

On Fri, 6 Nov 2015, Sujoy Saraswati wrote:

> > Shouldn't real_convert do this rather than the caller needing to do it?
> 
> Yes, it should be. I had started by doing this within real_convert but
> then saw that there are quite a few callers where I should add the
> check for flag_signaling_nans. This was making the patch bigger, so
> instead decided to change the caller in this particular case. I will
> try to make the change in real_convert now that we are planning to
> break the patch.

I think the general principle is:

* The caller decides whether folding is desirable (whether it would lose 
exceptions, for example).

* The real.c code is called only when the caller has decided that folding 
is desirable, and should always produce the correct output (which for a 
conversion means producing a quiet NaN from a signaling NaN).

So both places need changes, but real_convert is where the code that makes 
it a quiet NaN should go.

Another place in the patch that looks incorrect: the changes to 
fold-const-call.c calling real_powi and checking if the result is a 
signaling NaN.  The result of real_powi should never be a signaling NaN.  
Rather, real_powi should produce a quiet NaN if its input is a signaling 
NaN, and the callers should check if the argument is a signaling NaN when 
deciding whether to fold, not if the result is.

-- 
Joseph S. Myers
joseph@codesourcery.com