[IR][AutoUpgrade] Drop align attribute from void return types

Since D87304, `align` become an invalid attribute on none pointer types and
verifier will reject bitcode that has invalid `align` attribute.

The problem is before the change, DeadArgumentElimination can easily
turn a pointer return type into a void return type without removing
`align` attribute. Teach Autograde to remove invalid `align` attribute
from return types to maintain bitcode compatibility.

rdar://77022993

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D102201

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -5562,8 +5562,8 @@ Error BitcodeReader::materialize(GlobalValue *GV) {
     }
   }
 
-  // "Upgrade" older incorrect branch weights by dropping them.
   for (auto &I : instructions(F)) {
+    // "Upgrade" older incorrect branch weights by dropping them.
     if (auto *MD = I.getMetadata(LLVMContext::MD_prof)) {
       if (MD->getOperand(0) != nullptr && isa<MDString>(MD->getOperand(0))) {
         MDString *MDS = cast<MDString>(MD->getOperand(0));
@@ -5590,6 +5590,12 @@ Error BitcodeReader::materialize(GlobalValue *GV) {
           I.setMetadata(LLVMContext::MD_prof, nullptr);
       }
     }
+
+    // Remove align from return attribute on CallInst.
+    if (auto *CI = dyn_cast<CallInst>(&I)) {
+      if (CI->getFunctionType()->getReturnType()->isVoidTy())
+        CI->removeAttribute(0, Attribute::Alignment);
+    }
   }
 
   // Look for functions that rely on old function attribute behavior.

llvm/lib/IR/AutoUpgrade.cpp

@@ -4371,6 +4371,12 @@ void llvm::UpgradeFunctionAttributes(Function &F) {
     Attribute NewAttr = Attribute::getWithByValType(F.getContext(), ByValTy);
     F.addParamAttr(0, NewAttr);
   }
+
+  // If function has void return type, check it has align attribute. It has no
+  // affect on the return type and no longer passes the verifier.
+  if (F.getReturnType()->isVoidTy() &&
+      F.hasAttribute(AttributeList::ReturnIndex, Attribute::Alignment))
+    F.removeAttribute(AttributeList::ReturnIndex, Attribute::Alignment);
 }
 
 static bool isOldLoopArgument(Metadata *MD) {

llvm/test/Bitcode/upgrade-void-ret-attr-11.0.ll

@@ -0,0 +1,14 @@
+; Check upgrade is removing the incompatible attributes on void return type.
+
+; RUN: llvm-dis < %s.bc | FileCheck %s
+
+; CHECK: define void @f()
+define align 8 void @f() {
+  ret void
+}
+
+define void @g() {
+; CHECK: call void @f()
+  call align 8 void @f();
+  ret void
+}