[InstCombine] Fold (A - B) u>=/u< A --> B u>/u<= A iff B != 0

https://rise4fun.com/Alive/KtL

This also shows that the fold added in D67412 / r372257
was too specific, and the new fold allows those test cases
to be handled more generically, therefore i delete now-dead code.

This is yet again motivated by
D67122 "[UBSan][clang][compiler-rt] Applying non-zero offset to nullptr is undefined behaviour"

llvm-svn: 372912

1 files changed, 13 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 5bafab49032..b68266705e5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3638,7 +3638,8 @@ Value *InstCombiner::foldUnsignedMultiplicationOverflowCheck(ICmpInst &I) {
 /// TODO: A large part of this logic is duplicated in InstSimplify's
 /// simplifyICmpWithBinOp(). We should be able to share that and avoid the code
 /// duplication.
-Instruction *InstCombiner::foldICmpBinOp(ICmpInst &I) {
+Instruction *InstCombiner::foldICmpBinOp(ICmpInst &I, const SimplifyQuery &SQ) {
+  const SimplifyQuery Q = SQ.getWithInstruction(&I);
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
 
   // Special logic for binary operators.
@@ -3840,6 +3841,14 @@ Instruction *InstCombiner::foldICmpBinOp(ICmpInst &I) {
   // C u</u>= (C - D) --> C u</u>= D
   if (C == Op0 && (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_UGE))
     return new ICmpInst(Pred, C, D);
+  // (A - B) u>=/u< A --> B u>/u<= A  iff B != 0
+  if (A == Op1 && (Pred == ICmpInst::ICMP_UGE || Pred == ICmpInst::ICMP_ULT) &&
+      isKnownNonZero(B, Q.DL, /*Depth=*/0, Q.AC, Q.CxtI, Q.DT))
+    return new ICmpInst(CmpInst::getFlippedStrictnessPredicate(Pred), B, A);
+  // C u<=/u> (C - D) --> C u</u>= D  iff B != 0
+  if (C == Op0 && (Pred == ICmpInst::ICMP_ULE || Pred == ICmpInst::ICMP_UGT) &&
+      isKnownNonZero(D, Q.DL, /*Depth=*/0, Q.AC, Q.CxtI, Q.DT))
+    return new ICmpInst(CmpInst::getFlippedStrictnessPredicate(Pred), C, D);
 
   // icmp (A-B), (C-B) -> icmp A, C for equalities or if there is no overflow.
   if (B && D && B == D && NoOp0WrapProblem && NoOp1WrapProblem)
@@ -5346,6 +5355,7 @@ static Instruction *foldVectorCmp(CmpInst &Cmp,
 
 Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
   bool Changed = false;
+  const SimplifyQuery Q = SQ.getWithInstruction(&I);
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   unsigned Op0Cplxity = getComplexity(Op0);
   unsigned Op1Cplxity = getComplexity(Op1);
@@ -5360,8 +5370,7 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
     Changed = true;
   }
 
-  if (Value *V = SimplifyICmpInst(I.getPredicate(), Op0, Op1,
-                                  SQ.getWithInstruction(&I)))
+  if (Value *V = SimplifyICmpInst(I.getPredicate(), Op0, Op1, Q))
     return replaceInstUsesWith(I, V);
 
   // Comparing -val or val with non-zero is the same as just comparing val
@@ -5394,7 +5403,7 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
   if (Instruction *Res = foldICmpWithDominatingICmp(I))
     return Res;
 
-  if (Instruction *Res = foldICmpBinOp(I))
+  if (Instruction *Res = foldICmpBinOp(I, Q))
     return Res;
 
   if (Instruction *Res = foldICmpUsingKnownBits(I))