Respect the alignment attribute of functions.

lib/Target/CBackend/CBackend.cpp

@@ -2051,6 +2051,15 @@ static void defineAligns(raw_ostream &Out) {
   Out << "#endif\n\n";
 }
 
+static void defineFunctionAlign(raw_ostream &Out) {
+  Out << "#ifdef _MSC_VER\n";
+  Out << "#define __FUNCTIONALIGN__(X) /* WARNING: THIS FEATURE IS NOT "
+         "SUPPORTED BY MSVC! */ \n";
+  Out << "#else\n";
+  Out << "#define __FUNCTIONALIGN__(X) __attribute__((aligned(X)))\n";
+  Out << "#endif\n\n";
+}
+
 static void defineUnreachable(raw_ostream &Out) {
   Out << "#ifdef _MSC_VER\n";
   Out << "#define __builtin_unreachable() __assume(0)\n";
@@ -2355,6 +2364,8 @@ void CWriter::generateCompilerSpecificCode(raw_ostream &Out,
     defineUnalignedLoad(Out);
   if (headerIncAligns())
     defineAligns(Out);
+  if (headerIncFunctionAlign())
+    defineFunctionAlign(Out);
   if (headerIncNanInf())
     defineNanInf(Out);
   if (headerIncInt128())
@@ -2632,6 +2643,7 @@ void CWriter::generateHeader(Module &M) {
       headerUseAttributeWeak();
       Out << "__MSVC_INLINE__ ";
     }
+
     printFunctionProto(Out, &*I, GetValueName(&*I));
     printFunctionAttributes(Out, I->getAttributes());
     if (I->hasWeakLinkage() || I->hasLinkOnceLinkage()) {
@@ -2655,6 +2667,12 @@ void CWriter::generateHeader(Module &M) {
       Out << " __HIDDEN__";
     }
 
+    unsigned Alignment = I->getAlignment();
+    if (Alignment != 0) {
+      headerUseFunctionAlign();
+      Out << " __FUNCTIONALIGN__(" << Alignment << ") ";
+    }
+
     if (I->hasName() && I->getName()[0] == 1)
       Out << " __asm__ (\"" << I->getName().substr(1) << "\")";
 

lib/Target/CBackend/CBackend.h

@@ -112,6 +112,7 @@ class CWriter : public FunctionPass, public InstVisitor<CWriter> {
     bool AttributeList : 1;
     bool UnalignedLoad : 1;
     bool Aligns : 1;
+    bool FunctionAlign : 1;
     bool NanInf : 1;
     bool Int128 : 1;
     bool ThreadFence : 1;
@@ -148,6 +149,7 @@ class CWriter : public FunctionPass, public InstVisitor<CWriter> {
   USED_HEADERS_FLAG(AttributeList)
   USED_HEADERS_FLAG(UnalignedLoad)
   USED_HEADERS_FLAG(Aligns)
+  USED_HEADERS_FLAG(FunctionAlign)
   USED_HEADERS_FLAG(NanInf)
   USED_HEADERS_FLAG(Int128)
   USED_HEADERS_FLAG(ThreadFence)

test/cpp_tests/test_virtual_function_calls.cpp

@@ -0,0 +1,31 @@
+#ifndef _MSC_VER
+// Suppress "dereferencing type-punned pointer will break strict-aliasing rules"
+// gcc_extra_args: -Wno-strict-aliasing
+struct A {
+  int a;
+  int b;
+  int f1(int i) { return 1 + i; }
+  int f2(int i) { return 1 + i; }
+  typedef int (A::*Function)(int);
+  int function_wrapper(Function f, int i) __attribute__((noinline)) {
+    return (this->*f)(i);
+  }
+};
+
+int main() {
+  A a{};
+  if (a.function_wrapper(&A::f1, 1) != 2)
+    return 0;
+  if (a.function_wrapper(&A::f2, 1) != 2)
+    return 0;
+  return 6;
+}
+
+#else
+
+/* MSVC doesn't support adding alignment to functions, treat it as an expected
+ * failure */
+
+int main() { return 25; }
+
+#endif