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

[aemerson]

Summary

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.

[paquette]

LGTM

llvm/lib/CodeGen/GlobalISel/CallLowering.cpp

ArgInfo OrigArg{ArgRegs[i], Arg->getType(), ISD::ArgFlagsTy{},
                     i < NumFixedArgs};
     setArgFlags(OrigArg, i + AttributeList::FirstArgIndex, DL, CS);
     Info.OrigArgs.push_back(OrigArg);
     ++i;
   }
 
   if (const Function *F = CS.getCalledFunction())
     Info.Callee = MachineOperand::CreateGA(F, 0);
-  else
+  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())
     setArgFlags(Info.OrigRet, AttributeList::ReturnIndex, DL, CS);
 
   Info.KnownCallees =
       CS.getInstruction()->getMetadata(LLVMContext::MD_callees);
   Info.CallConv = CS.getCallingConv();

llvm/test/CodeGen/AArch64/GlobalISel/call-lowering-const-bitcast-func.ll

+; RUN: llc -O0 -global-isel -verify-machineinstrs %s -o - 2>&1 | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64-darwin-ios13.0"
+
+declare i8* @objc_msgSend(i8*, i8*, ...)
+define void @call_bitcast_ptr_const() {
+; CHECK-LABEL: @call_bitcast_ptr_const
+; CHECK: bl _objc_msgSend
+; CHECK-NOT: blr
+entry:
+  call void bitcast (i8* (i8*, i8*, ...)* @objc_msgSend to void (i8*, i8*, [2 x i32], i32, float)*)(i8* undef, i8* undef, [2 x i32] zeroinitializer, i32 0, float 1.000000e+00)
+  ret void
+}