[ValueTracking] Handle signbit idiom in computeKnownFPClassFromCond

Author: dtcxzyw

llvm/lib/Analysis/DomConditionCache.cpp

@@ -66,6 +66,10 @@ static void findAffectedValues(Value *Cond,
         // A > C3 && A < C4.
         if (match(A, m_Add(m_Value(X), m_ConstantInt())))
           AddAffected(X);
+        // Handle icmp slt/sgt (bitcast X to int), 0/-1
+        if (match(A, m_ElementWiseBitCast(m_Value(X))) &&
+            (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SGT))
+          Affected.push_back(X);
       }
     } else if (match(Cond, m_CombineOr(m_FCmp(Pred, m_Value(A), m_Constant()),
                                        m_Intrinsic<Intrinsic::is_fpclass>(

llvm/lib/Analysis/ValueTracking.cpp

@@ -4233,7 +4233,7 @@ static void computeKnownFPClassFromCond(const Value *V, Value *Cond,
   Value *LHS;
   uint64_t ClassVal = 0;
   const APFloat *CRHS;
-  // TODO: handle sign-bit check idiom
+  const APInt *RHS;
   if (match(Cond, m_FCmp(Pred, m_Value(LHS), m_APFloat(CRHS)))) {
     auto [CmpVal, MaskIfTrue, MaskIfFalse] = fcmpImpliesClass(
         Pred, *CxtI->getParent()->getParent(), LHS, *CRHS, LHS != V);
@@ -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;
+    if (TrueIfSigned == CondIsTrue)
+      KnownFromContext.signBitMustBeOne();
+    else
+      KnownFromContext.signBitMustBeZero();
   }
 }
 

llvm/test/Transforms/InstCombine/fpclass-from-dom-cond.ll

@@ -336,8 +336,7 @@ define float @test_signbit_check(float %x, i1 %cond) {
 ; CHECK-NEXT:    br label [[IF_END]]
 ; CHECK:       if.end:
 ; CHECK-NEXT:    [[VALUE:%.*]] = phi float [ [[FNEG]], [[IF_THEN1]] ], [ [[X]], [[IF_THEN2]] ], [ [[X]], [[IF_ELSE]] ]
-; CHECK-NEXT:    [[RET:%.*]] = call float @llvm.fabs.f32(float [[VALUE]])
-; CHECK-NEXT:    ret float [[RET]]
+; CHECK-NEXT:    ret float [[VALUE]]
 ;
   %i32 = bitcast float %x to i32
   %cmp = icmp slt i32 %i32, 0