MemoryBuiltins: accept non-TLI funcs with attribs as allocator funcs

[durin42]

Summary

This allows us to accept annotations from out-of-tree languages (the
example test is derived from Rust) so they can enjoy the benefits of
LLVM's optimizations without requiring LLVM to have language-specific
knowledge.

[bogner]

Seems completely reasonable.

[nikic]

LGTM

llvm/lib/Analysis/MemoryBuiltins.cpp

 
 Optional<StringRef> llvm::getAllocationFamily(const Value *I,
                                               const TargetLibraryInfo *TLI) {
   bool IsNoBuiltin;
   const Function *Callee = getCalledFunction(I, IsNoBuiltin);
   if (Callee == nullptr || IsNoBuiltin)
     return None;
   LibFunc TLIFn;
-  if (!TLI || !TLI->getLibFunc(*Callee, TLIFn) || !TLI->has(TLIFn))
-    return None;
+
+  if (TLI && TLI->getLibFunc(*Callee, TLIFn) && TLI->has(TLIFn)) {
+    // Callee is some known library function.
     const auto AllocData = getAllocationDataForFunction(Callee, AnyAlloc, TLI);
     if (AllocData)
       return mangledNameForMallocFamily(AllocData.value().Family);
     const auto FreeData = getFreeFunctionDataForFunction(Callee, TLIFn);
     if (FreeData)
       return mangledNameForMallocFamily(FreeData.value().Family);
+  }
+  // Callee isn't a known library function, still check attributes.
   if (checkFnAllocKind(I, AllocFnKind::Free | AllocFnKind::Alloc |
                               AllocFnKind::Realloc)) {
     Attribute Attr = cast<CallBase>(I)->getFnAttr("alloc-family");
     if (Attr.isValid())
       return Attr.getValueAsString();
   }
   return None;
 }

llvm/test/Transforms/InstCombine/out-of-tree-allocator-optimizes-away.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes=instcombine %s | FileCheck %s
+
+; Function Attrs: nonlazybind uwtable
+define void @alloc_elides_test(i32 %data){
+; CHECK-LABEL: @alloc_elides_test(
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    ret void
+;
+start:
+  %0 = tail call i8* @__rust_alloc(i64 4, i64 32)
+  tail call void @__rust_dealloc(i8* nonnull %0, i64 4, i64 32)
+  ret void
+}
+
+declare noalias i8* @__rust_alloc(i64, i64 allocalign) unnamed_addr nounwind nonlazybind allocsize(0) uwtable allockind("alloc,uninitialized,aligned") "alloc-family"="__rust_alloc"
+
+declare void @__rust_dealloc(i8* allocptr, i64, i64) unnamed_addr nounwind nonlazybind uwtable allockind("free") "alloc-family"="__rust_alloc"