Add directize-skip-type-mismatch pass argument#8475
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thiblahute wants to merge 1 commit intoWebAssembly:mainfrom
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Add directize-skip-type-mismatch pass argument#8475thiblahute wants to merge 1 commit intoWebAssembly:mainfrom
thiblahute wants to merge 1 commit intoWebAssembly:mainfrom
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When fpcast-emu will run later in the pipeline, directize should not replace type-mismatched call_indirect with unreachable. The new --pass-arg=directize-skip-type-mismatch tells directize to return Unknown instead of Trap for these cases, leaving the call_indirect intact for fpcast-emu to handle. Without this, emscripten's pass pipeline (-O2 then fpcast-emu) causes directize to kill type-mismatched calls before fpcast-emu can fix them, resulting in RuntimeError: unreachable at runtime. This affects any C code using -sEMULATE_FUNCTION_POINTER_CASTS that calls functions through cast function pointers (e.g. GLib's g_list_free_full casting GDestroyNotify to GFunc).
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Interesting problem, thanks for the PR! Thinking about this, two other options might be
In both cases we might need to measure the impact. Thoughts? |
thiblahute
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Mar 18, 2026
Move the `--fpcast-emu` binaryen pass earlier in the pass pipeline so it runs before -O2. Previously, directize (part of -O2) would see type-mismatched call_indirect entries and replace them with unreachable before fpcast-emu had a chance to rewrite the table entries with matching types. This caused crashes for common C idioms like calling a 1-arg function through a 2-arg function pointer (e.g. GLib's `g_list_free_full` pattern). This reordering also improves performance: on a real-world GStreamer test binary, the reordered pipeline produces a 1.8% smaller wasm output (5.30MB vs 5.40MB) and seems to run almost ~2x faster (not sure why). Also move the fpcast-emu block outside `if optimizing:` so it runs even without -O2 in the link flags as this is not an optimization pass in practice. Also add a regression test. See WebAssembly/binaryen#8475
thiblahute
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Mar 18, 2026
Move the `--fpcast-emu` binaryen pass earlier in the pass pipeline so it runs before -O2. Previously, directize (part of -O2) would see type-mismatched call_indirect entries and replace them with unreachable before fpcast-emu had a chance to rewrite the table entries with matching types. This caused crashes for common C idioms like calling a 1-arg function through a 2-arg function pointer (e.g. GLib's `g_list_free_full` pattern). This reordering also improves performance: on a real-world GStreamer test binary, the reordered pipeline produces a 1.8% smaller wasm output (5.30MB vs 5.40MB) and seems to run almost ~2x faster (not sure why). Also move the fpcast-emu block outside `if optimizing:` so it runs even without -O2 in the link flags as this is not an optimization pass in practice. Also add a regression test. See WebAssembly/binaryen#8475
thiblahute
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Mar 18, 2026
Move the `--fpcast-emu` binaryen pass earlier in the pass pipeline so it runs before -O2. Previously, directize (part of -O2) would see type-mismatched call_indirect entries and replace them with unreachable before fpcast-emu had a chance to rewrite the table entries with matching types. This caused crashes for common C idioms like calling a 1-arg function through a 2-arg function pointer (e.g. GLib's `g_list_free_full` pattern). This reordering also improves performance: on a real-world GStreamer test binary, the reordered pipeline produces a 1.8% smaller wasm output (5.30MB vs 5.40MB) and seems to run almost ~2x faster (not sure why). Also move the fpcast-emu block outside `if optimizing:` so it runs even without -O2 in the link flags as this is not an optimization pass in practice. Also add a regression test. See WebAssembly/binaryen#8475
kripken
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Mar 18, 2026
Move EMULATE_FUNCTION_POINTER_CASTS's pass earlier in the pipeline so it runs before -O2. Previously, directize (part of -O2) would see type-mismatched call_indirect entries and replace them with unreachable before fpcast-emu had a chance to rewrite the table entries with matching types. This caused crashes for common C idioms like calling a 1-arg function through a 2-arg function pointer (e.g. GLib's `g_list_free_full` pattern). This reordering also improves performance, on a real-world GStreamer test binary, the reordered pipeline produces a 1.8% smaller wasm output (5.30MB vs 5.40MB) and seems to run almost ~2x faster. Also move the fpcast-emu block outside `if optimizing:` so it runs even without -O2 in the link flags as this is not an optimization pass in practice. Also add a regression test. See WebAssembly/binaryen#8475
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When fpcast-emu will run later in the pipeline, directize should not replace type-mismatched call_indirect with unreachable. The new --pass-arg=directize-skip-type-mismatch tells directize to return Unknown instead of Trap for these cases, leaving the call_indirect intact for fpcast-emu to handle.
Without this, emscripten's pass pipeline (-O2 then fpcast-emu) causes directize to kill type-mismatched calls before fpcast-emu can fix them, resulting in RuntimeError: unreachable at runtime. This affects any C code using -sEMULATE_FUNCTION_POINTER_CASTS that calls functions through cast function pointers (e.g. GLib's g_list_free_full casting GDestroyNotify to GFunc).