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Auto merge of #119911 - NCGThompson:is-statically-known, r=oli-obk

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Replacement of #114390: Add new intrinsic `is_var_statically_known` and optimize pow for powers of two

This adds a new intrinsic `is_val_statically_known` that lowers to [``@llvm.is.constant.*`](https://llvm.org/docs/LangRef.html#llvm-is-constant-intrinsic).` It also applies the intrinsic in the int_pow methods to recognize and optimize the idiom `2isize.pow(x)`. See #114390 for more discussion.

While I have extended the scope of the power of two optimization from #114390, I haven't added any new uses for the intrinsic. That can be done in later pull requests.

Note: When testing or using the library, be sure to use `--stage 1` or higher. Otherwise, the intrinsic will be a noop and the doctests will be skipped. If you are trying out edits, you may be interested in [`--keep-stage 0`](https://rustc-dev-guide.rust-lang.org/building/suggested.html#faster-builds-with---keep-stage).

Fixes #47234
Resolves #114390
`@Centri3`
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bors 2024-01-25 05:16:53 +00:00
commit 039d887928
17 changed files with 634 additions and 154 deletions
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5
compiler/rustc_const_eval/src/const_eval/machine.rs
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@ -531,6 +531,11 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
)?;
}
}
// The intrinsic represents whether the value is known to the optimizer (LLVM).
// We're not doing any optimizations here, so there is no optimizer that could know the value.
// (We know the value here in the machine of course, but this is the runtime of that code,
// not the optimization stage.)
sym::is_val_statically_known => ecx.write_scalar(Scalar::from_bool(false), dest)?,
_ => {
throw_unsup_format!(
"intrinsic `{intrinsic_name}` is not supported at compile-time"