| commit | 4d1c82742302e205071a89ff42c0e90e548e861c | [log] [tgz] |
|---|---|---|
| author | Heejin Ahn <aheejin@gmail.com> | Wed Apr 02 03:15:29 2025 +0900 |
| committer | GitHub <noreply@github.com> | Wed Apr 02 03:15:29 2025 +0900 |
| tree | 0a5dec2db131775d50e594ce1708e76e13d746a1 | |
| parent | bd7585bea3906aa2d288f7238382e99b8d5506eb [diff] |
[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.
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