Summary: Add support to deduce a PHI node being a power of 2 if each incoming value is a power of 2.

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)) {
+    unsigned NewDepth = std::max(Depth, MaxAnalysisRecursionDepth - 1);
+    Query RecQ = Q;
+
+    // Recursively check all incoming values
+    return llvm::all_of(PN->operands(), [&](const Use &U) {
+      if (U.get() == PN)
+        return true;
+      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);
   }

llvm/test/Analysis/ValueTracking/known-power-of-two-urem.ll

 ; CHECK-NEXT:    br label [[COND_TRUE_END:%.*]]
 ; CHECK:       cond.true.false:
 ; CHECK-NEXT:    br label [[COND_TRUE_END]]
 ; CHECK:       cond.true.end:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi i64 [ 2, [[COND_TRUE_TRUE]] ], [ 4, [[COND_TRUE_FALSE]] ]
 ; CHECK-NEXT:    br label [[COND_END]]
 ; CHECK:       cond.end:
 ; CHECK-NEXT:    [[PHI1:%.*]] = phi i64 [ 4096, [[ENTRY:%.*]] ], [ [[PHI]], [[COND_TRUE_END]] ]
-; CHECK-NEXT:    [[UREM:%.*]] = urem i64 [[SIZE:%.*]], [[PHI1]]
+; CHECK-NEXT:    [[TMP0:%.*]] = add nsw i64 [[PHI1]], -1
+; CHECK-NEXT:    [[UREM:%.*]] = and i64 [[TMP0]], [[SIZE:%.*]]
 ; CHECK-NEXT:    ret i64 [[UREM]]
 ;
 entry:
   br i1 %cmp, label %cond.true, label %cond.end
 
 cond.true:
   br i1 %cmp1, label %cond.true.true, label %cond.true.false
 

 ; CHECK:       cond.true:
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl nuw i64 1, [[TMP0:%.*]]
 ; CHECK-NEXT:    br label [[COND_END:%.*]]
 ; CHECK:       cond.false:
 ; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[CMP1:%.*]], i64 2, i64 8
 ; CHECK-NEXT:    br label [[COND_END]]
 ; CHECK:       cond.end:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi i64 [ [[SELECT]], [[COND_FALSE]] ], [ [[TMP1]], [[COND_TRUE]] ], [ [[PHI]], [[COND_END]] ]
-; CHECK-NEXT:    [[UREM:%.*]] = urem i64 [[SIZE:%.*]], [[PHI]]
+; CHECK-NEXT:    [[TMP2:%.*]] = add i64 [[PHI]], -1
+; CHECK-NEXT:    [[UREM:%.*]] = and i64 [[TMP2]], [[SIZE:%.*]]
 ; CHECK-NEXT:    [[CMP2:%.*]] = icmp ult i64 [[UREM]], 10
 ; CHECK-NEXT:    br i1 [[CMP2]], label [[COND_END]], label [[END:%.*]]
 ; CHECK:       end:
 ; CHECK-NEXT:    ret i64 [[UREM]]
 ;
 entry:
   br i1 %cmp, label %cond.true, label %cond.false