PR36992: do not store beyond the dsize of a class object unless we know

the tail padding is not reused.

We track on the AggValueSlot (and through a couple of other
initialization actions) whether we're dealing with an object that might
share its tail padding with some other object, so that we can avoid
emitting stores into the tail padding if that's the case. We still
widen stores into tail padding when we can do so.

Differential Revision: https://reviews.llvm.org/D45306

llvm-svn: 329342

1 files changed, 34 insertions, 0 deletions

diff --git a/clang/test/CodeGenCXX/tail-padding.cpp b/clang/test/CodeGenCXX/tail-padding.cpp
new file mode 100644
index 00000000000..c3089bfe5e6
--- /dev/null
+++ b/clang/test/CodeGenCXX/tail-padding.cpp
@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -triple %itanium_abi_triple -emit-llvm -o - %s | FileCheck %s
+
+// PR36992
+namespace Implicit {
+  struct A { char c; A(const A&); };
+  struct B { int n; char c[3]; ~B(); };
+  struct C : B, virtual A {};
+  static_assert(sizeof(C) == sizeof(void*) + 8);
+  C f(C c) { return c; }
+
+  // CHECK: define {{.*}} @_ZN8Implicit1CC1EOS0_
+  // CHECK: call void @_ZN8Implicit1AC2ERKS0_(
+  // Note: this must memcpy 7 bytes, not 8, to avoid trampling over the virtual base class.
+  // CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* {{.*}}, i8* {{.*}}, i64 7, i1 false)
+  // CHECK: store i32 {{.*}} @_ZTVN8Implicit1CE
+}
+
+namespace InitWithinNVSize {
+  // This is the same as the previous test, except that the A base lies
+  // entirely within the nvsize of C. This makes it valid to copy at the
+  // full width.
+  struct A { char c; A(const A&); };
+  struct B { int n; char c[3]; ~B(); };
+  struct C : B, virtual A { char x; };
+  static_assert(sizeof(C) > sizeof(void*) + 8);
+  C f(C c) { return c; }
+
+  // CHECK: define {{.*}} @_ZN16InitWithinNVSize1CC1EOS0_
+  // CHECK: call void @_ZN16InitWithinNVSize1AC2ERKS0_(
+  // This copies over the 'C::x' member, but that's OK because we've not initialized it yet.
+  // CHECK: call void @llvm.memcpy.p0i8.p0i8.i64(i8* {{.*}}, i8* {{.*}}, i64 8, i1 false)
+  // CHECK: store i32 {{.*}} @_ZTVN16InitWithinNVSize1CE
+  // CHECK: store i8
+}