[SCEV] Generalize implication when signedness of FoundPred doesn't ma…

…tter

The implication logic for two values that are both negative or non-negative
says that it doesn't matter whether their predicate is signed and unsigned,
but only flips unsigned into signed for further inference. This patch adds
support for flipping a signed predicate into unsigned as well.

Differential Revision: https://reviews.llvm.org/D109959
Reviewed By: nikic

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -10704,8 +10704,13 @@ bool ScalarEvolution::isImpliedCondBalancedTypes(
 
   // Unsigned comparison is the same as signed comparison when both the operands
   // are non-negative or negative.
-  if (CmpInst::isUnsigned(FoundPred) &&
-      CmpInst::getSignedPredicate(FoundPred) == Pred &&
+  auto IsSignFlippedPredicate = [](CmpInst::Predicate P1,
+                                   CmpInst::Predicate P2) {
+    assert(P1 != P2 && "Handled earlier!");
+    return CmpInst::isRelational(P2) &&
+           P1 == CmpInst::getFlippedSignednessPredicate(P2);
+  };
+  if (IsSignFlippedPredicate(Pred, FoundPred) &&
       ((isKnownNonNegative(FoundLHS) && isKnownNonNegative(FoundRHS)) ||
        (isKnownNegative(FoundLHS) && isKnownNegative(FoundRHS))))
     return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS, CtxI);

llvm/test/Transforms/IndVarSimplify/negative_ranges.ll

@@ -4,7 +4,6 @@
 
 declare i1 @cond()
 
-; FIXME: 2nd check is implied by the 1st one as both values are negative.
 define i32 @test_01(i32* %p, i32* %s) {
 ; CHECK-LABEL: @test_01(
 ; CHECK-NEXT:  entry:
@@ -16,8 +15,7 @@ define i32 @test_01(i32* %p, i32* %s) {
 ; CHECK-NEXT:    [[C1:%.*]] = icmp slt i32 [[IV]], [[END]]
 ; CHECK-NEXT:    br i1 [[C1]], label [[GUARDED:%.*]], label [[SIDE_EXIT:%.*]]
 ; CHECK:       guarded:
-; CHECK-NEXT:    [[C2:%.*]] = icmp ult i32 [[IV]], [[END]]
-; CHECK-NEXT:    br i1 [[C2]], label [[BACKEDGE]], label [[SIDE_EXIT]]
+; CHECK-NEXT:    br i1 true, label [[BACKEDGE]], label [[SIDE_EXIT]]
 ; CHECK:       backedge:
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
 ; CHECK-NEXT:    [[LOOP_COND:%.*]] = call i1 @cond()