[llvm] [IVDescriptors] Identify min/max recurrences in single pass. (PR #163460)

[Ramkumar Ramachandra via llvm-commits]

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@@ -214,6 +214,174 @@ static bool checkOrderedReduction(RecurKind Kind, Instruction *ExactFPMathInst,
   return true;
 }
 
+// Helper to collect FMF from a value and its associated fcmp in select patterns
+static FastMathFlags collectMinMaxFMF(Value *V) {
+  FastMathFlags FMF = cast<FPMathOperator>(V)->getFastMathFlags();
+  if (auto *Sel = dyn_cast<SelectInst>(V)) {
+    // Accept FMF on either fcmp or select of a min/max idiom.
+    // TODO: This is a hack to work-around the fact that FMF may not be
+    //       assigned/propagated correctly. If that problem is fixed or we
+    //       standardize on fmin/fmax via intrinsics, this can be removed.
+    if (auto *FCmp = dyn_cast<FCmpInst>(Sel->getCondition()))
+      FMF |= FCmp->getFastMathFlags();
+  }
+  return FMF;
+}
+
+static std::optional<FastMathFlags>
+hasRequiredFastMathFlags(FPMathOperator *FPOp, RecurKind &RK,
+                         FastMathFlags FuncFMF) {
+  bool HasRequiredFMF =
+      (FuncFMF.noNaNs() && FuncFMF.noSignedZeros()) ||
+      (FPOp && FPOp->hasNoNaNs() && FPOp->hasNoSignedZeros()) ||
+      RK == RecurKind::FMinimum || RK == RecurKind::FMaximum ||
+      RK == RecurKind::FMinimumNum || RK == RecurKind::FMaximumNum;
+  if (!HasRequiredFMF) {
+    if (RK == RecurKind::FMax &&
+        match(FPOp, m_Intrinsic<Intrinsic::maxnum>(m_Value(), m_Value())))
+      RK = RecurKind::FMaxNum;
+    else if (RK == RecurKind::FMin &&
+             match(FPOp, m_Intrinsic<Intrinsic::minnum>(m_Value(), m_Value())))
+      RK = RecurKind::FMinNum;
+    else
+      return std::nullopt;
+  }
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artagnon wrote:

Could write this as a switch over RecurKind?

https://github.com/llvm/llvm-project/pull/163460