[llvm] 4d1c827 - [WebAssembly] Support parsing .lto_set_conditional (#126546)

[via llvm-commits]

Author: Heejin Ahn
Date: 2025-04-02T03:15:29+09:00
New Revision: 4d1c82742302e205071a89ff42c0e90e548e861c

URL: https://github.com/llvm/llvm-project/commit/4d1c82742302e205071a89ff42c0e90e548e861c
DIFF: https://github.com/llvm/llvm-project/commit/4d1c82742302e205071a89ff42c0e90e548e861c.diff

LOG: [WebAssembly] Support parsing .lto_set_conditional (#126546)

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

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

And when that promoted global value is a function, a
`.lto_set_conditional` entry is written to the original 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 `symbolA` 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 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`, which splits LTO units when
possible, defaults to false, but it defaults to true when
`-fwhole-program-vtables` is used.

Added: 
    llvm/test/MC/WebAssembly/lto-set-conditional.s

Modified: 
    llvm/lib/MC/WasmObjectWriter.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/MC/WasmObjectWriter.cpp b/llvm/lib/MC/WasmObjectWriter.cpp
index 9c919696a0ac2..9d5a290f70cad 100644
--- a/llvm/lib/MC/WasmObjectWriter.cpp
+++ b/llvm/lib/MC/WasmObjectWriter.cpp
@@ -1785,6 +1785,18 @@ uint64_t WasmObjectWriter::writeOneObject(MCAssembler &Asm,
       WS.setIndex(InvalidIndex);
       continue;
     }
+    // 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 symbol (symbolA here) has been renamed to the new name
+    // created by attaching its module ID, so the original symbol does not
+    // appear in the bitcode anymore, and thus not in DataLocations. We should
+    // ignore them.
+    if (WS.isData() && WS.isDefined() && !DataLocations.count(&WS))
+      continue;
     LLVM_DEBUG(dbgs() << "adding to symtab: " << WS << "\n");
 
     uint32_t Flags = 0;

diff  --git a/llvm/test/MC/WebAssembly/lto-set-conditional.s b/llvm/test/MC/WebAssembly/lto-set-conditional.s
new file mode 100644
index 0000000000000..c9519e232c2d8
--- /dev/null
+++ b/llvm/test/MC/WebAssembly/lto-set-conditional.s
@@ -0,0 +1,8 @@
+# RUN: llvm-mc -triple=wasm32-unknown-unknown
+
+# Tests if `.lto_set_conditional` directives are parsed without crashing.
+.lto_set_conditional a, a.new
+.type  a.new, at function
+a.new:
+  .functype  a.new () -> ()
+  end_function