[SCEV] Opportunistically interpret unsigned constraints as signed

Summary:
An unsigned comparision is equivalent to is corresponding signed version
if both the operands being compared are positive.  Teach SCEV to use
this fact when profitable.

Reviewers: atrick, hfinkel, reames, nlewycky

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13687

llvm-svn: 251051

llvm/include/llvm/IR/InstrTypes.h

@@ -1033,6 +1033,19 @@ public:
     return isUnsigned(getPredicate());
   }
 
+  /// For example, ULT->SLT, ULE->SLE, UGT->SGT, UGE->SGE, SLT->Failed assert
+  /// @returns the signed version of the unsigned predicate pred.
+  /// @brief return the signed version of a predicate
+  static Predicate getSignedPredicate(Predicate pred);
+
+  /// For example, ULT->SLT, ULE->SLE, UGT->SGT, UGE->SGE, SLT->Failed assert
+  /// @returns the signed version of the predicate for this instruction (which
+  /// has to be an unsigned predicate).
+  /// @brief return the signed version of a predicate
+  Predicate getSignedPredicate() {
+    return getSignedPredicate(getPredicate());
+  }
+
   /// This is just a convenience.
   /// @brief Determine if this is true when both operands are the same.
   bool isTrueWhenEqual() const {

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -7509,6 +7509,13 @@ bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
                                    RHS, LHS, FoundLHS, FoundRHS);
   }
 
+  // Unsigned comparison is the same as signed comparison when both the operands
+  // are non-negative.
+  if (CmpInst::isUnsigned(FoundPred) &&
+      CmpInst::getSignedPredicate(FoundPred) == Pred &&
+      isKnownNonNegative(FoundLHS) && isKnownNonNegative(FoundRHS))
+    return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS);
+
   // Check if we can make progress by sharpening ranges.
   if (FoundPred == ICmpInst::ICMP_NE &&
       (isa<SCEVConstant>(FoundLHS) || isa<SCEVConstant>(FoundRHS))) {

llvm/lib/IR/Instructions.cpp

@@ -3558,6 +3558,23 @@ CmpInst::Predicate CmpInst::getSwappedPredicate(Predicate pred) {
   }
 }
 
+CmpInst::Predicate CmpInst::getSignedPredicate(Predicate pred) {
+  assert(CmpInst::isUnsigned(pred) && "Call only with signed predicates!");
+
+  switch (pred) {
+  default:
+    llvm_unreachable("Unknown predicate!");
+  case CmpInst::ICMP_ULT:
+    return CmpInst::ICMP_SLT;
+  case CmpInst::ICMP_ULE:
+    return CmpInst::ICMP_SLE;
+  case CmpInst::ICMP_UGT:
+    return CmpInst::ICMP_SGT;
+  case CmpInst::ICMP_UGE:
+    return CmpInst::ICMP_SGE;
+  }
+}
+
 bool CmpInst::isUnsigned(unsigned short predicate) {
   switch (predicate) {
     default: return false;

llvm/test/Transforms/IndVarSimplify/eliminate-comparison.ll

@@ -535,5 +535,55 @@ define void @func_23(i32* %length.ptr) {
   ret void
 }
 
+define void @func_24(i32* %length.ptr) {
+; CHECK-LABEL: @func_24(
+ entry:
+  %length = load i32, i32* %length.ptr, !range !0
+  %entry.cond = icmp ult i32 4, %length
+  br i1 %entry.cond, label %loop, label %leave
+
+ loop:
+; CHECK: loop:
+  %iv = phi i32 [ 4, %entry ], [ %iv.inc, %be ]
+  %iv.inc = add i32 %iv, 1
+  %range.check = icmp slt i32 %iv, %length
+  br i1 %range.check, label %be, label %leave
+; CHECK:   br i1 true, label %be, label %leave.loopexit
+; CHECK: be:
+
+ be:
+  call void @side_effect()
+  %be.cond = icmp slt i32 %iv.inc, %length
+  br i1 %be.cond, label %loop, label %leave
+
+ leave:
+  ret void
+}
+
+define void @func_25(i32* %init.ptr) {
+; CHECK-LABEL: @func_25(
+ entry:
+  %init = load i32, i32* %init.ptr, !range !0
+  %entry.cond = icmp ugt i32 %init, 4
+  br i1 %entry.cond, label %loop, label %leave
+
+ loop:
+; CHECK: loop:
+  %iv = phi i32 [ %init, %entry ], [ %iv.dec, %be ]
+  %iv.dec = add i32 %iv, -1
+  %range.check = icmp sgt i32 %iv, 4
+  br i1 %range.check, label %be, label %leave
+; CHECK:   br i1 true, label %be, label %leave.loopexit
+; CHECK: be:
+
+ be:
+  call void @side_effect()
+  %be.cond = icmp sgt i32 %iv.dec, 4
+  br i1 %be.cond, label %loop, label %leave
+
+ leave:
+  ret void
+}
+
 
 !0 = !{i32 0, i32 2147483647}