[ValueTracking] Added support to deduce PHI Nodes values being a power of 2

[huangjd]

Summary

Add Value Tracking support to deduce induction variable being a power of 2, allowing urem optimizations

[huangjd]

Added back the revision previously in D124890

[spatel]

The test changes resulting from only this code change should be included in this patch.

[nikic]

I think you accidentally uploaded a partial diff (that just addresses review feedback, without the base patch).

[huangjd]

Added updated unit tests

[spatel]

LGTM - do you have commit privileges?

Note: it might be worth comparing this code to the SelectionDAG equivalent; that ends by calling computeKnownBits to catch all of the non-special cases while this code just gives up (returns false).

[thakis]

Looks like this breaks tests: http://45.33.8.238/linux/76547/step_12.txt

[thakis]

reverted in 304a5a7a14dd26ae93194b336fef9a1213fa44b5 for now

[huangjd]

I think arc is confused about the git commits to include and the unit test changes are missing again. I will make a clean diff

llvm/lib/Analysis/ValueTracking.cpp

if (OrZero || Q.IIQ.hasNoUnsignedWrap(VOBO) ||
       if ((~(LHSBits.Zero & RHSBits.Zero)).isPowerOf2())
         // If OrZero isn't set, we cannot give back a zero result.
         // Make sure either the LHS or RHS has a bit set.
         if (OrZero || RHSBits.One.getBoolValue() || LHSBits.One.getBoolValue())
           return true;
     }
   }
 
+  // A PHI node is power of two if all incoming values are power of two.
+  if (const PHINode *PN = dyn_cast<PHINode>(V)) {
+    Query RecQ = Q;
+
+    // Recursively check all incoming values. Limit recursion to 2 levels, so
+    // that search complexity is limited to number of operands^2.
+    unsigned NewDepth = std::max(Depth, MaxAnalysisRecursionDepth - 1);
+    return llvm::all_of(PN->operands(), [&](const Use &U) {
+      // Value is power of 2 if it is coming from PHI node itself by induction.
+      if (U.get() == PN)
+        return true;
+
+      // Change the context instruction to the incoming block where it is
+      // evaluated.
+      RecQ.CxtI = PN->getIncomingBlock(U)->getTerminator();
+      return isKnownToBeAPowerOfTwo(U.get(), OrZero, NewDepth, RecQ);
+    });
+  }
+
   // An exact divide or right shift can only shift off zero bits, so the result
   // is a power of two only if the first operand is a power of two and not
   // copying a sign bit (sdiv int_min, 2).
   if (match(V, m_Exact(m_LShr(m_Value(), m_Value()))) ||
       match(V, m_Exact(m_UDiv(m_Value(), m_Value())))) {
     return isKnownToBeAPowerOfTwo(cast<Operator>(V)->getOperand(0), OrZero,
                                   Depth, Q);
   }