Teach fast-isel to avoid loading a value from memory when it's already

available in a register. This is pretty primitive, but it reduces the
number of instructions in common testcases by 4%.

llvm-svn: 107380

1 files changed, 24 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index 2d6b78840f5..56009f39031 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -48,6 +48,7 @@
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/Analysis/DebugInfo.h"
+#include "llvm/Analysis/Loads.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetLowering.h"
@@ -716,8 +717,31 @@ FastISel::SelectFNeg(const User *I) {
 }
 
 bool
+FastISel::SelectLoad(const User *I) {
+  LoadInst *LI = const_cast<LoadInst *>(cast<LoadInst>(I));
+
+  // For a load from an alloca, make a limited effort to find the value
+  // already available in a register, avoiding redundant loads.
+  if (!LI->isVolatile() && isa<AllocaInst>(LI->getPointerOperand())) {
+    BasicBlock::iterator ScanFrom = LI;
+    if (const Value *V = FindAvailableLoadedValue(LI->getPointerOperand(),
+                                                  LI->getParent(), ScanFrom)) {
+      unsigned ResultReg = getRegForValue(V);
+      if (ResultReg != 0) {
+        UpdateValueMap(I, ResultReg);
+        return true;
+      }
+    }
+  }
+
+  return false;
+}
+
+bool
 FastISel::SelectOperator(const User *I, unsigned Opcode) {
   switch (Opcode) {
+  case Instruction::Load:
+    return SelectLoad(I);
   case Instruction::Add:
     return SelectBinaryOp(I, ISD::ADD);
   case Instruction::FAdd: