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C++ PATCH for c++/70259 (-flifetime-dse vs. empty bases)
- From: Jason Merrill <jason at redhat dot com>
- To: gcc-patches List <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 16 Mar 2016 14:47:09 -0400
- Subject: C++ PATCH for c++/70259 (-flifetime-dse vs. empty bases)
- Authentication-results: sourceware.org; auth=none
The constructor for an empty class can't do the -flifetime-dse clobber
because when the class is used as a base it might be assigned the same
offset as a real base, so the clobber would mess with real data.
Tested x86_64-pc-linux-gnu, applying to trunk.
commit e1a5f038350d1881153d8f65359bd883f7452237
Author: Jason Merrill <jason@redhat.com>
Date: Wed Mar 16 13:46:32 2016 -0400
PR c++/70259
* decl.c (start_preparsed_function): Don't clobber an empty base.
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 4ee4ccc..e783163 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -14125,6 +14125,8 @@ start_preparsed_function (tree decl1, tree attrs, int flags)
&& (flag_lifetime_dse > 1)
&& DECL_CONSTRUCTOR_P (decl1)
&& !DECL_CLONED_FUNCTION_P (decl1)
+ /* Clobbering an empty base is harmful if it overlays real data. */
+ && !is_empty_class (current_class_type)
/* We can't clobber safely for an implicitly-defined default constructor
because part of the initialization might happen before we enter the
constructor, via AGGR_INIT_ZERO_FIRST (c++/68006). */
diff --git a/gcc/testsuite/g++.dg/opt/flifetime-dse5.C b/gcc/testsuite/g++.dg/opt/flifetime-dse5.C
new file mode 100644
index 0000000..2c49021
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/flifetime-dse5.C
@@ -0,0 +1,13 @@
+// PR c++/70259
+// { dg-options -O2 }
+// { dg-do run }
+
+struct Empty { };
+struct A { A() : a(true) { } bool a; };
+struct B : Empty { B() : Empty() { } };
+struct C : A, B { C() : A(), B() { } };
+int main() {
+ C c;
+ if ( c.a == false )
+ __builtin_abort();
+};