[SelectionDAG] Use KnownBits::computeForAddSub/computeForAddCarry

Summary:
Use KnownBits::computeForAddSub/computeForAddCarry
in SelectionDAG::computeKnownBits when doing value
tracking for addition/subtraction.

This should improve the precision of the known bits,
as we only used to make a simple estimate of known
zeroes. The KnownBits support functions are also
able to deduce bits that are known to be one in the
result.

Reviewers: spatel, RKSimon, nikic, lebedev.ri

Reviewed By: nikic

Subscribers: nikic, javed.absar, lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60460

llvm-svn: 358372

1 files changed, 42 insertions, 0 deletions

diff --git a/llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp b/llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp
index ae4c76ecf15..3ac32aebeac 100644
--- a/llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp
+++ b/llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp
@@ -157,4 +157,46 @@ TEST_F(AArch64SelectionDAGTest, SimplifyDemandedVectorElts_EXTRACT_SUBVECTOR) {
             false);
 }
 
+// Piggy-backing on the AArch64 tests to verify SelectionDAG::computeKnownBits.
+TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_ADD) {
+  if (!TM)
+    return;
+  SDLoc Loc;
+  auto IntVT = EVT::getIntegerVT(Context, 8);
+  auto UnknownOp = DAG->getRegister(0, IntVT);
+  auto Mask = DAG->getConstant(0x8A, Loc, IntVT);
+  auto N0 = DAG->getNode(ISD::AND, Loc, IntVT, Mask, UnknownOp);
+  auto N1 = DAG->getConstant(0x55, Loc, IntVT);
+  auto Op = DAG->getNode(ISD::ADD, Loc, IntVT, N0, N1);
+  // N0 = ?000?0?0
+  // N1 = 01010101
+  //  =>
+  // Known.One  = 01010101 (0x55)
+  // Known.Zero = 00100000 (0x20)
+  KnownBits Known = DAG->computeKnownBits(Op);
+  EXPECT_EQ(Known.Zero, APInt(8, 0x20));
+  EXPECT_EQ(Known.One, APInt(8, 0x55));
+}
+
+// Piggy-backing on the AArch64 tests to verify SelectionDAG::computeKnownBits.
+TEST_F(AArch64SelectionDAGTest, ComputeKnownBits_SUB) {
+  if (!TM)
+    return;
+  SDLoc Loc;
+  auto IntVT = EVT::getIntegerVT(Context, 8);
+  auto N0 = DAG->getConstant(0x55, Loc, IntVT);
+  auto UnknownOp = DAG->getRegister(0, IntVT);
+  auto Mask = DAG->getConstant(0x2e, Loc, IntVT);
+  auto N1 = DAG->getNode(ISD::AND, Loc, IntVT, Mask, UnknownOp);
+  auto Op = DAG->getNode(ISD::SUB, Loc, IntVT, N0, N1);
+  // N0 = 01010101
+  // N1 = 00?0???0
+  //  =>
+  // Known.One  = 00000001 (0x1)
+  // Known.Zero = 10000000 (0x80)
+  KnownBits Known = DAG->computeKnownBits(Op);
+  EXPECT_EQ(Known.Zero, APInt(8, 0x80));
+  EXPECT_EQ(Known.One, APInt(8, 0x1));
+}
+
 } // end anonymous namespace