Question about fold C_MAYBE_CONST_EXPR expressions

[Joseph Myers]

On Sun, 24 Jul 2016, Bin.Cheng wrote:

> Hi,
> I ran into a problem that C frontend (in function
> build_conditional_expr) creates expression like (C_MAYBE_CONST_EXPR
> (NULL, x + const)).  The inner expression (and its operands) have
> unsigned int type.  After that, the expression needs to be casted to
> result type which is unsigned short.  In this case,
> convert_to_integer/convert_to_integer_1 doesn't fold into
> C_MAYBE_CONST_EXPR and just returns (unsigned
> short)(C_MAYBE_CONST_EXPR (NULL, x + const)), as a result, (unsigned
> short)(x + const) won't be simplified as (unsigned short)x + const.
> My questions are, 1) Is it possible to fold type conversion into
> C_MAYBE_CONST_EXPR's inner expression in convert_to_integer_1?  2) If
> not, looks like there are couple of places the mentioned fold can be
> done, for example, after stripping C_MAYBE_CONST_EXPR and before (or
> during) gimplify.  So which place is preferred?

The folding should occur from, or after, c_fully_fold.  In general, 
convert_to_* should do less optimization; the optimizations there should 
be moved to the match-and-simplify infrastructure.

-- 
Joseph S. Myers
joseph@codesourcery.com