Introduce enum value for previously defined metadata -- make.implicit

[• chenli]

Summary

This patch adds enum value for an existing metadata type -- make.implicit. Using preassigned enum will be helpful to get compile time type checking and avoid string construction and comparison. The patch also changes uses of make.implicit from string metadata to enum metadata. There is no functionality change.

[reames]

This patch appears incomplete. In particular, look at the analogous code in LLVMContext.cpp.

Also, would you mind talking with Sanjoy and landing a follow up patch with verifier support?

[• chenli]

Update w.r.t Philip's comments.

[• chenli]

In D11698#216393, @reames wrote:

This patch appears incomplete. In particular, look at the analogous code in LLVMContext.cpp.

Also, would you mind talking with Sanjoy and landing a follow up patch with verifier support?

I will talk to Sanjoy about follow up patches both on verifier and llvm doc.

[reames]

LGTM

include/llvm/IR/LLVMContext.h

enum {
     MD_tbaa_struct = 5, // "tbaa.struct"
     MD_invariant_load = 6, // "invariant.load"
     MD_alias_scope = 7, // "alias.scope"
     MD_noalias = 8, // "noalias",
     MD_nontemporal = 9, // "nontemporal"
     MD_mem_parallel_loop_access = 10, // "llvm.mem.parallel_loop_access"
     MD_nonnull = 11, // "nonnull"
     MD_dereferenceable = 12, // "dereferenceable"
-    MD_dereferenceable_or_null = 13 // "dereferenceable_or_null"
+    MD_dereferenceable_or_null = 13, // "dereferenceable_or_null"
+    MD_make_implicit = 14 // "make.implicit"
   };
 
   /// getMDKindID - Return a unique non-zero ID for the specified metadata kind.
   /// This ID is uniqued across modules in the current LLVMContext.
   unsigned getMDKindID(StringRef Name) const;
 
   /// getMDKindNames - Populate client supplied SmallVector with the name for
   /// custom metadata IDs registered in this LLVMContext.

lib/CodeGen/ImplicitNullChecks.cpp

 #include "llvm/CodeGen/MachineMemOperand.h"
 #include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Instruction.h"
+#include "llvm/IR/LLVMContext.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
 #include "llvm/Target/TargetInstrInfo.h"
 
 using namespace llvm;
 
 static cl::opt<unsigned> PageSize("imp-null-check-page-size",

 /// implicit null check.  If yes, append a description of the said null check to
 /// NullCheckList and return true, else return false.
 bool ImplicitNullChecks::analyzeBlockForNullChecks(
     MachineBasicBlock &MBB, SmallVectorImpl<NullCheck> &NullCheckList) {
   typedef TargetInstrInfo::MachineBranchPredicate MachineBranchPredicate;
 
   MDNode *BranchMD =
       MBB.getBasicBlock()
-          ? MBB.getBasicBlock()->getTerminator()->getMetadata("make.implicit")
+          ? MBB.getBasicBlock()->getTerminator()->getMetadata(LLVMContext::MD_make_implicit)
           : nullptr;
   if (!BranchMD)
     return false;
 
   MachineBranchPredicate MBP;
 
   if (TII->AnalyzeBranchPredicate(MBB, MBP, true))
     return false;

lib/IR/LLVMContext.cpp

assert(DereferenceableID == MD_dereferenceable &&
          "dereferenceable kind id drifted");
   (void)DereferenceableID;
   
   // Create the 'dereferenceable_or_null' metadata kind.
   unsigned DereferenceableOrNullID = getMDKindID("dereferenceable_or_null");
   assert(DereferenceableOrNullID == MD_dereferenceable_or_null && 
          "dereferenceable_or_null kind id drifted");
   (void)DereferenceableOrNullID;
+
+  // Create the 'make.implicit' metadata kind.
+  unsigned MakeImplicitID = getMDKindID("make.implicit");
+  assert(MakeImplicitID == MD_make_implicit &&
+         "make.implicit kind id drifted");
+  (void)MakeImplicitID;
 }
 LLVMContext::~LLVMContext() { delete pImpl; }
 
 void LLVMContext::addModule(Module *M) {
   pImpl->OwnedModules.insert(M);
 }
 
 void LLVMContext::removeModule(Module *M) {