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, 13 insertions, 7 deletions

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index d4e9a5043d..c9289add79 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)