Re: [PATCH] [C FE] Fold trivial exprs that refer to const vars

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Sun, Dec 6, 2015 at 11:50 PM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> There is a minor inconsistency in the folding behavior within the C
> frontend.  The C frontend does not currently fold the expression "x",
> where x is a const int, yet the FE does fold the expression "x + 0".
>
> This happens because decl_constant_value is called in c_fully_fold only
> while recursing over the operands of the expression being folded, i.e.
> there is no top-level call to decl_constant_value to handle the case
> where the expression being folded happens to be a singular expression
> such as "x", as opposed to "x + 5" (where x is a const variable).
>
> To fix this inconsistency, this patch calls decl_constant_value in
> c_fully fold after folding the given expression.
>
> Bootstrap + regtest in progress on x86_64-pc-linux-gnu, OK to commit if
> testing succeeds?

It just occurred to me that this change is not completely safe because
calling c_fully_fold on an lvalue can now return an rvalue. Callers of
c_fully_fold are not prepared to handle this.  Indeed, this patch
causes a couple of regressions in the handling asm() memory operands
due to this implicit lvalue-rvalue conversion.