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

[mkazantsev]

Summary

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 it,
but only flips unsigned into signed for further inference. This patch adds support
for flipping a signed predicate into unsigned.

[nikic]

LGTM

llvm/lib/Analysis/ScalarEvolution.cpp

10711

You should be able to simply this to CmpInst::isRelational(P2) && P1 == CmpInst::getFlippedSignednessPredicate(P2).

[mkazantsev]

llvm/lib/Analysis/ScalarEvolution.cpp

10711

That's a good point, thanks!

llvm/lib/Analysis/ScalarEvolution.cpp

if (!FoundLHS->getType()->isPointerTy() &&
                               getNotSCEV(FoundRHS), CtxI))
       return true;
 
     return false;
   }
 
   // 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);
 
   // Check if we can make progress by sharpening ranges.
   if (FoundPred == ICmpInst::ICMP_NE &&
       (isa<SCEVConstant>(FoundLHS) || isa<SCEVConstant>(FoundRHS))) {
 

llvm/test/Transforms/IndVarSimplify/negative_ranges.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -indvars -S < %s | FileCheck %s
 ; RUN: opt -passes=indvars -S < %s | FileCheck %s
 
 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:
 ; CHECK-NEXT:    [[START:%.*]] = load i32, i32* [[P:%.*]], align 4, !range [[RNG0:![0-9]+]]
 ; CHECK-NEXT:    [[END:%.*]] = load i32, i32* [[S:%.*]], align 4, !range [[RNG0]]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[START]], [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
 ; 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()
 ; CHECK-NEXT:    br i1 [[LOOP_COND]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret i32 1
 ; CHECK:       side_exit:
 ; CHECK-NEXT:    ret i32 0