[GlobalISel][KnownBits] Implement G_CTPOP

Implementation copied almost verbatim from ValueTracking.

Differential revision: https://reviews.llvm.org/D107606

Author: jroelofs

llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

@@ -9,7 +9,7 @@
 /// Provides analysis for querying information about KnownBits during GISel
 /// passes.
 //
-//===------------------
+//===----------------------------------------------------------------------===//
 #include "llvm/CodeGen/GlobalISel/GISelKnownBits.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/CodeGen/GlobalISel/Utils.h"
@@ -510,6 +510,18 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
     Known = Known.reverseBits();
     break;
   }
+  case TargetOpcode::G_CTPOP: {
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
+                         Depth + 1);
+    // We can bound the space the count needs.  Also, bits known to be zero can't
+    // contribute to the population.
+    unsigned BitsPossiblySet = Known2.countMaxPopulation();
+    unsigned LowBits = Log2_32(BitsPossiblySet)+1;
+    Known.Zero.setBitsFrom(LowBits);
+    // TODO: we could bound Known.One using the lower bound on the number of
+    // bits which might be set provided by popcnt KnownOne2.
+    break;
+  }
   case TargetOpcode::G_UBFX: {
     KnownBits SrcOpKnown, OffsetKnown, WidthKnown;
     computeKnownBitsImpl(MI.getOperand(1).getReg(), SrcOpKnown, DemandedElts,

llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp

@@ -1672,6 +1672,63 @@ TEST_F(AArch64GISelMITest, TestKnownBitsAssertZext) {
   EXPECT_EQ(0xFFFFFFFFFFFFFFF8u, Res.Zero.getZExtValue());
 }
 
+TEST_F(AArch64GISelMITest, TestKnownBitsCTPOP) {
+  StringRef MIRString = R"(
+   %src:_(s32) = COPY $w0
+   %unknown:_(s32) = G_CTPOP %src
+   %constant_4294967295:_(s32) = G_CONSTANT i32 4294967295
+   %thirtytwo:_(s32) = G_CTPOP %constant_4294967295
+   %thirtytwo_copy:_(s32) = COPY %thirtytwo
+   %constant_15:_(s32) = G_CONSTANT i32 15
+   %four:_(s32) = G_CTPOP %constant_15
+   %four_copy:_(s32) = COPY %four
+   %constant_1:_(s32) = G_CONSTANT i32 1
+   %one:_(s32) = G_CTPOP %constant_1
+   %one_copy:_(s32) = COPY %one
+)";
+  setUp(MIRString);
+  if (!TM)
+    return;
+
+  Register UnknownCopy = Copies[Copies.size() - 4];
+  Register ThirtytwoCopy = Copies[Copies.size() - 3];
+  Register FourCopy = Copies[Copies.size() - 2];
+  Register OneCopy = Copies[Copies.size() - 1];
+
+  GISelKnownBits Info(*MF);
+  MachineInstr *Copy;
+  Register SrcReg;
+  KnownBits Res;
+
+  Copy = MRI->getVRegDef(UnknownCopy);
+  SrcReg = Copy->getOperand(1).getReg();
+  Res = Info.getKnownBits(SrcReg);
+  EXPECT_EQ(1u, Res.getBitWidth());
+  EXPECT_EQ(0u, Res.One.getZExtValue());
+  EXPECT_EQ(0u, Res.Zero.getZExtValue());
+
+  Copy = MRI->getVRegDef(ThirtytwoCopy);
+  SrcReg = Copy->getOperand(1).getReg();
+  Res = Info.getKnownBits(SrcReg);
+  EXPECT_EQ(32u, Res.getBitWidth());
+  EXPECT_EQ(0u, Res.One.getZExtValue());
+  EXPECT_EQ(0xFFFFFFC0u, Res.Zero.getZExtValue());
+
+  Copy = MRI->getVRegDef(FourCopy);
+  SrcReg = Copy->getOperand(1).getReg();
+  Res = Info.getKnownBits(SrcReg);
+  EXPECT_EQ(32u, Res.getBitWidth());
+  EXPECT_EQ(0u, Res.One.getZExtValue());
+  EXPECT_EQ(0xFFFFFFF8u, Res.Zero.getZExtValue());
+
+  Copy = MRI->getVRegDef(OneCopy);
+  SrcReg = Copy->getOperand(1).getReg();
+  Res = Info.getKnownBits(SrcReg);
+  EXPECT_EQ(32u, Res.getBitWidth());
+  EXPECT_EQ(0u, Res.One.getZExtValue());
+  EXPECT_EQ(0xFFFFFFFEu, Res.Zero.getZExtValue());
+}
+
 TEST_F(AMDGPUGISelMITest, TestKnownBitsUBFX) {
   StringRef MIRString = "  %3:_(s32) = G_IMPLICIT_DEF\n"
                         "  %4:_(s32) = G_CONSTANT i32 12\n"