Re: [PATCH] c-family: Honor -Wno-init-self for cv-qual vars [PR102633]

[Marek Polacek <polacek@redhat.com>]

On Wed, Jul 27, 2022 at 06:41:09AM +0000, Richard Biener via Gcc-patches wrote:
> On Tue, 26 Jul 2022, Marek Polacek wrote:
> 
> > Since r11-5188-g32934a4f45a721, we drop qualifiers during l-to-r
> > conversion by creating a NOP_EXPR.  For e.g.
> > 
> >   const int i = i;
> > 
> > that means that the DECL_INITIAL is '(int) i' and not 'i' anymore.
> > Consequently, we don't suppress_warning here:
> > 
> > 711     case DECL_EXPR:
> > 715       if (VAR_P (DECL_EXPR_DECL (*expr_p))
> > 716           && !DECL_EXTERNAL (DECL_EXPR_DECL (*expr_p))
> > 717           && !TREE_STATIC (DECL_EXPR_DECL (*expr_p))
> > 718           && (DECL_INITIAL (DECL_EXPR_DECL (*expr_p)) == DECL_EXPR_DECL (*expr_p))
> > 719           && !warn_init_self)
> > 720         suppress_warning (DECL_EXPR_DECL (*expr_p), OPT_Winit_self);
> > 
> > because of the check on line 718 -- (int) i is not i.  So -Wno-init-self
> > doesn't disable the warning as it's supposed to.
> > 
> > The following patch fixes it...except it doesn't, for volatile variables
> > in C++.  The problem is that for
> > 
> >   volatile int k = k;
> > 
> > we see that the initializer has TREE_SIDE_EFFECTS, so we perform dynamic
> > initialization.  So there's no DECL_INITIAL and the suppress_warning
> > call above is never done.  I suppose we could amend get_no_uninit_warning
> > to return true for volatile-qualified expressions.  I mean, can we ever
> > say for a fact that a volatile variable is uninitialized?
> 
> As I said in the bug the bug is probably that we emit uninitialized
> diagnostics for volatiles at all?

Non-volatile const variables also have this problem, so I think we still
need this patch (or something like it).

> OTOH 'volatile' is recommended
> for vars live around setjmp/longjmp and there diagnostics would be
> welcome.  It's probably the difference between "compiler-hands-off"
> and "hardware-controlled" :/

Ah, you're right.  Then I guess it's better to leave the warning be,
(but we still have a discrepancy between C and C++).

Marek