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Auto merge of #96285 - flip1995:pk-vfe, r=nagisa

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Introduce `-Zvirtual-function-elimination` codegen flag

Fixes #68262

This PR adds a codegen flag `-Zvirtual-function-elimination` to enable the VFE optimization in LLVM. To make this work, additonal  information has to be added to vtables ([`!vcall_visibility` metadata](https://llvm.org/docs/TypeMetadata.html#vcall-visibility-metadata) and a `typeid` of the trait). Furthermore, instead of just `load`ing functions, the [`llvm.type.checked.load` intrinsic](https://llvm.org/docs/LangRef.html#llvm-type-checked-load-intrinsic) has to be used to map functions to vtables.

For technical details of the changes, see the commit messages.

I also tested this flag on https://github.com/tock/tock on different boards to verify that this fixes the issue https://github.com/tock/tock/issues/2594. This flag is able to improve the size of the resulting binary by about 8k-9k bytes by removing the unused debug print functions.

[Rendered documentation update](https://github.com/flip1995/rust/blob/pk-vfe/src/doc/rustc/src/codegen-options/index.md#virtual-function-elimination)
This commit is contained in:
bors 2022-06-14 21:37:11 +00:00
commit 2d1e075079
20 changed files with 431 additions and 32 deletions
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3
compiler/rustc_session/src/options.rs
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@ -1585,6 +1585,9 @@ options! {
"in general, enable more debug printouts (default: no)"),
verify_llvm_ir: bool = (false, parse_bool, [TRACKED],
"verify LLVM IR (default: no)"),
virtual_function_elimination: bool = (false, parse_bool, [TRACKED],
"enables dead virtual function elimination optimization. \
Requires `-Clto[=[fat,yes]]`"),
wasi_exec_model: Option<WasiExecModel> = (None, parse_wasi_exec_model, [TRACKED],
"whether to build a wasi command or reactor"),