[PATCH] [InstCombine] Teach how to fold a select into a cttz/ctlz with the 'is_zero_undef' flag cleared.

[David Majnemer]

================
Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:465-468
@@ +464,6 @@
+  // Check that a constant is propagated when CmpLHS is zero.
+  if (Pred == ICmpInst::ICMP_EQ && match(TrueVal, m_ConstantInt(CI)))
+    Count = FalseVal;
+  else if (Pred == ICmpInst::ICMP_NE && match(FalseVal, m_ConstantInt(CI)))
+    Count = TrueVal;
+  else
----------------
`@llvm.cttz.*` and `@llvm.ctlz.*` both accept vector types as arguments.  You may want to switch from `m_ConstantInt` to `m_APInt` because it will match against a splatted `ConstantVector` as well.

You could go even further and use `m_SpecificInt(II->getType()->getScalarSizeInBits())` to save you a check later on.

================
Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:478-483
@@ +477,8 @@
+
+  if (match(Count, m_Intrinsic<Intrinsic::cttz>(m_Value(V))) ||
+      match(Count, m_Intrinsic<Intrinsic::ctlz>(m_Value(V)))) {
+    // The value in input to the cttz/ctlz must be the same value in input to
+    // the compare instruction.
+    if (V != CmpLHS)
+      return nullptr;
+
----------------
You could use `m_Specific(CmpLHS)` instead of `m_Value(V)` and `V != CmpLHS`.

================
Comment at: lib/Transforms/InstCombine/InstCombineSelect.cpp:492-498
@@ +491,9 @@
+        // Explicitly set the 'undef_on_zero' flag to zero.
+        Type *Ty = II->getArgOperand(0)->getType();
+        Value *Args[] = { II->getArgOperand(0),
+                          ConstantInt::getFalse(II->getContext()) };
+        Module *M = II->getParent()->getParent()->getParent();
+        Value *IF = Intrinsic::getDeclaration(M, II->getIntrinsicID(), Ty);
+        CallInst *NewI = Builder.CreateCall(IF, Args);
+        NewI->setTailCallKind(II->getTailCallKind());
+        Count = NewI;
----------------
I wonder if it might be nicer to clone `II` and then fixup the clone.
Something like:
  NewI = II->clone();
  NewI->setOperand(1, Constant::getNullValue(II->getArgOperand(1)->getType()));

This formulation also has the advantage of working with vector types.

http://reviews.llvm.org/D6891

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/