Simplify LTO flag passing in get_clang_flags - #27407
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Previously, `get_clang_flags` in `tools/compile.py` handled LTO and non-LTO modes separately: - In non-LTO mode, `building.llvm_backend_args()` was passed to `clang` via `-mllvm`. - In LTO mode (`settings.LTO`), `building.llvm_backend_args()` was not passed to `clang` (as backend flags are passed to `wasm-ld` at link time). Instead, `-mexception-handling` was explicitly appended when `settings.SUPPORT_LONGJMP == 'wasm'` so that `clang` added `target-features=+exception-handling` to LLVM IR attributes. Also, if `-flto` was not in `user_args`, `-flto=` was appended. This change simplifies `get_clang_flags` by unconditionally passing `building.llvm_backend_args()` to `clang` via `-mllvm` in both LTO and non-LTO modes. Because `building.llvm_backend_args()` includes `-wasm-enable-sjlj` when `settings.SUPPORT_LONGJMP == 'wasm'`, the `clang` driver automatically adds `target-features=+exception-handling` to LLVM IR, removing the need for the special `-mexception-handling` case. It also removes the fallback `-flto=` injection from `get_clang_flags`.
sbc100
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Jul 23, 2026
| flags.append('-fvisibility=default') | ||
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| if settings.LTO: | ||
| if not any(a.startswith('-flto') for a in user_args): |
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I think this probably dates back to when users could do -sLTO maybe, so the setting might be enabled without -flto being on the command line.
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Jul 28, 2026
Followup to emscripten-core#27407 Because `settings.LTO` was an internal setting not used anywhere in JavaScript libraries or runtime code. This change removes `LTO` from `src/settings_internal.js` and stores `-flto` / `-fno-lto` state in `options.lto` on `EmccOptions` instead.
sbc100
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Jul 28, 2026
Followup to emscripten-core#27407 Because `settings.LTO` was an internal setting not used anywhere in JavaScript libraries or runtime code. This change removes `LTO` from `src/settings_internal.js` and stores `-flto` / `-fno-lto` state in `options.lto` on `EmccOptions` instead.
sbc100
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Jul 30, 2026
Followup to emscripten-core#27407 Because `settings.LTO` was an internal setting not used anywhere in JavaScript libraries or runtime code. This change removes `LTO` from `src/settings_internal.js` and stores `-flto` / `-fno-lto` state in `options.lto` on `EmccOptions` instead.
sbc100
added a commit
that referenced
this pull request
Jul 30, 2026
Followup to #27407 Because `settings.LTO` was an internal setting not used anywhere in JavaScript libraries or runtime code. This change removes `LTO` from `src/settings_internal.js` and stores `-flto` / `-fno-lto` state in `options.lto` on `EmccOptions` instead. In the long run I hope to use internal settings only for values that need to flow out the the JS compiler.
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Previously,
get_clang_flagsintools/compile.pyhandled LTO and non-LTO modes separately:building.llvm_backend_args()was passed toclangvia-mllvm.settings.LTO),building.llvm_backend_args()was not passed toclang(as backend flags are passed towasm-ldat link time). Instead,-mexception-handlingwas explicitly appended whensettings.SUPPORT_LONGJMP == 'wasm'so thatclangaddedtarget-features=+exception-handlingto LLVM IR attributes. Also, if-fltowas not inuser_args,-flto=was appended.This change simplifies
get_clang_flagsby unconditionally passingbuilding.llvm_backend_args()toclangvia-mllvmin both LTO and non-LTO modes. Becausebuilding.llvm_backend_args()includes-wasm-enable-sjljwhensettings.SUPPORT_LONGJMP == 'wasm', theclangdriver automatically addstarget-features=+exception-handlingto LLVM IR, removing the need for the special-mexception-handlingcase. It also removes the fallback-flto=injection fromget_clang_flags.