[PATCH] Add save_expr langhook (PR c/68513)
Mon Nov 30 12:58:00 GMT 2015
On Fri, Nov 27, 2015 at 10:43:42PM +0000, Joseph Myers wrote:
> On Fri, 27 Nov 2015, Marek Polacek wrote:
> > I didn't know where to put setting of in_late_processing. With the current
> > placement, we won't (for valid programs) call c_save_expr from c_genericize
> > or c_gimplify_expr.
> Well, the placement in this patch (in c_parser_compound_statement) is
> certainly wrong. It doesn't even save and restore, so after one compound
> statement inside another, parsing would continue with in_late_processing
> wrongly set. And c_save_expr is logically right for any parsing outside
> compound statements as well (arbitrary expressions can occur in sizeof
> outside functions and in VLA parameter sizes and should follow the normal
> rules for what's a constant expression - there's a known bug that
> statement expressions are wrongly rejected in such contexts).
Indeed. I don't know what I was thinking. :/
> Starting from first principles: parsing takes place from within
> c_parse_file as the sole external entry point to the parser. So you could
> have a parsing_input variable that starts off as false, and where
> c_parse_file saves it, sets to true, and restores the saved value at the
> end. Then you'd use c_save_expr if parsing_input && !in_late_binary_op.
> If that doesn't work, it means there are cases where the hook gets called
> from folding that takes place during parsing, on expressions that will not
> subsequently go through c_fully_fold, but without in_late_binary_op set.
> Knowing what those cases are might help work out any fix for them that is
I'm not sanguine about doing this reliably in stage3. I think I'll try the
other approach mentioned later in this thread.
> > I suppose I should also modify save_expr in fold-const.c to call it via the
> > langhook, if this approach is sane. Dunno.
> That's a complication. When the folding is taking place from within
> c_fully_fold (and so the sub-expressions have already been folded, and had
> their C_MAYBE_CONST_EXPRs removed, and the result of folding will not be
> re-folded), it should be using save_expr not c_save_expr. So maybe the
> hook needs to say: use c_save_expr, if parsing, not in_late_binary_op and
> not folding from within c_fully_fold.
Oh, I see :(.
> Again long term we should aim for the representation during parsing not to
> need SAVE_EXPRs and for the folding that creates them (and the other
> folding for optimization in general) to happen only after parsing....
Yeah, let's strike that for gcc7.
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