[PATCH] Fix urem instruction with power of two to compute known bit.

[Andrey Kuharev]

For power of two urem similiar to and instruction

http://reviews.llvm.org/D3823

Files:
  lib/CodeGen/SelectionDAG/SelectionDAG.cpp
  test/CodeGen/Generic/computeKnownBits_urem.ll

Index: lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2192,8 +2192,14 @@
       const APInt &RA = Rem->getAPIntValue();
       if (RA.isPowerOf2()) {
         APInt LowBits = (RA - 1);
+        computeKnownBits(Op.getOperand(0), KnownZero2, KnownOne2, Depth + 1);
+
+        // The low bits of the first operand are unchanged by the urem.
+        KnownZero = KnownZero2 & LowBits;
+        KnownOne = KnownOne2 & LowBits;
+
+        // The upper bits are all zero.
         KnownZero |= ~LowBits;
-        computeKnownBits(Op.getOperand(0), KnownZero, KnownOne,Depth+1);
         break;
       }
     }
Index: test/CodeGen/Generic/computeKnownBits_urem.ll
===================================================================
--- test/CodeGen/Generic/computeKnownBits_urem.ll
+++ test/CodeGen/Generic/computeKnownBits_urem.ll
@@ -0,0 +1,14 @@
+; RUN: llc -debug %s -o /dev/null 2>&1 | FileCheck %s
+define i32 @main() #0 {
+entry:
+  %a = alloca i32, align 4
+  store i32 1, i32* %a, align 4
+  %0 = load i32* %a, align 4
+  %or = or i32 1, %0
+  %and = and i32 1, %or
+  %rem = urem i32 %and, 1
+  %add = add i32 %rem, 1
+  ret i32 %add
+}
+; CHECK: Replacing.2 0x{{[0-9a-f]+}}: i32 = urem 0x{{[0-9a-f]+}}, 0x{{[0-9a-f]+}} [ORD=7]
+; CHECK: With: 0x{{[0-9a-f]+}}: i32 = Constant<0>
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