Branch: refs/heads/main Home: https://github.com/WebKit/WebKit Commit: cee499e5516c550b861c4bda2657023d5e076c06 https://github.com/WebKit/WebKit/commit/cee499e5516c550b861c4bda2657023d5e076c06 Author: Mark Lam <mark....@apple.com> Date: 2023-01-02 (Mon, 02 Jan 2023)
Changed paths: M Source/JavaScriptCore/llint/WebAssembly.asm Log Message: ----------- Wasm Tail Call needs to restore callee save registers preserved by caller. https://bugs.webkit.org/show_bug.cgi?id=249979 <rdar://problem/103795203> Reviewed by Yusuke Suzuki. In a normal call sequence where we have functions A, B, and C, and A calls B, which in turn calls C, the following sequence of events occur: 1. On entry to B from A, B preserves A's callee save registers. 2. On entry to C from B, C preserves B's callee save registers. 3. On exit from C to B, C restores B's callee save registers. 4. On exit from B to A, B restores A's callee save registers. However, when we have a tail call sequence like this instead: A calls B, and B tail calls C, the following events should occur: 1. On entry to B from A, B preserves A's callee save registers. 2. Before B tails call to C, B restores A's callee save registers. 3. On entry to C from B, C preserves A's callee save registers (which was restored in (2)). 4. On exit from C to A, C restores A's callee save registers. In our current implementation, we have a bug where step (2) is missing for one of the callee save registers i.e. the wasmInstance register. This only affects useFastTLS mode. For NoTLS mode, the wasmInstance register is used as a global register (similar to how TLS is used), and is not meant to be preserved / restored like a callee save register. This has been tested with ASSERTs in vmEntryToJavaScript (and friends) that verifies that callee save registers have been restored on exit. However, these ASSERTs are only in my local builds and relies on other dependencies. So, I'll have to defer landing these ASSERTs to a subsequent patch. * Source/JavaScriptCore/llint/WebAssembly.asm: Canonical link: https://commits.webkit.org/258378@main _______________________________________________ webkit-changes mailing list webkit-changes@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-changes