[llvm] [InstCombineCompare] Use known bits to insert assume intrinsics. (PR #96017)

[Mikhail Gudim via llvm-commits]

================
@@ -6333,6 +6337,125 @@ bool InstCombinerImpl::replacedSelectWithOperand(SelectInst *SI,
   return false;
 }
 
+static void computeClosestIntsSatisfyingKnownBits(
+    APInt Target, KnownBits &Known, unsigned BitWidth, bool IsSigned,
+    APInt &ClosestSmaller, APInt &ClosestBigger) {
+  int KnownZeroMaskLength = BitWidth - Known.Zero.countLeadingZeros();
+  if (KnownZeroMaskLength == 0)
+    return;
+
+  APInt PowOf2(BitWidth, 1 << KnownZeroMaskLength);
+  if (!IsSigned || Target.isNonNegative()) {
+    ClosestSmaller =
+        PowOf2 * APIntOps::RoundingUDiv(Target, PowOf2, APInt::Rounding::DOWN);
+    ClosestBigger =
+        PowOf2 * APIntOps::RoundingUDiv(Target, PowOf2, APInt::Rounding::UP);
+  } else {
+    ClosestSmaller =
+        PowOf2 * APIntOps::RoundingSDiv(Target, PowOf2, APInt::Rounding::UP);
+    ClosestBigger =
+        PowOf2 * APIntOps::RoundingSDiv(Target, PowOf2, APInt::Rounding::DOWN);
+  }
+}
+
+static void insertAssumeICmp(BasicBlock *BB, ICmpInst::Predicate Pred,
+                             Value *LHS, Value *RHS, LLVMContext &Ctx) {
+  IRBuilder<> Builder(Ctx);
+  Builder.SetInsertPoint(BB, BB->getFirstInsertionPt());
+  auto *Cmp = Builder.CreateICmp(Pred, LHS, RHS);
+  Builder.CreateAssumption(Cmp);
+  return;
+}
+
+static void tryToInsertAssumeBasedOnICmpAndKnownBits(ICmpInst &I,
+                                                     KnownBits Op0Known,
+                                                     KnownBits Op1Known,
+                                                     unsigned BitWidth) {
+  if (!BitWidth)
+    return;
+  if (!(Op1Known.isConstant() && Op0Known.Zero.isMask()))
+    return;
+
+  SmallVector<BasicBlock *> TBBs;
+  SmallVector<BasicBlock *> FBBs;
+  for (Use &U : I.uses()) {
+    Instruction *UI = cast<Instruction>(U.getUser());
+    if (BranchInst *BrUse = dyn_cast<BranchInst>(UI)) {
+      if (BrUse->isUnconditional())
+        continue;
+      TBBs.push_back(BrUse->getSuccessor(0));
+      FBBs.push_back(BrUse->getSuccessor(1));
+    }
+  }
----------------
mgudim wrote:

We insert assume at both true and false destinations of all branches which use the compare.

https://github.com/llvm/llvm-project/pull/96017