Respect the alignment attribute of functions (#186)

Respect the alignment attribute of functions. This is necessary to
support calls to function pointer calls to member functions (which need
to be aligned to 2, and the generated code crashes if this happens to
be not the case)

MSVC doesn't support adding alignment to functions, so create a
separate #define for this, which is a no-op on MSVC Given that this
occurs in several tests, and the MSVC tests have error-on-warning, this
is a comment rather than a #warning

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