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

[Marek Polacek <polacek@redhat.com>]

On Tue, Jul 26, 2022 at 04:24:18PM -0400, Jason Merrill wrote:
> On 7/26/22 15:03, 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?
> > 
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > 	PR middle-end/102633
> > 
> > gcc/c-family/ChangeLog:
> > 
> > 	* c-gimplify.cc (c_gimplify_expr): Strip NOPs of DECL_INITIAL.
> 
> I wonder if we want to handle this i = i case earlier, like in finish_decl.

I could, something like

@@ -5381,7 +5381,14 @@ finish_decl (tree decl, location_t init_loc, tree init,
     init = NULL_TREE;

   if (init)
-    store_init_value (init_loc, decl, init, origtype);
+    {
+      /* In the self-init case, undo the artificial NOP_EXPR we may have
+    added in convert_lvalue_to_rvalue so that c_gimplify_expr/DECL_EXPR
+    can perform suppress_warning.  */
+      if (TREE_CODE (init) == NOP_EXPR && TREE_OPERAND (init, 0) == decl)
+   init = TREE_OPERAND (init, 0);
+      store_init_value (init_loc, decl, init, origtype);
+    }

but then I'd have to do the same thing in cp_finish_decl because
decay_conversion also adds a NOP_EXPR for cv-qualified non-class prvalues.
Is that what we want?  To me that seems less clean than having c_gimplify_expr
see through NOP_EXPRs.

Marek