[PATCH] D43264: [WebAssembly] Add explicit symbol table

[Nicholas Wilson via Phabricator via llvm-commits]

ncw added a comment.

In https://reviews.llvm.org/D43264#1008031, @ruiu wrote:

> So, InputFunction is a function, InputSegment is a data segment, and what is "wasm global" for? It sounds like it is more like data than a function, but I wonder how "wasm global" is different from regular data section.


It's basically a special type of data section; what makes it special is that it's accessed more like a register than a memory region.

The InputGlobal object represents the allocation of the memory/storage for the symbol, and contains the region of memory to be copied into the output binary with the initial value of the data.

You're right, it's very similar to a data symbol. The InputGlobal has really the same role for a GlobalSymbol as an InputSection does for a DataSymbol.

  // Will be stored in ordinary memory, addressable down to individual bytes.
  // Creates a 12-byte data section (InputSegment), holding its initial value, which
  // the data Symbol points to as its definition.
  std::tuple<int,int,int> global_variable = {1,2,3};
  
  // Will create a 4-byte section, but using a "Wasm global". Not addressable down
  // to individual bytes, it's like a register. Can't take address-of, can only assign
  // and read from the storage. Requests the WebAssembly virtual machine to
  // allocate a thread-local register. Constrained to be of register type (int32/float32/
  // int64/float64) so the size of an InputGlobal is always a 4- or 8-byte section.
  //
  // Unlike x86 registers, you can have have as many as you want, they are storage
  // slots allocated at program startup, just like ordinary sections are mapped to
  // RAM. (NB The WebAssembly interpreter will spill the globals to RAM if the "real"
  // CPU doesn't have enough spare registers to use for the Wasm globals). So it's like
  // data, in that these are variables with a value, but the mechanics of the storage
  // slots are different. There's no analogue in ELF, because x86 has a fixed number of
  // registers, and the binary doesn't have to ask for them to be created.
  //
  // The 4-byte section containing the initial value is represented by an InputGlobal,
  // which the Wasm-global symbol points to as its definition.  It's written out in a
  // different place in the final Wasm object, since it has distinct semantics.
  //
  // NB. This attribute isn't actually implemented in the C frontend ("Wasm globals"
  // are only accessible via intrinsics). Conceptually these are just variables though.
  int __attribute__((wasm_global)) wasm_global = 123;


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D43264