Currently, codegen may spent some time in SDISel passes even if an entire

function is successfully handled by fast-isel. That's because function
arguments are *always* handled by SDISel. Introduce FastLowerArguments to
allow each target to provide hook to handle formal argument lowering.

As a proof-of-concept, add ARMFastIsel::FastLowerArguments to handle
functions with 4 or fewer scalar integer (i8, i16, or i32) arguments. It
completely eliminates the need for SDISel for trivial functions.

rdar://13163905


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174855 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 74 insertions, 0 deletions

diff --git a/lib/Target/ARM/ARMFastISel.cpp b/lib/Target/ARM/ARMFastISel.cpp
index 4b7978abd2..022fe97026 100644
--- a/lib/Target/ARM/ARMFastISel.cpp
+++ b/lib/Target/ARM/ARMFastISel.cpp
@@ -146,6 +146,7 @@ class ARMFastISel : public FastISel {
     virtual unsigned TargetMaterializeAlloca(const AllocaInst *AI);
     virtual bool TryToFoldLoad(MachineInstr *MI, unsigned OpNo,
                                const LoadInst *LI);
+    virtual bool FastLowerArguments();
   private:
   #include "ARMGenFastISel.inc"
 
@@ -2884,6 +2885,79 @@ unsigned ARMFastISel::ARMLowerPICELF(const GlobalValue *GV,
   return DestReg2;
 }
 
+bool ARMFastISel::FastLowerArguments() {
+  if (!FuncInfo.CanLowerReturn)
+    return false;
+
+  const Function *F = FuncInfo.Fn;
+  if (F->isVarArg())
+    return false;
+
+  CallingConv::ID CC = F->getCallingConv();
+  switch (CC) {
+  default:
+    return false;
+  case CallingConv::Fast:
+  case CallingConv::C:
+  case CallingConv::ARM_AAPCS_VFP:
+  case CallingConv::ARM_AAPCS:
+  case CallingConv::ARM_APCS:
+    break;
+  }
+
+  // Only handle simple cases. i.e. Up to 4 i8/i16/i32 scalar arguments
+  // which are passed in r0 - r3.
+  unsigned Idx = 1;
+  for (Function::const_arg_iterator I = F->arg_begin(), E = F->arg_end();
+       I != E; ++I, ++Idx) {
+    if (Idx > 4)
+      return false;
+
+    if (F->getAttributes().hasAttribute(Idx, Attribute::InReg) ||
+        F->getAttributes().hasAttribute(Idx, Attribute::StructRet) ||
+        F->getAttributes().hasAttribute(Idx, Attribute::ByVal))
+      return false;
+
+    Type *ArgTy = I->getType();
+    if (ArgTy->isStructTy() || ArgTy->isArrayTy() || ArgTy->isVectorTy())
+      return false;
+
+    EVT ArgVT = TLI.getValueType(ArgTy);
+    switch (ArgVT.getSimpleVT().SimpleTy) {
+    case MVT::i8:
+    case MVT::i16:
+    case MVT::i32:
+      break;
+    default:
+      return false;
+    }
+  }
+
+
+  static const uint16_t GPRArgRegs[] = {
+    ARM::R0, ARM::R1, ARM::R2, ARM::R3
+  };
+
+  const TargetRegisterClass *RC = TLI.getRegClassFor(MVT::i32);
+  Idx = 0;
+  for (Function::const_arg_iterator I = F->arg_begin(), E = F->arg_end();
+       I != E; ++I, ++Idx) {
+    if (I->use_empty())
+      continue;
+    unsigned SrcReg = GPRArgRegs[Idx];
+    unsigned DstReg = FuncInfo.MF->addLiveIn(SrcReg, RC);
+    // FIXME: Unfortunately it's necessary to emit a copy from the livein copy.
+    // Without this, EmitLiveInCopies may eliminate the livein if its only
+    // use is a bitcast (which isn't turned into an instruction).
+    unsigned ResultReg = createResultReg(RC);
+    BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DL, TII.get(TargetOpcode::COPY),
+            ResultReg).addReg(DstReg, getKillRegState(true));
+    UpdateValueMap(I, ResultReg);
+  }
+
+  return true;
+}
+
 namespace llvm {
   FastISel *ARM::createFastISel(FunctionLoweringInfo &funcInfo,
                                 const TargetLibraryInfo *libInfo) {