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

2 files changed, 40 insertions, 8 deletions

diff --git a/lib/CodeGen/SelectionDAG/FastISel.cpp b/lib/CodeGen/SelectionDAG/FastISel.cpp
index 0d90a07..6c41e1b 100644
--- a/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -87,6 +87,27 @@ void FastISel::startNewBlock() {
   LastLocalValue = EmitStartPt;
 }
 
+bool FastISel::LowerArguments() {
+  if (!FuncInfo.CanLowerReturn)
+    // Fallback to SDISel argument lowering code to deal with sret pointer
+    // parameter.
+    return false;
+  
+  if (!FastLowerArguments())
+    return false;
+
+  // Enter non-dead arguments into ValueMap for uses in non-entry BBs.
+  for (Function::const_arg_iterator I = FuncInfo.Fn->arg_begin(),
+         E = FuncInfo.Fn->arg_end(); I != E; ++I) {
+    if (!I->use_empty()) {
+      DenseMap<const Value *, unsigned>::iterator VI = LocalValueMap.find(I);
+      assert(VI != LocalValueMap.end() && "Missed an argument?");
+      FuncInfo.ValueMap[I] = VI->second;
+    }
+  }
+  return true;
+}
+
 void FastISel::flushLocalValueMap() {
   LocalValueMap.clear();
   LastLocalValue = EmitStartPt;
@@ -836,7 +857,8 @@ FastISel::SelectInstruction(const Instruction *I) {
 void
 FastISel::FastEmitBranch(MachineBasicBlock *MSucc, DebugLoc DL) {
 
-  if (FuncInfo.MBB->getBasicBlock()->size() > 1 && FuncInfo.MBB->isLayoutSuccessor(MSucc)) {
+  if (FuncInfo.MBB->getBasicBlock()->size() > 1 &&
+      FuncInfo.MBB->isLayoutSuccessor(MSucc)) {
     // For more accurate line information if this is the only instruction
     // in the block then emit it, otherwise we have the unconditional
     // fall-through case, which needs no instructions.
@@ -1067,6 +1089,10 @@ FastISel::FastISel(FunctionLoweringInfo &funcInfo,
 
 FastISel::~FastISel() {}
 
+bool FastISel::FastLowerArguments() {
+  return false;
+}
+
 unsigned FastISel::FastEmit_(MVT, MVT,
                              unsigned) {
   return 0;
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index d4e9a50..c9289ad 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -1032,10 +1032,6 @@ void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {
     if (FuncInfo->MBB->isLandingPad())
       PrepareEHLandingPad();
 
-    // Lower any arguments needed in this block if this is the entry block.
-    if (LLVMBB == &Fn.getEntryBlock())
-      LowerArguments(LLVMBB);
-
     // Before doing SelectionDAG ISel, see if FastISel has been requested.
     if (FastIS) {
       FastIS->startNewBlock();
@@ -1043,9 +1039,15 @@ void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {
       // Emit code for any incoming arguments. This must happen before
       // beginning FastISel on the entry block.
       if (LLVMBB == &Fn.getEntryBlock()) {
-        CurDAG->setRoot(SDB->getControlRoot());
-        SDB->clear();
-        CodeGenAndEmitDAG();
+        // Lower any arguments needed in this block if this is the entry block.
+        if (!FastIS->LowerArguments()) {
+          // Call target indepedent SDISel argument lowering code if the target
+          // specific routine is not successful.
+          LowerArguments(LLVMBB);
+          CurDAG->setRoot(SDB->getControlRoot());
+          SDB->clear();
+          CodeGenAndEmitDAG();
+        }
 
         // If we inserted any instructions at the beginning, make a note of
         // where they are, so we can be sure to emit subsequent instructions
@@ -1156,6 +1158,10 @@ void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {
       }
 
       FastIS->recomputeInsertPt();
+    } else {
+      // Lower any arguments needed in this block if this is the entry block.
+      if (LLVMBB == &Fn.getEntryBlock())
+        LowerArguments(LLVMBB);
     }
 
     if (Begin != BI)