[BPF] Allow selectively disable compiler builtins for BPF target (#4)

* [BPF] Add method to check whether a triple is a BPF architecture

* [BPF] Make rust allocation builtins unavailable for BPF target

- the calls to rust alloc/dealloc routines need to be preserved and
not replaced by compiler generated builtin inlined code

llvm/include/llvm/ADT/Triple.h

@@ -706,6 +706,11 @@ class Triple {
     return getArch() == Triple::aarch64 || getArch() == Triple::aarch64_be;
   }
 
+  /// Tests whether the target is BPF (little and big endian).
+  bool isBPF() const {
+    return getArch() == Triple::bpfel || getArch() == Triple::bpfeb;
+  }
+
   /// Tests whether the target is MIPS 32-bit (little and big endian).
   bool isMIPS32() const {
     return getArch() == Triple::mips || getArch() == Triple::mipsel;

llvm/lib/Analysis/TargetLibraryInfo.cpp

@@ -550,6 +550,12 @@ static void initialize(TargetLibraryInfoImpl &TLI, const Triple &T,
     TLI.setUnavailable(LibFunc_nvvm_reflect);
   }
 
+  if (T.isBPF()) {
+    TLI.setUnavailable(LibFunc_rust_alloc);
+    TLI.setUnavailable(LibFunc_rust_dealloc);
+    TLI.setUnavailable(LibFunc_rust_realloc);
+  }
+
   TLI.addVectorizableFunctionsFromVecLib(ClVectorLibrary);
 }
 
@@ -1503,9 +1509,9 @@ bool TargetLibraryInfoImpl::getLibFunc(const Function &FDecl,
                                        LibFunc &F) const {
   // Intrinsics don't overlap w/libcalls; if our module has a large number of
   // intrinsics, this ends up being an interesting compile time win since we
-  // avoid string normalization and comparison. 
+  // avoid string normalization and comparison.
   if (FDecl.isIntrinsic()) return false;
-  
+
   const DataLayout *DL =
       FDecl.getParent() ? &FDecl.getParent()->getDataLayout() : nullptr;
   return getLibFunc(FDecl.getName(), F) &&