implement rdar://6653118 - fastisel should fold loads where possible.

Since mem2reg isn't run at -O0, we get a ton of reloads from the stack,
for example, before, this code:

int foo(int x, int y, int z) {
  return x+y+z;
}

used to compile into:

_foo:                                   ## @foo
	subq	$12, %rsp
	movl	%edi, 8(%rsp)
	movl	%esi, 4(%rsp)
	movl	%edx, (%rsp)
	movl	8(%rsp), %edx
	movl	4(%rsp), %esi
	addl	%edx, %esi
	movl	(%rsp), %edx
	addl	%esi, %edx
	movl	%edx, %eax
	addq	$12, %rsp
	ret

Now we produce:

_foo:                                   ## @foo
	subq	$12, %rsp
	movl	%edi, 8(%rsp)
	movl	%esi, 4(%rsp)
	movl	%edx, (%rsp)
	movl	8(%rsp), %edx
	addl	4(%rsp), %edx    ## Folded load
	addl	(%rsp), %edx     ## Folded load
	movl	%edx, %eax
	addq	$12, %rsp
	ret

Fewer instructions and less register use = faster compiles.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113102 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 13 insertions, 1 deletions

diff --git a/include/llvm/CodeGen/FastISel.h b/include/llvm/CodeGen/FastISel.h
index 79b1554..d7506be 100644
--- a/include/llvm/CodeGen/FastISel.h
+++ b/include/llvm/CodeGen/FastISel.h
@@ -39,6 +39,7 @@ class TargetLowering;
 class TargetMachine;
 class TargetRegisterClass;
 class TargetRegisterInfo;
+class LoadInst;
 
 /// FastISel - This is a fast-path instruction selection class that
 /// generates poor code and doesn't support illegal types or non-trivial
@@ -102,7 +103,16 @@ public:
   /// index value.
   std::pair<unsigned, bool> getRegForGEPIndex(const Value *V);
 
-  /// recomputeInsertPt - Reset InsertPt to prepare for insterting instructions
+  /// TryToFoldLoad - The specified machine instr operand is a vreg, and that
+  /// vreg is being provided by the specified load instruction.  If possible,
+  /// try to fold the load as an operand to the instruction, returning true if
+  /// possible.
+  virtual bool TryToFoldLoad(MachineInstr * /*MI*/, unsigned /*OpNo*/,
+                             const LoadInst * /*LI*/) {
+    return false;
+  }
+  
+  /// recomputeInsertPt - Reset InsertPt to prepare for inserting instructions
   /// into the current block.
   void recomputeInsertPt();
 
diff --git a/include/llvm/CodeGen/SelectionDAGISel.h b/include/llvm/CodeGen/SelectionDAGISel.h
index 01d05dd..51895a6 100644
--- a/include/llvm/CodeGen/SelectionDAGISel.h
+++ b/include/llvm/CodeGen/SelectionDAGISel.h
@@ -34,6 +34,7 @@ namespace llvm {
   class ScheduleHazardRecognizer;
   class GCFunctionInfo;
   class ScheduleDAGSDNodes;
+  class LoadInst;
  
 /// SelectionDAGISel - This is the common base class used for SelectionDAG-based
 /// pattern-matching instruction selectors.
@@ -282,6 +283,7 @@ private:
   
   void PrepareEHLandingPad();
   void SelectAllBasicBlocks(const Function &Fn);
+  bool TryToFoldFastISelLoad(const LoadInst *LI, FastISel *FastIS);
   void FinishBasicBlock();
 
   void SelectBasicBlock(BasicBlock::const_iterator Begin,