Re: [PATCH] c++: suppress all warnings on memper pointers to work around dICE [PR101219]

[Sergei Trofimovich <slyfox@gentoo.org>]

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On Wed, 11 Aug 2021 15:19:58 -0400
Jason Merrill <jason@redhat.com> wrote:

> On 8/6/21 11:34 AM, Sergei Trofimovich wrote:
> > On Thu, 29 Jul 2021 11:41:39 -0400
> > Jason Merrill <jason@redhat.com> wrote:
> >   
> >> On 7/22/21 7:15 PM, Sergei Trofimovich wrote:  
> >>> From: Sergei Trofimovich <siarheit@google.com>
> >>>
> >>> r12-1804 ("cp: add support for per-location warning groups.") among other
> >>> things removed warning suppression from a few places including ptrmemfuncs.
> >>>
> >>> Currently ptrmemfuncs don't have valid BINFO attached which causes ICEs
> >>> in access checks:
> >>>
> >>>       crash_signal
> >>>           gcc/toplev.c:328
> >>>       perform_or_defer_access_check(tree_node*, tree_node*, tree_node*, int, access_failure_info*)
> >>>           gcc/cp/semantics.c:490
> >>>       finish_non_static_data_member(tree_node*, tree_node*, tree_node*)
> >>>           gcc/cp/semantics.c:2208
> >>>       ...
> >>>
> >>> The change suppresses warnings again until we provide BINFOs for ptrmemfuncs.  
> >>
> >> We don't need BINFOs for PMFs, we need to avoid paths that expect them.
> >>
> >> It looks like the problem is with tsubst_copy_and_build calling
> >> finish_non_static_data_member instead of build_ptrmemfunc_access_expr.  
> > 
> > Sounds good. I'm not sure what would be the best way to match it. Here is
> > my attempt seems to survive all regtests:
> > 
> > --- a/gcc/cp/pt.c
> > +++ b/gcc/cp/pt.c
> > @@ -20530,7 +20530,13 @@ tsubst_copy_and_build (tree t,
> >          if (member == error_mark_node)
> >            RETURN (error_mark_node);
> > 
> > -       if (TREE_CODE (member) == FIELD_DECL)
> > +       if (object_type && TYPE_PTRMEMFUNC_P(object_type)
> > +           && TREE_CODE (member) == FIELD_DECL)
> > +         {
> > +           r = build_ptrmemfunc_access_expr (object, DECL_NAME(member));
> > +           RETURN (r);
> > +         }
> > +       else if (TREE_CODE (member) == FIELD_DECL)
> >            {
> >              r = finish_non_static_data_member (member, object, NULL_TREE);
> >              if (TREE_CODE (r) == COMPONENT_REF)
> >   
> >>> 	PR c++/101219
> >>>
> >>> gcc/cp/ChangeLog:
> >>>
> >>> 	* typeck.c (build_ptrmemfunc_access_expr): Suppress all warnings
> >>> 	to avoid ICE.
> >>>
> >>> gcc/testsuite/ChangeLog:
> >>>
> >>> 	* g++.dg/torture/pr101219.C: New test.  
> >>
> >> This doesn't need to be in torture; it has nothing to do with optimization.  
> > 
> > Aha, moved to gcc/testsuite/g++.dg/warn/pr101219.C.
> > 
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/warn/pr101219.C
> > @@ -0,0 +1,11 @@
> > +/* PR c++/101219 - ICE on use of uninitialized memfun pointer
> > +   { dg-do compile }
> > +   { dg-options "-Wall" } */
> > +
> > +struct S { void m(); };
> > +
> > +template <int> bool f() {
> > +  void (S::*mp)();
> > +
> > +  return &S::m == mp; // no warning emitted here (no instantiation)
> > +}
> > 
> > Another question: Is it expected that gcc generates no warnings here?
> > It's an uninstantiated function (-1 for warn), but from what I
> > understand it's guaranteed to generate comparison with uninitialized
> > data if it ever gets instantiated. Given that we used to ICE in
> > warning code gcc could possibly flag it? (+1 for warn)  
> 
> Generally it's desirable to diagnose templates for which no valid 
> instantiation is possible.  It seems reasonable in most cases to also 
> warn about templates for which all instantiations would warn.
> 
> But uninitialized warnings rely on flow analysis that we only do on 
> instantiated functions, and in any case the ICE doesn't depend on mp 
> being uninitialized; I get the same crash if I add = 0 to the declaration.

