[PATCH] D85375: [GlobalISel] Fix computing known bits for loads with range metadata

[Jessica Paquette via Phabricator via llvm-commits]

paquette created this revision.
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In GlobalISel, if you have a load into a small type with a range, you'll hit an assert if you try to compute known bits on it starting at a larger type.

e.g.

  %x:_(s8) = G_LOAD %whatever(p0) :: (load 1 ... !range !n)
  ...
  %y:_(s32) = G_SOMETHING %x

When we walk through G_SOMETHING and hit the load, the width of our known bits is 32. However, the width of the range is going to be 8. This will cause us to hit an assert.

To fix this, make computeKnownBitsFromRangeMetadata zero extend or truncate the range type to match the bitwidth of the known bits we're calculating.

Add a testcase in CodeGen/GlobalISel/KnownBitsTest.cpp to reflect that this works now.


https://reviews.llvm.org/D85375

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp


Index: llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp
===================================================================
--- llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp
+++ llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp
@@ -463,3 +463,51 @@
   EXPECT_EQ(Align(4), Info.computeKnownAlignment(CopyImplicitArgPtr));
   EXPECT_EQ(Align(4), Info.computeKnownAlignment(CopyImplicitBufferPtr));
 }
+
+TEST_F(AArch64GISelMITest, TestMetadata) {
+  StringRef MIRString = "  %imp:_(p0) = G_IMPLICIT_DEF\n"
+                        "  %load:_(s8) = G_LOAD %imp(p0) :: (load 1)\n"
+                        "  %ext:_(s32) = G_ZEXT %load(s8)\n"
+                        "  %cst:_(s32) = G_CONSTANT i32 1\n"
+                        "  %and:_(s32) = G_AND %ext, %cst\n"
+                        "  %copy:_(s32) = COPY %and(s32)\n";
+  setUp(MIRString);
+  if (!TM)
+    return;
+
+  Register CopyReg = Copies[Copies.size() - 1];
+  MachineInstr *FinalCopy = MRI->getVRegDef(CopyReg);
+  Register SrcReg = FinalCopy->getOperand(1).getReg();
+
+  // We need a load with a metadata range for this to break. Fudge the load in
+  // the string and replace it with something we can work with.
+  MachineInstr *And = MRI->getVRegDef(SrcReg);
+  MachineInstr *Ext = MRI->getVRegDef(And->getOperand(1).getReg());
+  MachineInstr *Load = MRI->getVRegDef(Ext->getOperand(1).getReg());
+  IntegerType *Int8Ty = Type::getInt8Ty(Context);
+
+  // Value must be in [0, 2)
+  Metadata *LowAndHigh[] = {
+      ConstantAsMetadata::get(ConstantInt::get(Int8Ty, 0)),
+      ConstantAsMetadata::get(ConstantInt::get(Int8Ty, 2))};
+  auto NewMDNode = MDNode::get(Context, LowAndHigh);
+  const MachineMemOperand *OldMMO = *Load->memoperands_begin();
+  MachineMemOperand NewMMO(OldMMO->getPointerInfo(), OldMMO->getFlags(),
+                           OldMMO->getSizeInBits(), OldMMO->getAlign(),
+                           OldMMO->getAAInfo(), NewMDNode);
+  MachineIRBuilder MIB(*Load);
+  MIB.buildLoad(Load->getOperand(0), Load->getOperand(1), NewMMO);
+  Load->eraseFromParent();
+
+  GISelKnownBits Info(*MF);
+  KnownBits Res = Info.getKnownBits(And->getOperand(1).getReg());
+
+  // We don't know what the result of the load is, so we don't know any ones.
+  EXPECT_TRUE(Res.One.isNullValue());
+
+  // We know that the value is in [0, 2). So, we don't know if the first bit
+  // is 0 or not. However, we do know that every other bit must be 0.
+  APInt Mask(Res.getBitWidth(), 1);
+  Mask.flipAllBits();
+  EXPECT_EQ(Mask.getZExtValue(), Res.Zero.getZExtValue());
+}
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -549,10 +549,10 @@
     // The first CommonPrefixBits of all values in Range are equal.
     unsigned CommonPrefixBits =
         (Range.getUnsignedMax() ^ Range.getUnsignedMin()).countLeadingZeros();
-
     APInt Mask = APInt::getHighBitsSet(BitWidth, CommonPrefixBits);
-    Known.One &= Range.getUnsignedMax() & Mask;
-    Known.Zero &= ~Range.getUnsignedMax() & Mask;
+    APInt UnsignedMax = Range.getUnsignedMax().zextOrTrunc(BitWidth);
+    Known.One &= UnsignedMax & Mask;
+    Known.Zero &= ~UnsignedMax & Mask;
   }
 }
 


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