Revert "[IndVars] Smart hard uses detection"

This reverts commit 2f425e9c7946b9d74e64ebbfa33c1caa36914402.

It seems that the check that we still should do the transform if we
know the result is constant is missing in this code. So the logic that
has been deleted by this change is still sometimes accidentally useful.
I revert the change to see what can be done about it. The motivating
case is the following:

@Y = global [400 x i16] zeroinitializer, align 1

define i16 @foo() {
entry:
  br label %for.body

for.body:                                         ; preds = %entry, %for.body
  %i = phi i16 [ 0, %entry ], [ %inc, %for.body ]

  %arrayidx = getelementptr inbounds [400 x i16], [400 x i16]* @Y, i16 0, i16 %i
  store i16 0, i16* %arrayidx, align 1
  %inc = add nuw nsw i16 %i, 1
  %cmp = icmp ult i16 %inc, 400
  br i1 %cmp, label %for.body, label %for.end

for.end:                                          ; preds = %for.body
  %inc.lcssa = phi i16 [ %inc, %for.body ]
  ret i16 %inc.lcssa
}

We should be able to figure out that the result is constant, but the patch
breaks it.

Differential Revision: https://reviews.llvm.org/D51584

llvm-svn: 346198

1 files changed, 13 insertions, 26 deletions

diff --git a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
index 3e4e0f46ca3..ec51ad71abc 100644
--- a/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -145,7 +145,6 @@ class IndVarSimplify {
   bool canLoopBeDeleted(Loop *L, SmallVector<RewritePhi, 8> &RewritePhiSet);
   bool rewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter);
   bool rewriteFirstIterationLoopExitValues(Loop *L);
-  bool hasHardUserWithinLoop(const Loop *L, const Instruction *I) const;
 
   bool linearFunctionTestReplace(Loop *L, const SCEV *BackedgeTakenCount,
                                  PHINode *IndVar, SCEVExpander &Rewriter);
@@ -525,29 +524,6 @@ struct RewritePhi {
 // As a side effect, reduces the amount of IV processing within the loop.
 //===----------------------------------------------------------------------===//
 
-bool IndVarSimplify::hasHardUserWithinLoop(const Loop *L, const Instruction *I) const {
-  SmallPtrSet<const Instruction *, 8> Visited;
-  SmallVector<const Instruction *, 8> WorkList;
-  Visited.insert(I);
-  WorkList.push_back(I);
-  while (!WorkList.empty()) {
-    const Instruction *Curr = WorkList.pop_back_val();
-    // This use is outside the loop, nothing to do.
-    if (!L->contains(Curr))
-      continue;
-    // Do we assume it is a "hard" use which will not be eliminated easily?
-    if (Curr->mayHaveSideEffects())
-      return true;
-    // Otherwise, add all its users to worklist.
-    for (auto U : Curr->users()) {
-      auto *UI = cast<Instruction>(U);
-      if (Visited.insert(UI).second)
-        WorkList.push_back(UI);
-    }
-  }
-  return false;
-}
-
 /// Check to see if this loop has a computable loop-invariant execution count.
 /// If so, this means that we can compute the final value of any expressions
 /// that are recurrent in the loop, and substitute the exit values from the loop
@@ -622,8 +598,19 @@ bool IndVarSimplify::rewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
         // Computing the value outside of the loop brings no benefit if it is
         // definitely used inside the loop in a way which can not be optimized
         // away.
-        if (hasHardUserWithinLoop(L, Inst))
-          continue;
+        if (ExitValue->getSCEVType()>=scMulExpr) {
+          bool HasHardInternalUses = false;
+          for (auto *IB : Inst->users()) {
+            Instruction *UseInstr = cast<Instruction>(IB);
+            unsigned Opc = UseInstr->getOpcode();
+            if (L->contains(UseInstr) && Opc == Instruction::Call) {
+              HasHardInternalUses = true;
+              break;
+            }
+          }
+          if (HasHardInternalUses)
+            continue;
+        }
 
         bool HighCost = Rewriter.isHighCostExpansion(ExitValue, L, Inst);
         Value *ExitVal = Rewriter.expandCodeFor(ExitValue, PN->getType(), Inst);