Aha. That makes sense. Let's just fix ICE then.

> > +	if (object_type && TYPE_PTRMEMFUNC_P(object_type)  
> 
> Missing space before (.
> 
> > +	    && TREE_CODE (member) == FIELD_DECL)
> > +	  {
> > +	    r = build_ptrmemfunc_access_expr (object, DECL_NAME(member));  
> 
> And here.

Added both. Attached as v3.

-- 

  Sergei

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From dbb17a22383faa7837bdd2ea9c902bfab53fa8f2 Mon Sep 17 00:00:00 2001
From: Sergei Trofimovich <siarheit@google.com>
Date: Fri, 6 Aug 2021 16:14:16 +0100
Subject: [PATCH v3] c++: fix ptrmemfunc template instantiation [PR101219]

r12-1804 ("cp: add support for per-location warning groups.") among other
things removed warning suppression from a few places including ptrmemfuncs.

This exposed a bug in warning detection code as a reference to missing
BINFO (it's intentionally missing for ptrmemfunc types):

    crash_signal
        gcc/toplev.c:328
    perform_or_defer_access_check(tree_node*, tree_node*, tree_node*, int, access_failure_info*)
        gcc/cp/semantics.c:490
    finish_non_static_data_member(tree_node*, tree_node*, tree_node*)
        gcc/cp/semantics.c:2208
    ...

The change special cases ptrmemfuncs in templace substitution by using
build_ptrmemfunc_access_expr() instead of finish_non_static_data_member().

        PR c++/101219

gcc/cp/ChangeLog:

        * pt.c (tsubst_copy_and_build): Use build_ptrmemfunc_access_expr
        to construct ptrmemfunc expression instantiation.

gcc/testsuite/ChangeLog:

        * g++.dg/warn/pr101219.C: New test.
---
Change since v2: fix whitespace around macros.
 gcc/cp/pt.c                          |  8 +++++++-
 gcc/testsuite/g++.dg/warn/pr101219.C | 11 +++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/warn/pr101219.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index b396ddd0089..42ea51cebc0 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -20530,7 +20530,13 @@ tsubst_copy_and_build (tree t,
 	if (member == error_mark_node)
 	  RETURN (error_mark_node);
 
-	if (TREE_CODE (member) == FIELD_DECL)
+	if (object_type && TYPE_PTRMEMFUNC_P (object_type)
+	    && TREE_CODE (member) == FIELD_DECL)
+	  {
+	    r = build_ptrmemfunc_access_expr (object, DECL_NAME (member));
+	    RETURN (r);
+	  }
+	else if (TREE_CODE (member) == FIELD_DECL)
 	  {
 	    r = finish_non_static_data_member (member, object, NULL_TREE);
 	    if (TREE_CODE (r) == COMPONENT_REF)
diff --git a/gcc/testsuite/g++.dg/warn/pr101219.C b/gcc/testsuite/g++.dg/warn/pr101219.C
new file mode 100644
index 00000000000..0d23d73c9ec
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/pr101219.C
@@ -0,0 +1,11 @@
+/* PR c++/101219 - ICE on use of uninitialized memfun pointer
+   { dg-do compile }
+   { dg-options "-Wall" } */
+
+struct S { void m(); };
+
+template <int> bool f() {
+  void (S::*mp)();
+
+  return &S::m == mp; // no warning emitted here (no instantiation)
+}
-- 
2.32.0