Revert r1296656, "Fix rdar://9289512 - not folding load into compare at -O0...",

which broke a couple GCC test suite tests at -O0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129914 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 15 insertions, 41 deletions

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index af31888..25dd97d 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -746,31 +746,9 @@ void SelectionDAGISel::PrepareEHLandingPad() {
 
 
 
-/// TryToFoldFastISelLoad - We're checking to see if we can fold the specified
-/// load into the specified FoldInst.  Note that we could have a sequence where
-/// multiple LLVM IR instructions are folded into the same machineinstr.  For
-/// example we could have:
-///   A: x = load i32 *P
-///   B: y = icmp A, 42
-///   C: br y, ...
-///
-/// In this scenario, LI is "A", and FoldInst is "C".  We know about "B" (and
-/// any other folded instructions) because it is between A and C.
-///
-/// If we succeed in folding the load into the operation, return true.
-///
+
 bool SelectionDAGISel::TryToFoldFastISelLoad(const LoadInst *LI,
-                                             const Instruction *FoldInst,
                                              FastISel *FastIS) {
-  SmallPtrSet<const Instruction*, 4> FoldedInsts;
-  for (BasicBlock::const_iterator II = FoldInst; &*II != LI; --II)
-    FoldedInsts.insert(II);
-  
-  // We know that the load has a single use, but don't know what it is.  If it
-  // isn't one of the folded instructions, then we can't succeed here.
-  if (!FoldedInsts.count(LI->use_back()))
-    return false;
-  
   // Don't try to fold volatile loads.  Target has to deal with alignment
   // constraints.
   if (LI->isVolatile()) return false;
@@ -855,10 +833,10 @@ static void CheckLineNumbers(const MachineBasicBlock *MBB) {
 /// Return false if it needs to be emitted.
 static bool isFoldedOrDeadInstruction(const Instruction *I,
                                       FunctionLoweringInfo *FuncInfo) {
-  return !I->mayWriteToMemory() && // Side-effecting instructions aren't folded.
-         !isa<TerminatorInst>(I) && // Terminators aren't folded.
-         !isa<DbgInfoIntrinsic>(I) &&  // Debug instructions aren't folded.
-         !FuncInfo->isExportedInst(I); // Exported instrs must be computed.
+  return !I->mayWriteToMemory() &&
+         !isa<TerminatorInst>(I) &&
+         !isa<DbgInfoIntrinsic>(I) &&
+         !FuncInfo->isExportedInst(I);
 }
 
 void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {
@@ -950,20 +928,16 @@ void SelectionDAGISel::SelectAllBasicBlocks(const Function &Fn) {
 
         // Try to select the instruction with FastISel.
         if (FastIS->SelectInstruction(Inst)) {
-          // If fast isel succeeded, skip over all the folded instructions, and
-          // then see if there is a load right before the selected instructions.
-          // Try to fold the load if so.
-          const Instruction *BeforeInst = Inst;
-          while (BeforeInst != Begin) {
-            BeforeInst = llvm::prior(BasicBlock::const_iterator(BeforeInst));
-            if (!isFoldedOrDeadInstruction(BeforeInst, FuncInfo))
-              break;
-          }
-          if (BeforeInst != Inst && isa<LoadInst>(BeforeInst) &&
-              BeforeInst->hasOneUse() &&
-              TryToFoldFastISelLoad(cast<LoadInst>(BeforeInst), Inst, FastIS))
-            // If we succeeded, don't re-select the load.
-            BI = llvm::next(BasicBlock::const_iterator(BeforeInst));
+          // If fast isel succeeded, check to see if there is a single-use
+          // non-volatile load right before the selected instruction, and see if
+          // the load is used by the instruction.  If so, try to fold it.
+          const Instruction *BeforeInst = 0;
+          if (Inst != Begin)
+            BeforeInst = llvm::prior(llvm::prior(BI));
+          if (BeforeInst && isa<LoadInst>(BeforeInst) &&
+              BeforeInst->hasOneUse() && *BeforeInst->use_begin() == Inst &&
+              TryToFoldFastISelLoad(cast<LoadInst>(BeforeInst), FastIS))
+            --BI; // If we succeeded, don't re-select the load.
           continue;
         }