[CodeGen] Fix hashing for MO_ExternalSymbol MachineOperands.

We were hashing the string pointer, not the string, so two instructions
could be identical (isIdenticalTo), but have different hash codes.

This showed up as a very rare, non-deterministic assertion failure
rehashing a DenseMap constructed by MachineOutliner.  So there's no
"real" testcase, just a unittest which checks that the hash function
behaves correctly.

I'm a little scared fixing this is going to cause a regression in
outlining or MachineCSE, but hopefully we won't run into any issues.

Differential Revision: https://reviews.llvm.org/D61975



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362281 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineOperand.cpp

@@ -361,7 +361,7 @@ hash_code llvm::hash_value(const MachineOperand &MO) {
     return hash_combine(MO.getType(), MO.getTargetFlags(), MO.getIndex());
   case MachineOperand::MO_ExternalSymbol:
     return hash_combine(MO.getType(), MO.getTargetFlags(), MO.getOffset(),
-                        MO.getSymbolName());
+                        StringRef(MO.getSymbolName()));
   case MachineOperand::MO_GlobalAddress:
     return hash_combine(MO.getType(), MO.getTargetFlags(), MO.getGlobal(),
                         MO.getOffset());

unittests/CodeGen/MachineOperandTest.cpp

@@ -398,4 +398,14 @@ TEST(MachineOperandTest, PrintPredicate) {
   ASSERT_TRUE(OS.str() == "intpred(eq)");
 }
 
+TEST(MachineOperandTest, HashValue) {
+  char SymName1[] = "test";
+  char SymName2[] = "test";
+  MachineOperand MO1 = MachineOperand::CreateES(SymName1);
+  MachineOperand MO2 = MachineOperand::CreateES(SymName2);
+  ASSERT_NE(SymName1, SymName2);
+  ASSERT_EQ(hash_value(MO1), hash_value(MO2));
+  ASSERT_TRUE(MO1.isIdenticalTo(MO2));
+}
+
 } // end namespace