[WebAssembly][MC] Fix placement of table section

[wingo]

Summary

The table section goes after functions.

[sbc100]

llvm/lib/MC/WasmObjectWriter.cpp

1790

I don't understand how moving the table section before Events and Globals relates to being able to use externref and funcref as block types.

Aren't all types defined in the type section? And also.. the blocks themselves are all defined in Code section which is after all of these isn't it?

[wingo]

llvm/lib/MC/WasmObjectWriter.cpp

1790

They are only related in the sense that they were just two small things that I needed to fix to assemble and link https://github.com/Igalia/ref-cpp/blob/master/milestones/m3/test.S.

Regarding this specific block, the table section must go after functions and before events. The existing tests only validated before because the tests didn't include events or globals. Once you start linking things with globals, you run into validation problems, both in wasm-ld when it parses object files, and in external tools.

llvm/lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

343 ↗

(On Diff #308364)

I assume this will be subsumed by https://reviews.llvm.org/D92359

[sbc100]

llvm/lib/MC/WasmObjectWriter.cpp

1790

Wait.. are you saying that any llvm-produced object file today that contains even one global or event will not validate?

[wingo]

llvm/lib/MC/WasmObjectWriter.cpp

1790

Any LLVM-produced object file which defines a table and which contains even one global will not validate, correct.

We don't hit this case right now because LLVM doesn't produce object files that define tables, unless in a .s file -- there is only __indirect_function_table which is an import, not a definition.

[wingo]

llvm/lib/MC/WasmObjectWriter.cpp

1790

Reference: https://webassembly.github.io/reference-types/core/binary/modules.html#binary-module

[sbc100]

Oh I see .. up until now we import the table.

[sbc100]

I guess we wait for D92359 and then land this on top of that.

Have you applied for commit access yet?

[wingo]

llvm/lib/MC/WasmObjectWriter.cpp

1790

Tables go before events: https://github.com/WebAssembly/exception-handling/blob/master/document/core/binary/modules.rst#modules-1

[wingo]

In D92323#2425008, @sbc100 wrote:

I guess we wait for D92359 and then land this on top of that.

SGTM

Have you applied for commit access yet?

Not yet, will do that now. Thanks for the poke.

[wingo]

Rebase

llvm/lib/MC/WasmObjectWriter.cpp

uint64_t WasmObjectWriter::writeOneObject(MCAssembler &Asm,
   // Write out the Wasm header.
   writeHeader(Asm);
 
   uint32_t CodeSectionIndex, DataSectionIndex;
   if (Mode != DwoMode::DwoOnly) {
     writeTypeSection(Signatures);
     writeImportSection(Imports, DataSize, TableElems.size());
     writeFunctionSection(Functions);
+    writeTableSection(Tables);
     // Skip the "memory" section; we import the memory instead.
     writeEventSection(Events);
     writeGlobalSection(Globals);
-    writeTableSection(Tables);
     writeExportSection(Exports);
     writeElemSection(TableElems);
     writeDataCountSection();
 
     CodeSectionIndex = writeCodeSection(Asm, Layout, Functions);
     DataSectionIndex = writeDataSection(Layout);
   }