cleanup: add some comments per review feedback

Author: durin42

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -1438,6 +1438,9 @@ LLVMRustCreateThinLTOData(LLVMRustThinLTOModule *modules,
   };
 
 #if LLVM_VERSION_GE(13,0)
+  // Uses FromPrevailing visibility scheme which works for many binary
+  // formats. We probably could and should use ELF visibility scheme for many of
+  // our targets, however.
   lto::Config conf;
   thinLTOResolvePrevailingInIndex(conf, Ret->Index, isPrevailing, recordNewLinkage,
                                   Ret->GUIDPreservedSymbols);

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -383,6 +383,9 @@ LLVMRustBuildAtomicCmpXchg(LLVMBuilderRef B, LLVMValueRef Target,
                            LLVMAtomicOrdering Order,
                            LLVMAtomicOrdering FailureOrder, LLVMBool Weak) {
 #if LLVM_VERSION_GE(13,0)
+  // Rust probably knows the alignment of the target value and should be able to
+  // specify something more precise than MaybeAlign here. See also
+  // https://reviews.llvm.org/D97224 which may be a useful reference.
   AtomicCmpXchgInst *ACXI = unwrap(B)->CreateAtomicCmpXchg(
       unwrap(Target), unwrap(Old), unwrap(Source), llvm::MaybeAlign(), fromRust(Order),
       fromRust(FailureOrder));