How to not fold constants?

[Toon Moene]

Joern Rennecke wrote:

> > I think an easier (and more obvious IMHO) solution is to have the C
> > frontend "Do The Right Thing", i.e. call build(fold(...)) in case of a
> > static initializer and build(...) otherwise [or is there a snag in there
> > that I'm overlooking ?]

> Yes.  There might be integer expressions inside that we want to fold.

Is that true - shouldn't the *frontend* decide how to interpret mixed
mode expressions ?  Or is fold-const.c so tied up with C that the
Fortran frontend probably shouldn't use it ?!?

> We also want to fold floating point if this can be done without an
> inexact operation or generating NaNs.

The challenge is to determine that the outcome after folding has a
single correct result, independent of the rounding mode (round to
nearest, round to zero, round up or round down - note, I got the first
one wrong in my original message) applied to the constituent numerical
operations.  This condition is necessary, because the compiler cannot
know what the rounding mode _is_, at run time [Off-hand, I'd think that
trying to determine this is equivalent to the halting problem].

Seems like quite a challenge - I wouldn't be surprised if there weren't
_that_ many expressions for which this holds.

-- 
Toon Moene (toon@moene.indiv.nluug.nl)
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
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PS: wish you a sunny eclipse :-)