Implement a trivial form of store->load forwarding where the store and the

load are exactly consequtive.  This is picked up by other passes, but this
triggers thousands of times in fortran programs that use static locals
(and is thus a compile-time speedup).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23320 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 67dca06a420531ab685f7738a7dfa1c0e83195a4..67144b93cfe39471d79adc4c503ed0a07f1f2d37 100644 (file)
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -4925,6 +4925,15 @@ Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
 
   // None of the following transforms are legal for volatile loads.
   if (LI.isVolatile()) return 0;
+  
+  // If the instruction immediately before this is a store to the same address,
+  // do a simple form of store->load forwarding.
+  if (&LI.getParent()->front() != &LI) {
+    BasicBlock::iterator BBI = &LI; --BBI;
+    if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
+      if (SI->getOperand(1) == LI.getOperand(0))
+        return ReplaceInstUsesWith(LI, SI->getOperand(0));
+  }
 
   if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op))
     if (isa<ConstantPointerNull>(GEPI->getOperand(0)) ||