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Cleanup number handling in match exhaustiveness #116281

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Oct 1, 2023
Merged

Cleanup number handling in match exhaustiveness #116281

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fac50e8
Evaluate float consts eagerly
Nadrieril Sep 29, 2023
d6e9b32
No need to carry `bias` in `IntRange`
Nadrieril Sep 29, 2023
0a6d794
Cleanup number literal evaluation
Nadrieril Sep 29, 2023
eac7bcd
Move `eval_bits` optimization upstream
Nadrieril Sep 30, 2023
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11 changes: 10 additions & 1 deletion compiler/rustc_middle/src/mir/consts.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -288,7 +288,16 @@ impl<'tcx> Const<'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
) -> Option<ScalarInt> {
self.try_eval_scalar(tcx, param_env)?.try_to_int().ok()
match self {
// If the constant is already evaluated, we shortcut here.
Const::Ty(c) if let ty::ConstKind::Value(valtree) = c.kind() => {
valtree.try_to_scalar_int()
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Are you sure this helps? self.eval will return immediately for ty::ConstKind::Value.

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I measured a small change on one benchmark yeah. I think looking through the code it always goes through a query

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Ah, there's the valtree_to_const_val. But then we should really have this as a fast-path in try_eval_scalar and it should apply on all Const::Ty, not just for things that are already valtrees.

#116457

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I considered it, but the optimization is only correct if we know the normal path will only find Valtree::Leaf I think. We talked about it here #116281 (comment)

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I mostly felt out of my depth on this ngl

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Such discussion should definitely have resulted in a comment in the code...

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This is still a change in behavior. Previously, try_eval_scalar_int would have worked on struct newtypes around integers; now it might return None since those are branch valtrees (under our current representation).

Also, previously if this accidentally got called on a reference it would have returned None, but now when called on &i32 it will return the integer value. That seems bad.

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@Nadrieril Nadrieril Oct 5, 2023 •
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Oh that's not good at all. I did run all tests once with an assert checking that it's only called on numeric types fwiw. Also there was almost no difference when I loved it into try_eval_scalar_int. We could leave it in deconstruct_pat and document the choices made

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@RalfJung RalfJung Oct 5, 2023 •
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I did run all tests once with an assert checking that it's only called on numeric types fwiw.

That doesn't help guard against new uses introduced tomorrow.

#116457 should fix this.

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Ah yeah that's much better

},
// This is a more general form of the previous case.
_ => {
self.try_eval_scalar(tcx, param_env)?.try_to_int().ok()
},
}
}

#[inline]
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