Re: [PATCH] PR optimization/9325: Saturating FP-to-int overflow.

["Zack Weinberg" <zack at codesourcery dot com>]

Roger Sayle <roger@eyesopen.com> writes:

> The following patch resolves PR optimization/9325 and PR java/6391.
> Tom Tromey suggested that I investigate the latter as an example of
> the middle-end not respecting java semantics, which turned out to be
> identical to PR9325 which is reported against the C-like front-ends.
...

> The solution for the middle-end is to perform constant folding using
> the Java semantics, i.e. saturating overflow, as allowed by the C and
> C++ language standards.  Indeed PR9325 isn't actually a "bug" at all,
> it invokes unspecified C/C++ behaviour that produces surprising results.
> Implementing saturating overflow semantics, at compile-time atleast,
> produces less surprising results.
...

> 	PR optimization/9325, PR java/6391
> 	* fold-const.c (fold_convert): For floating point to integer
> 	conversions, return the maximum/minimum representable integer
> 	value if the real constant overflows the destination type.
> 	* tree.c (real_value_from_int_cst): Allow the type to be NULL,
> 	meaning don't truncate the result to a floating point mode.
> 	Simplify the logic by calling real_from_integer directly.
> 	* simplify-rtx.c (simplify_unary_operation):  Implement the
> 	same semantics for folding floating point to integer conversions
> 	in RTL.

I would say that this is OK provided that you document in the source
code that the middle-end implements the Java semantics for these
operations, which is also acceptable for C and C++.  If you want to be
thorough, check what the Ada standard says about these operations.  In
any case please wait 24 hours for the numerical folks to weigh in.

zw