[llvm] [InstCombine] Fold copysign of selects from sign comparison to sign operand (PR #85627)

[Matt Arsenault via llvm-commits]

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@@ -2479,6 +2479,47 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
     if (match(Mag, m_FAbs(m_Value(X))) || match(Mag, m_FNeg(m_Value(X))))
       return replaceOperand(*II, 0, X);
 
+    Value *A, *B;
+    CmpInst::Predicate Pred;
+    const APFloat *TC, *FC;
+    if (!match(Sign, m_Select((m_And(m_Value(B),
+                                     m_FCmp(Pred, m_Value(A), m_PosZeroFP()))),
+                              m_APFloat(TC), m_APFloat(FC))))
+      return nullptr;
+    // Match select ?, TC, FC where the constants are equal but negated.
+    // Check for these 8 conditions
+    /*
+    copysign(Mag, B & (A < 0.0) ? -TC : TC) --> copysign(Mag, A) B->true, A<0.
+    copysign(Mag, B & (A < 0.0) ? -TC : TC) --> copysign(Mag, A) B->true, A>0.
+    copysign(Mag, B & (A < 0.0) ? -TC : TC) --> copysign(Mag, A) B->false, A>0.
+    copysign(Mag, B & (A < 0.0) ? -TC : TC) --> copysign(Mag, -A) B->false, A<0.
+    */
+    /*
+    copysign(Mag, B & (A > 0.0) ? -TC : TC) --> copysign(Mag, -A) B->true, A<0.
+    copysign(Mag, B & (A > 0.0) ? -TC : TC) --> copysign(Mag, -A) B->true, A>0.
+    copysign(Mag, B & (A > 0.0) ? -TC : TC) --> copysign(Mag, A) B->false, A>0.
+    copysign(Mag, B & (A > 0.0) ? -TC : TC) --> copysign(Mag, -A) B->false, A<0.
+    */
+    assert(TC != FC);
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arsenm wrote:

I don't think this is guaranteed to hold here 

https://github.com/llvm/llvm-project/pull/85627