[PATCH] Fix pattern causing C_MAYBE_CONST_EXPRs leak into gimplifier (PR c/68513)

[Richard Biener]

On Wed, 25 Nov 2015, Jakub Jelinek wrote:

> On Wed, Nov 25, 2015 at 03:35:10PM +0100, Marek Polacek wrote:
> > Look how 'x' is duplicated in the result of the pattern, and since (because of
> > the volatile 'e') it has TREE_SIDE_EFFECTS, we need to wrap it in a SAVE_EXPR,
> > which means the convert() produces
> > 
> > (short int) (((SAVE_EXPR <c_maybe_const_expr <i> != 0 ? (unsigned short)
> >                c_maybe_const_expr <*e > : 0>)
> >               ^ (unsigned short) i) & (unsigned short) u
> >              ^ (SAVE_EXPR <c_maybe_const_expr <i > != 0 ? (unsigned short)
> >                 c_maybe_const_expr <*e > : 0>))
> > 
> > What's important is that we end up with a C_MAYBE_CONST_EXPR in a SAVE_EXPR.
> > 
> > Down the line, we get into c_process_expr_stmt, where there's
> > 
> >   expr = c_fully_fold (expr, false, NULL);
> > 
> > so we try to fully-fold the whole expression.  However, c_fully_fold just
> > gives up when it sees a SAVE_EXPR, so it doesn't scrub out C_MAYBE_CONST_EXPR.
> > It then leaks into the gimplifier and crashes.
> 
> Ran into similar issues many times in the past, the main issue is that the C
> FE wants that before c_fully_fold is called on some expression that nobody
> wraps anything into a SAVE_EXPR except by calling c_save_expr.
> So, unless we get rid of that (severe) limitation, we should make sure
> that either match.pd never creates SAVE_EXPRs when called from the "early"
> spots in the C FE, or that it uses c_save_expr instead of save_expr.
> But finding out if you have an expression on which c_fully_fold has been
> already called or not is non-trivial too.
> 
> Wonder if we couldn't use some FE specific bit on the SAVE_EXPR to say
> whether c_fully_fold_internal has already processed it or not, and just
> get rid of c_save_expr, in c_fully_fold* recurse into SAVE_EXPRs too, but
> only if that bit is not yet already set, and set it afterwards.

c_gimplify_expr and SAVE_EXPR_RESOLVED_P would work as well I guess?

Richard.