InstCombine rule to fold truncs whose value is available

We can fold truncs whose operand feeds from a load, if the trunc value
is available through a prior load/store.

This change is from: http://reviews.llvm.org/D21246, which folded the
trunc but missed the bitcast or ptrtoint/inttoptr required in the RAUW
call, when the load type didnt match the prior load/store type.

Differential Revision: http://reviews.llvm.org/D21791

llvm-svn: 274853

1 files changed, 23 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 20556157188..395e83661d5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -13,9 +13,10 @@
 
 #include "InstCombineInternal.h"
 #include "llvm/Analysis/ConstantFolding.h"
+#include "llvm/Analysis/Loads.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/PatternMatch.h"
-#include "llvm/Analysis/TargetLibraryInfo.h"
 using namespace llvm;
 using namespace PatternMatch;
 
@@ -575,6 +576,27 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
   if (Instruction *I = foldVecTruncToExtElt(CI, *this, DL))
     return I;
 
+  // When trunc operand is a widened load, see if we can get the value from a
+  // previous store/load
+  if (auto *LI = dyn_cast<LoadInst>(Src)) {
+    BasicBlock::iterator BBI(*LI);
+
+    // Scan a few instructions up from LI and if we find a partial load/store
+    // of Type DestTy that feeds into LI, we can replace all uses of the trunc
+    // with the load/store value.
+    // This replacement can be done only in the case of non-volatile loads, with
+    // ordering at most unordered. If the load is atomic, its only use should be
+    // the trunc instruction. We don't want to allow other users of LI to see a
+    // value that is out of sync with the value we're folding the trunc to (in
+    // case of a race).
+    if (LI->isUnordered() && (!LI->isAtomic() || LI->hasOneUse()))
+      if (Value *AvailableVal = FindAvailableLoadedValue(
+              LI->getPointerOperand(), DestTy, LI->isAtomic(), LI->getParent(),
+              BBI, DefMaxInstsToScan))
+        return replaceInstUsesWith(
+            CI, Builder->CreateBitOrPointerCast(AvailableVal, CI.getType(),
+                                                CI.getName() + ".cast"));
+  }
   return nullptr;
 }