[llvm] [WebAssembly] Support parsing .lto_set_conditional (PR #126546)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-webassembly

Author: Heejin Ahn (aheejin)

<details>
<summary>Changes</summary>

In the split-LTO-unit mode in ThinLTO, a compilation module is split into two and global variables that meet a [specific criteria](https://github.com/llvm/llvm-project/blob/d21fc58aeeaa7f0369a24dbe70a0360e0edbf76f/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp#L315-L366) is moved to the second module.

And if there is an originally local-linkage global value defined in the first module and referenced in the second module or the vice versa, that value is [_promoted_](https://github.com/llvm/llvm-project/blob/d21fc58aeeaa7f0369a24dbe70a0360e0edbf76f/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp#L46-L100) by attaching a module has ID to their names in order to prevent name clashes because now they can be referenced from other modules.

And when that promoted global value is a function, a `.lto_set_conditional` entry is written to the first module to avoid breaking references from inline assembly:
https://github.com/llvm/llvm-project/blob/d21fc58aeeaa7f0369a24dbe70a0360e0edbf76f/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp#L84-L91

The syntax of this is, if the original function name is `a` and the module ID is `123`,
```ll
module asm ".lto_set_conditional symbolA,symbolA.123"
```
These symbols are parsed here:
https://github.com/llvm/llvm-project/blob/648981f913431749c4656268ed670677a88511f6/llvm/lib/MC/MCParser/AsmParser.cpp#L6467

The first function symbol in this `.lto_set_conditional` do not exist as a function in the bitcode anymore because it was renamed to the second. So they are not [assigned as function symbols](https://github.com/llvm/llvm-project/blob/648981f913431749c4656268ed670677a88511f6/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp#L355) but they are not really data either, so the object writer crashes here:
https://github.com/llvm/llvm-project/blob/5b9e6c7993359c16b4d645c851bb7fe2fd7b78c7/llvm/lib/MC/WasmObjectWriter.cpp#L1820

This PR makes the object writer just skip those symbols.

---

This problem was discovered when I was testing with `-fwhole-program-vtables`.
The reason we didn't have this problem before with ThinLTO was because [`-fsplit-lto-unit`](https://clang.llvm.org/docs/UsersManual.html#cmdoption-f-no-split-lto-unit), which splits LTO units when possible, defaults to false, but it defaults to true when `-fwhole-program-vtables` is used.

---
Full diff: https://github.com/llvm/llvm-project/pull/126546.diff


2 Files Affected:

- (modified) llvm/lib/MC/WasmObjectWriter.cpp (+12-1) 
- (added) llvm/test/MC/WebAssembly/lto-set-conditional.ll (+13) 


``````````diff
diff --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index c5a95cb3da54331..d51bbd642196acb 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -1817,7 +1817,18 @@ uint64_t WasmObjectWriter::writeOneObject(MCAssembler &Asm,
       assert(WasmIndices.count(&WS) > 0);
       Info.ElementIndex = WasmIndices.find(&WS)->second;
     } else if (WS.isDefined()) {
-      assert(DataLocations.count(&WS) > 0);
+      if (!DataLocations.count(&WS))
+        // In bitcode generated by split-LTO-unit mode in ThinLTO, these lines
+        // can appear:
+        // module asm ".lto_set_conditional symbolA,symbolA.[moduleId]"
+        // ...
+        // (Here [moduleId] will be replaced by a real module hash ID)
+        //
+        // Here the original symbols (symbolA here) have been renamed to symbol
+        // new names created by attaching their module IDs and the original
+        // symbols do not appear in the bitcode anymore, and thus not in
+        // DataLocations. We should ignore them.
+        continue;
       Info.DataRef = DataLocations.find(&WS)->second;
     }
     WS.setIndex(SymbolInfos.size());
diff --git a/llvm/test/MC/WebAssembly/lto-set-conditional.ll b/llvm/test/MC/WebAssembly/lto-set-conditional.ll
new file mode 100644
index 000000000000000..84270aea60bdebe
--- /dev/null
+++ b/llvm/test/MC/WebAssembly/lto-set-conditional.ll
@@ -0,0 +1,13 @@
+; RUN: llc %s -filetype=obj
+
+target triple = "wasm32-unknown-unknown"
+
+; In .lto_set_conditional, the first symbol is renamed to the second symbol, so
+; the first symbol does not exist anymore in the file. Object writer should not
+; crash when .lto_set_conditional is present.
+
+module asm ".lto_set_conditional a,a.new"
+
+define hidden void @a.new() {
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/126546