From 74ce7112c3fccccfa7edb134c0a2d8fe2aab462f Mon Sep 17 00:00:00 2001
From: Richard Smith <richard-llvm@metafoo.co.uk>
Date: Mon, 7 Oct 2019 02:45:12 +0000
Subject: [PATCH] Fix behavior of __builtin_bit_cast when the From and To types
 are the same.

We were missing the lvalue-to-rvalue conversion entirely in this case,
and in fact still need the full CK_LValueToRValueBitCast conversion to
perform a load with no TBAA.

llvm-svn: 373874
---
 clang/include/clang/AST/OperationKinds.def       |  5 +++--
 clang/lib/Sema/SemaCast.cpp                      |  5 -----
 .../test/CodeGenCXX/builtin-bit-cast-no-tbaa.cpp |  5 +++++
 .../test/SemaCXX/constexpr-builtin-bit-cast.cpp  | 16 ++++++++++++++++
 4 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/clang/include/clang/AST/OperationKinds.def b/clang/include/clang/AST/OperationKinds.def
index 9af92c1ae7ff..f29664e8eb33 100644
--- a/clang/include/clang/AST/OperationKinds.def
+++ b/clang/include/clang/AST/OperationKinds.def
@@ -66,8 +66,9 @@ CAST_OPERATION(BitCast)
 ///    bool b; reinterpret_cast<char&>(b) = 'a';
 CAST_OPERATION(LValueBitCast)
 
-/// CK_LValueToRValueBitCast - A conversion that causes us to reinterpret an
-/// lvalue as an rvalue of a different type. Created by __builtin_bit_cast.
+/// CK_LValueToRValueBitCast - A conversion that causes us to reinterpret the
+/// object representation of an lvalue as an rvalue. Created by
+/// __builtin_bit_cast.
 CAST_OPERATION(LValueToRValueBitCast)
 
 /// CK_LValueToRValue - A conversion which causes the extraction of
diff --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index 71e5e8e4286f..8c6abc448d97 100644
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/clang/lib/Sema/SemaCast.cpp
@@ -2835,11 +2835,6 @@ void CastOperation::CheckBuiltinBitCast() {
     return;
   }
 
-  if (Self.Context.hasSameUnqualifiedType(DestType, SrcType)) {
-    Kind = CK_NoOp;
-    return;
-  }
-
   Kind = CK_LValueToRValueBitCast;
 }
 
diff --git a/clang/test/CodeGenCXX/builtin-bit-cast-no-tbaa.cpp b/clang/test/CodeGenCXX/builtin-bit-cast-no-tbaa.cpp
index 3f0e490a5f94..b26e519bee3d 100644
--- a/clang/test/CodeGenCXX/builtin-bit-cast-no-tbaa.cpp
+++ b/clang/test/CodeGenCXX/builtin-bit-cast-no-tbaa.cpp
@@ -15,5 +15,10 @@ void test_scalar2() {
   // CHECK: load i32, i32* {{.*}}, align 4, !tbaa ![[MAY_ALIAS_TBAA]]
 }
 
+int test_same_type(int &r) {
+  // CHECK: load i32, i32* {{.*}}, align 4, !tbaa ![[MAY_ALIAS_TBAA]]
+  return __builtin_bit_cast(int, r);
+}
+
 // CHECK: ![[CHAR_TBAA:.*]] = !{!"omnipotent char", {{.*}}, i64 0}
 // CHECK: ![[MAY_ALIAS_TBAA]] = !{![[CHAR_TBAA]], ![[CHAR_TBAA]], i64 0}
diff --git a/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp b/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp
index 0a12e7eebe45..06771f8f3252 100644
--- a/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp
+++ b/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp
@@ -381,3 +381,19 @@ constexpr bool test_pad_buffer() {
   return x.a == z.a && x.b == z.b;
 }
 static_assert(test_pad_buffer());
+
+constexpr unsigned char identity1a = 42;
+constexpr unsigned char identity1b = __builtin_bit_cast(unsigned char, identity1a);
+static_assert(identity1b == 42);
+
+struct IdentityInStruct {
+  unsigned char n;
+};
+constexpr IdentityInStruct identity2a = {42};
+constexpr unsigned char identity2b = __builtin_bit_cast(unsigned char, identity2a.n);
+
+union IdentityInUnion {
+  unsigned char n;
+};
+constexpr IdentityInUnion identity3a = {42};
+constexpr unsigned char identity3b = __builtin_bit_cast(unsigned char, identity3a.n);