[globalisel] Support trivial COPY in GISelKnownBits

[dsanders]

Summary

Allow GISelKnownBits to look through the trivial case of TargetOpcode::COPY

llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

if (!DemandedElts)
     return; // No demanded elts, better to assume we don't know anything.
 
   KnownBits Known2;
 
   switch (Opcode) {
   default:
     TL.computeKnownBitsForTargetInstr(R, Known, DemandedElts, MRI, Depth);
     break;
+  case TargetOpcode::COPY: {
+    MachineOperand Dst = MI.getOperand(0);
+    MachineOperand Src = MI.getOperand(1);
+    // Look through trivial copies.
+    // We can't use NoSubRegister by name as it's defined by each target but
+    // it's always defined to be 0 by tablegen.
+    if (Dst.getSubReg() == 0 /*NoSubRegister*/ && Src.getReg().isVirtual() &&
+        Src.getSubReg() == 0 /*NoSubRegister*/) {
+      // Don't increment Depth for this one since we didn't do any work.
+      computeKnownBitsImpl(Src.getReg(), Known, DemandedElts, Depth);
+    }
+    break;
+  }
   case TargetOpcode::G_CONSTANT: {
     auto CstVal = getConstantVRegVal(R, MRI);
     Known.One = *CstVal;
     Known.Zero = ~Known.One;
     break;
   }
   case TargetOpcode::G_FRAME_INDEX: {
     computeKnownBitsForFrameIndex(R, Known, DemandedElts);

llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp

TEST_F(GISelMITest, TestKnownBitsCst) {
   StringRef MIRString = "  %3:_(s8) = G_CONSTANT i8 1\n"
                         "  %4:_(s8) = COPY %3\n";
   setUp(MIRString);
   if (!TM)
     return;
   unsigned CopyReg = Copies[Copies.size() - 1];
   MachineInstr *FinalCopy = MRI->getVRegDef(CopyReg);
   unsigned SrcReg = FinalCopy->getOperand(1).getReg();
+  unsigned DstReg = FinalCopy->getOperand(1).getReg();
   GISelKnownBits Info(*MF);
   KnownBits Res = Info.getKnownBits(SrcReg);
   EXPECT_EQ((uint64_t)1, Res.One.getZExtValue());
   EXPECT_EQ((uint64_t)0xfe, Res.Zero.getZExtValue());
+
+  KnownBits Res2 = Info.getKnownBits(DstReg);
+  EXPECT_EQ(Res.One.getZExtValue(), Res2.One.getZExtValue());
+  EXPECT_EQ(Res.Zero.getZExtValue(), Res2.Zero.getZExtValue());
 }
+
 TEST_F(GISelMITest, TestKnownBitsPtrToIntViceVersa) {
   StringRef MIRString = "  %3:_(s16) = G_CONSTANT i16 256\n"
                         "  %4:_(p0) = G_INTTOPTR %3\n"
                         "  %5:_(s32) = G_PTRTOINT %4\n"
                         "  %6:_(s32) = COPY %5\n";
   setUp(MIRString);
   if (!TM)
     return;