[InstCombine] cttz(zext(x)) -> zext(cttz(x)) if the 'ZeroIsUndef' parameter is 'true' (PR50172)

[xbolva00]

Summary

Zext doesn't change the number of trailing zeros, so narrow cttz(zext(x)) -> zext(cttz(x)) if the 'ZeroIsUndef' parameter is 'true'.

Proofs:
https://alive2.llvm.org/ce/z/o2dnjY

Solves https://bugs.llvm.org/show_bug.cgi?id=50172

[xbolva00]

Added tests

llvm/test/Transforms/InstCombine/cttz.ll

8 ↗

(On Diff #341680)

Enough tests?

[xbolva00]

TODO: cttz(sext(x)) -> zext(cttz(x))

[xbolva00]

Handle also cttz(sext(x)) -> zext(cttz(x)) if the 'ZeroIsUndef' parameter is 'true'

[spatel]

Feel free to add tests as preliminary steps - no pre-commit review needed for that.

It would be more flexible to make a "sext -> zext" transform as an independent fold - it doesn't require that zero is undef?
https://alive2.llvm.org/ce/z/kZ7BR6

[RKSimon]

@xbolva00 Please don't forget to generate the diffs with context

[xbolva00]

Thanks for feedback, sext case will be in second patch.

[xbolva00]

Precommited tests. Rebased.

[xbolva00]

X86 backend patch landed https://reviews.llvm.org/D101662 to avoid regressions.

[xbolva00]

Fixed typo in test name.

[spatel]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

 }
 
 static Instruction *foldCttzCtlz(IntrinsicInst &II, InstCombinerImpl &IC) {
   assert((II.getIntrinsicID() == Intrinsic::cttz ||
           II.getIntrinsicID() == Intrinsic::ctlz) &&
          "Expected cttz or ctlz intrinsic");
   bool IsTZ = II.getIntrinsicID() == Intrinsic::cttz;
   Value *Op0 = II.getArgOperand(0);
+  Value *Op1 = II.getArgOperand(1);
   Value *X;
   // ctlz(bitreverse(x)) -> cttz(x)
   // cttz(bitreverse(x)) -> ctlz(x)
   if (match(Op0, m_BitReverse(m_Value(X)))) {
     Intrinsic::ID ID = IsTZ ? Intrinsic::ctlz : Intrinsic::cttz;
     Function *F = Intrinsic::getDeclaration(II.getModule(), ID, II.getType());
     return CallInst::Create(F, {X, II.getArgOperand(1)});
   }
 
   if (IsTZ) {
     // cttz(-x) -> cttz(x)
     if (match(Op0, m_Neg(m_Value(X))))
       return IC.replaceOperand(II, 0, X);
 
+    // Zext doesn't change the number of trailing zeros, so narrow:
+    // cttz(zext(x)) -> zext(cttz(x)) if the 'ZeroIsUndef' parameter is 'true'.
+    if (match(Op0, m_OneUse(m_ZExt(m_Value(X)))) && match(Op1, m_One())) {
+      auto *Cttz = IC.Builder.CreateBinaryIntrinsic(Intrinsic::cttz, X,
+                                                     IC.Builder.getTrue());
+      auto *ZextCttz = IC.Builder.CreateZExt(Cttz, II.getType());
+      return IC.replaceInstUsesWith(II, ZextCttz);
+    }
+
     // cttz(abs(x)) -> cttz(x)
     // cttz(nabs(x)) -> cttz(x)
     Value *Y;
     SelectPatternFlavor SPF = matchSelectPattern(Op0, X, Y).Flavor;
     if (SPF == SPF_ABS || SPF == SPF_NABS)
       return IC.replaceOperand(II, 0, X);
 
     if (match(Op0, m_Intrinsic<Intrinsic::abs>(m_Value(X))))