[clang] d2e7dca - [CodeGen] Fix Bug 47499: __unaligned extension inconsistent behaviour with C and C++

[Reid Kleckner via cfe-commits]

Author: Jan Ole Hüser
Date: 2020-11-05T12:57:17-08:00
New Revision: d2e7dca5ca92c655e451d6fcb806df38d7f2d56b

URL: https://github.com/llvm/llvm-project/commit/d2e7dca5ca92c655e451d6fcb806df38d7f2d56b
DIFF: https://github.com/llvm/llvm-project/commit/d2e7dca5ca92c655e451d6fcb806df38d7f2d56b.diff

LOG: [CodeGen] Fix Bug 47499: __unaligned extension inconsistent behaviour with C and C++

For the language C++ the keyword __unaligned (a Microsoft extension) had no effect on pointers.

The reason, why there was a difference between C and C++ for the keyword __unaligned:
For C, the Method getAsCXXREcordDecl() returns nullptr. That guarantees that hasUnaligned() is called.
If the language is C++, it is not guaranteed, that hasUnaligend() is called and evaluated.

Here are some links:

The Bug: https://bugs.llvm.org/show_bug.cgi?id=47499
Thread on the cfe-dev mailing list: http://lists.llvm.org/pipermail/cfe-dev/2020-September/066783.html
Diff, that introduced the check hasUnaligned() in getNaturalTypeAlignment(): https://reviews.llvm.org/D30166

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D90630

Added: 
    clang/test/CodeGen/unaligned-struct-copy.c

Modified: 
    clang/lib/CodeGen/CodeGenModule.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 24c067539f83..ccf5d24bb9eb 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -6197,16 +6197,17 @@ CharUnits CodeGenModule::getNaturalTypeAlignment(QualType T,
     *BaseInfo = LValueBaseInfo(AlignmentSource::Type);
 
   CharUnits Alignment;
-  // For C++ class pointees, we don't know whether we're pointing at a
-  // base or a complete object, so we generally need to use the
-  // non-virtual alignment.
   const CXXRecordDecl *RD;
-  if (forPointeeType && !AlignForArray && (RD = T->getAsCXXRecordDecl())) {
+  if (T.getQualifiers().hasUnaligned()) {
+    Alignment = CharUnits::One();
+  } else if (forPointeeType && !AlignForArray &&
+             (RD = T->getAsCXXRecordDecl())) {
+    // For C++ class pointees, we don't know whether we're pointing at a
+    // base or a complete object, so we generally need to use the
+    // non-virtual alignment.
     Alignment = getClassPointerAlignment(RD);
   } else {
     Alignment = getContext().getTypeAlignInChars(T);
-    if (T.getQualifiers().hasUnaligned())
-      Alignment = CharUnits::One();
   }
 
   // Cap to the global maximum type alignment unless the alignment

diff  --git a/clang/test/CodeGen/unaligned-struct-copy.c b/clang/test/CodeGen/unaligned-struct-copy.c
new file mode 100644
index 000000000000..45a9670bc234
--- /dev/null
+++ b/clang/test/CodeGen/unaligned-struct-copy.c
@@ -0,0 +1,32 @@
+// RUN: %clang_cc1 -xc   -O2 -triple thumbv7a-unknown-windows-eabi -fms-extensions -emit-llvm < %s | FileCheck %s
+// RUN: %clang_cc1 -xc++ -O2 -triple thumbv7a-unknown-windows-eabi -fms-extensions -emit-llvm < %s | FileCheck %s
+// RUN: %clang_cc1 -xc   -O2 -triple x86_64-unknown-linux-gnu -fms-extensions -emit-llvm < %s | FileCheck %s
+// RUN: %clang_cc1 -xc++ -O2 -triple x86_64-unknown-linux-gnu -fms-extensions -emit-llvm < %s | FileCheck %s
+
+struct S1 {
+  unsigned long x;
+};
+
+// CHECK: define
+// CHECK-SAME: void
+// CHECK-SAME: test1
+
+void test1(__unaligned struct S1 *out) {
+  // CHECK: store
+  // CHECK-SAME: align 1
+  out->x = 5;
+  // CHECK: ret void
+}
+
+// CHECK: define
+// CHECK-SAME: void
+// CHECK-SAME: test2
+
+void test2(__unaligned struct S1 *out, __unaligned struct S1 *in) {
+  // CHECK: load
+  // CHECK-SAME: align 1
+  // CHECK: store
+  // CHECK-SAME: align 1
+  *out = *in;
+  // CHECK: ret void
+}