[GlobalISel][CallLowering] Look through bitcasts from constant function pointers.

Calls to ObjC's objc_msgSend function are done by bitcasting the function global
to the required function type signature. This patch looks through this bitcast
so that we can do a direct call with bl on arm64 instead of using an indirect blr.

Differential Revision: https://reviews.llvm.org/D74241

1 files changed, 12 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/CallLowering.cpp b/llvm/lib/CodeGen/GlobalISel/CallLowering.cpp
index 294e089..752f26b 100644
--- a/llvm/lib/CodeGen/GlobalISel/CallLowering.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CallLowering.cpp
@@ -52,8 +52,18 @@ bool CallLowering::lowerCall(MachineIRBuilder &MIRBuilder, ImmutableCallSite CS,
 
   if (const Function *F = CS.getCalledFunction())
     Info.Callee = MachineOperand::CreateGA(F, 0);
-  else
-    Info.Callee = MachineOperand::CreateReg(GetCalleeReg(), false);
+  else {
+    // Try looking through a bitcast from one function type to another.
+    // Commonly happens with calls to objc_msgSend().
+    const Value *CalleeV = CS.getCalledValue();
+    if (auto *BC = dyn_cast<ConstantExpr>(CalleeV)) {
+      if (const auto *F = dyn_cast<Function>(BC->getOperand(0))) {
+        Info.Callee = MachineOperand::CreateGA(F, 0);
+      }
+    } else {
+      Info.Callee = MachineOperand::CreateReg(GetCalleeReg(), false);
+    }
+  }
 
   Info.OrigRet = ArgInfo{ResRegs, CS.getType(), ISD::ArgFlagsTy{}};
   if (!Info.OrigRet.Ty->isVoidTy())