[llvm] Optimize fptrunc(x)>=C1 --> x>=C2 (PR #99475)

[Matt Arsenault via llvm-commits]

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@@ -7882,6 +7892,79 @@ static Instruction *foldFCmpReciprocalAndZero(FCmpInst &I, Instruction *LHSI,
   return new FCmpInst(Pred, LHSI->getOperand(1), RHSC, "", &I);
 }
 
+// Fold trunc(x) < constant --> x < constant if possible.
+static Instruction *foldFCmpFpTrunc(FCmpInst &I, Instruction *LHSI,
+                                    Constant *RHSC) {
+  FCmpInst::Predicate Pred = I.getPredicate();
+  bool RoundDown = false;
+
+  if ((Pred == FCmpInst::FCMP_OGE) || (Pred == FCmpInst::FCMP_UGE) ||
+      (Pred == FCmpInst::FCMP_OLT) || (Pred == FCmpInst::FCMP_ULT))
+    RoundDown = true;
+  else if ((Pred == FCmpInst::FCMP_OGT) || (Pred == FCmpInst::FCMP_UGT) ||
+           (Pred == FCmpInst::FCMP_OLE) || (Pred == FCmpInst::FCMP_ULE))
+    RoundDown = false;
+  else
+    return nullptr;
+
+  const APFloat *RValue;
+  if (!match(RHSC, m_APFloat(RValue)))
+    return nullptr;
+
+  Type *LType = LHSI->getOperand(0)->getType();
+  Type *RType = RHSC->getType();
+  Type *LEleType = LType->getScalarType();
+  Type *REleType = RType->getScalarType();
+
+  APFloat NextRValue = *RValue;
+  NextRValue.next(RoundDown);
+
+  // Round RValue to suitable value
+  APFloat ExtRValue = *RValue;
+  APFloat ExtNextRValue = NextRValue;
+  bool lossInfo;
+  ExtRValue.convert(LEleType->getFltSemantics(), APFloat::rmNearestTiesToEven,
+                    &lossInfo);
+  ExtNextRValue.convert(LEleType->getFltSemantics(),
+                        APFloat::rmNearestTiesToEven, &lossInfo);
+
+  APFloat RoundValue{LEleType->getFltSemantics()};
+  {
+    APFloat Two{LEleType->getFltSemantics(), 2};
+    APFloat LowBound = RoundDown ? ExtNextRValue : ExtRValue;
+    APFloat UpBound = RoundDown ? ExtRValue : ExtNextRValue;
+
+    while (true) {
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arsenm wrote:

This loop is the hard to review part and needs some comments explaining what constants are legal 

https://github.com/llvm/llvm-project/pull/99475