[PATCH] Handle resolvable branches in complete loop unroll heuristic.

[Michael Zolotukhin]

- In two-operand instructions use simplified values only if both operands have the same base pointer (or don't have it at all).


http://reviews.llvm.org/D10206

Files:
  lib/Transforms/Scalar/LoopUnrollPass.cpp

Index: lib/Transforms/Scalar/LoopUnrollPass.cpp
===================================================================
--- lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -361,6 +361,8 @@
     if (SCEVSimplify(&I))
       return true;
     Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
+    if (!hasSamePointerBase(LHS, RHS))
+      return false;
     if (!isa<Constant>(LHS))
       if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
         LHS = SimpleLHS;
@@ -421,6 +423,53 @@
 
     return true;
   }
+
+  bool visitCmpInst(CmpInst &I) {
+    if (SCEVSimplify(&I))
+      return true;
+    Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
+    if (!hasSamePointerBase(LHS, RHS))
+      return false;
+
+    // First try to handle simplified comparisons.
+    if (!isa<Constant>(LHS))
+      if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
+        LHS = SimpleLHS;
+    if (!isa<Constant>(RHS))
+      if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
+        RHS = SimpleRHS;
+    if (Constant *CLHS = dyn_cast<Constant>(LHS)) {
+      if (Constant *CRHS = dyn_cast<Constant>(RHS))
+        if (Constant *C = ConstantExpr::getCompare(I.getPredicate(), CLHS, CRHS)) {
+          SimplifiedValues[&I] = C;
+          return true;
+        }
+    }
+
+    return false;
+  }
+
+  /// \brief Check if \p LHS and \p RHS has either the same pointer base, or
+  /// don't have it at all.
+  ///
+  /// If \p LHS and \p RHS are GEP-instructions, compare their base pointers
+  /// saved in GEPPointerBases map. They should be either the same, or be absent
+  /// for both values.
+  /// If both values aren't GEPs, \return true.
+  /// This check is necessary, because for GEPs with a base pointer
+  /// SimplifiedValues contains constant offset, and for GEPs without a base
+  /// pointer we save its plain value (constant). It's incorrect to compare
+  /// simplified values for GEPs of these two different kinds.
+  bool hasSamePointerBase(Value *LHS, Value *RHS) {
+    Value *LHSBase = nullptr, *RHSBase = nullptr;
+    GetElementPtrInst *LHSGEP = dyn_cast<GetElementPtrInst>(LHS);
+    GetElementPtrInst *RHSGEP = dyn_cast<GetElementPtrInst>(RHS);
+    if (LHSGEP)
+      LHSBase = GEPPointerBases.lookup(LHSGEP);
+    if (RHSGEP)
+      RHSBase = GEPPointerBases.lookup(RHSGEP);
+    return LHSBase == RHSBase;
+  }
 };
 } // namespace
 
@@ -499,6 +548,28 @@
           return None;
       }
 
+      TerminatorInst *TI = BB->getTerminator();
+
+      // Add in the live successors by first checking whether we have terminator
+      // that may be simplified based on the values simplified by this call.
+      if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
+        if (BI->isConditional()) {
+          Value *Cond = BI->getCondition();
+          if (ConstantInt *SimpleCond
+              = dyn_cast_or_null<ConstantInt>(SimplifiedValues.lookup(Cond))) {
+            BBWorklist.insert(BI->getSuccessor(SimpleCond->isZero() ? 1 : 0));
+            continue;
+          }
+        }
+      } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
+        Value *Cond = SI->getCondition();
+        if (ConstantInt *SimpleCond
+            = dyn_cast_or_null<ConstantInt>(SimplifiedValues.lookup(Cond))) {
+          BBWorklist.insert(SI->findCaseValue(SimpleCond).getCaseSuccessor());
+          continue;
+        }
+      }
+
       // Add BB's successors to the worklist.
       for (BasicBlock *Succ : successors(BB))
         if (L->contains(Succ))

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