[ARM] prologue instructions emitted for naked function with >64 byte argument

Summary:

The naked function attribute is meant to suppress all function
prologue/epilogue instructions.

On ARM, some are still emitted if an argument greater than 64 bytes in size
(the threshold for using the byval attribute in IR) is passed partially
in registers.

Perform the check for Attribute::Naked and early exit in
SelectionDAGISel::LowerArguments().

Checking in ARMFrameLowering::determineCalleeSaves() is too late.

A test case is included.

Reviewers: llvm-commits, olista01, danielkiss

Reviewed By: danielkiss

Subscribers: kristof.beyls, hiraditya, danielkiss

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80715

Change-Id: Icedecf2a4ad31bc3c35ab0df7489a9d346e1f7cc

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -9607,6 +9607,10 @@ void SelectionDAGISel::LowerArguments(const Function &F) {
   const DataLayout &DL = DAG.getDataLayout();
   SmallVector<ISD::InputArg, 16> Ins;
 
+  // In Naked functions we aren't going to save any registers.
+  if (F.hasFnAttribute(Attribute::Naked))
+    return;
+
   if (!FuncInfo->CanLowerReturn) {
     // Put in an sret pointer parameter before all the other parameters.
     SmallVector<EVT, 1> ValueVTs;

test/CodeGen/ARM/naked-no-prolog.ll

@@ -0,0 +1,13 @@
+; RUN: llc -mtriple=arm-eabi -mcpu=cortex-a7 -verify-machineinstrs %s -o - | FileCheck %s
+
+%struct.S = type { [65 x i8] }
+
+define void @naked_no_prologue(%struct.S* byval(%struct.S) align 4 %0) naked noinline nounwind optnone {
+; CHECK-NOT: stm
+; CHECK-NOT: str
+
+entry:
+  ret void
+  unreachable
+}
+