[llvm] [ValueTracking] Compute known FPClass from signbit idiom (PR #80740)

[Matt Arsenault via llvm-commits]

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@@ -4243,6 +4243,19 @@ static void computeKnownFPClassFromCond(const Value *V, Value *Cond,
                              m_Value(LHS), m_ConstantInt(ClassVal)))) {
     FPClassTest Mask = static_cast<FPClassTest>(ClassVal);
     KnownFromContext.knownNot(CondIsTrue ? ~Mask : Mask);
+  } else if (match(Cond, m_ICmp(Pred, m_ElementWiseBitCast(m_Value(LHS)),
+                                m_APInt(RHS)))) {
+    bool TrueIfSigned;
+    if (Pred == ICmpInst::ICMP_SLT && RHS->isZero())
+      TrueIfSigned = true;
+    else if (Pred == ICmpInst::ICMP_SGT && RHS->isAllOnes())
+      TrueIfSigned = false;
+    else
+      return;
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arsenm wrote:

I vaguely recall seeing a helper for this somewhere but I can't seem to find it 

https://github.com/llvm/llvm-project/pull/80740