[llvm] [ValueTracking] Compute known FPClass from dominating condition (PR #80941)

[Matt Arsenault via llvm-commits]

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@@ -4213,9 +4213,56 @@ llvm::fcmpImpliesClass(CmpInst::Predicate Pred, const Function &F, Value *LHS,
   return fcmpImpliesClass(Pred, F, LHS, *ConstRHS, LookThroughSrc);
 }
 
-static FPClassTest computeKnownFPClassFromAssumes(const Value *V,
-                                                  const SimplifyQuery &Q) {
-  FPClassTest KnownFromAssume = fcAllFlags;
+static void computeKnownFPClassFromCond(const Value *V, Value *Cond,
+                                        bool CondIsTrue,
+                                        const Instruction *CxtI,
+                                        KnownFPClass &KnownFromContext) {
+  CmpInst::Predicate Pred;
+  Value *LHS;
+  Value *RHS;
+  uint64_t ClassVal = 0;
+  // TODO: handle sign-bit check idiom
+  if (match(Cond, m_FCmp(Pred, m_Value(LHS), m_Value(RHS)))) {
+    const APFloat *CRHS;
+    if (match(RHS, m_APFloat(CRHS))) {
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arsenm wrote:

Could move this directly into the initial m_FCmp match; however I do have a patch I was working on to generalize fcmpImpliesClass to handle non-constant cases 

https://github.com/llvm/llvm-project/pull/80941