[LLVM-C] Turn a ShuffleVector Constant Into a Getter.

[CodaFi]

Summary

It is not a good idea to expose raw constants in the LLVM C API. Replace this with an explicit getter.

[CodaFi]

@cdisselkoen I believe we were talking past each other in the other revision - sorry about the confusion I may have caused. Hopefully this is clearer.

[cdisselkoen]

I have no problem with this revision. I apologize for my lack of understanding in the other revision (D88190); I was just trying to understand why having an extern const int in the C API wouldn't work. This proposal seems fine and provides more future flexibility.

llvm/include/llvm-c/Core.h

LLVMValueRef LLVMBuildAtomicCmpXchg(LLVMBuilderRef B, LLVMValueRef Ptr,
                                     LLVMBool SingleThread);
 
 /**
  * Get the number of elements in the mask of a ShuffleVector instruction.
  */
 unsigned LLVMGetNumMaskElements(LLVMValueRef ShuffleVectorInst);
 
 /**
+ * \returns a constant that specifies that the result of a \c ShuffleVectorInst
+ * is undefined.
+ */
+int LLVMGetUndefMaskElem(void);
+
+/**
  * Get the mask value at position Elt in the mask of a ShuffleVector
- * instruction. Return LLVMUndefMaskElem if the mask value is undef at that
- * position.
+ * instruction.
+ *
+ * \Returns the result of \c LLVMGetUndefMaskElem() if the mask value is undef
+ * at that position.
  */
 int LLVMGetMaskValue(LLVMValueRef ShuffleVectorInst, unsigned Elt);
-extern const int LLVMUndefMaskElem;
 
 LLVMBool LLVMIsAtomicSingleThread(LLVMValueRef AtomicInst);
 void LLVMSetAtomicSingleThread(LLVMValueRef AtomicInst, LLVMBool SingleThread);
 
 LLVMAtomicOrdering LLVMGetCmpXchgSuccessOrdering(LLVMValueRef CmpXchgInst);
 void LLVMSetCmpXchgSuccessOrdering(LLVMValueRef CmpXchgInst,
                                    LLVMAtomicOrdering Ordering);
 LLVMAtomicOrdering LLVMGetCmpXchgFailureOrdering(LLVMValueRef CmpXchgInst);

llvm/lib/IR/Core.cpp

unsigned LLVMGetNumMaskElements(LLVMValueRef SVInst) {
   return I->getShuffleMask().size();
 }
 
 int LLVMGetMaskValue(LLVMValueRef SVInst, unsigned Elt) {
   Value *P = unwrap<Value>(SVInst);
   ShuffleVectorInst *I = cast<ShuffleVectorInst>(P);
   return I->getMaskValue(Elt);
 }
-const int LLVMUndefMaskElem =
-    -1; // not actually accessible as ShuffleVectorInst::UndefMaskElem, so we
-        // hardcode it here
+
+int LLVMGetUndefMaskElem(void) { return UndefMaskElem; }
 
 LLVMBool LLVMIsAtomicSingleThread(LLVMValueRef AtomicInst) {
   Value *P = unwrap<Value>(AtomicInst);
 
   if (AtomicRMWInst *I = dyn_cast<AtomicRMWInst>(P))
     return I->getSyncScopeID() == SyncScope::SingleThread;
   return cast<AtomicCmpXchgInst>(P)->getSyncScopeID() ==
              SyncScope::SingleThread;

llvm/tools/llvm-c-test/echo.cpp

switch(Op) {
       }
       case LLVMShuffleVector: {
         LLVMValueRef Agg0 = CloneValue(LLVMGetOperand(Src, 0));
         LLVMValueRef Agg1 = CloneValue(LLVMGetOperand(Src, 1));
         SmallVector<LLVMValueRef, 8> MaskElts;
         unsigned NumMaskElts = LLVMGetNumMaskElements(Src);
         for (unsigned i = 0; i < NumMaskElts; i++) {
           int Val = LLVMGetMaskValue(Src, i);
-          if (Val == LLVMUndefMaskElem) {
+          if (Val == LLVMGetUndefMaskElem()) {
             MaskElts.push_back(LLVMGetUndef(LLVMInt64Type()));
           } else {
             MaskElts.push_back(LLVMConstInt(LLVMInt64Type(), Val, true));
           }
         }
         LLVMValueRef Mask = LLVMConstVector(MaskElts.data(), NumMaskElts);
         Dst = LLVMBuildShuffleVector(Builder, Agg0, Agg1, Mask, Name);
         break;