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Optimize try_eval_bits to avoid layout queries

This specifically targets match checking, but is possibly more widely
useful as well. In code with large, single-value match statements, we
were previously spending a lot of time running layout_of for the
primitive types (integers, chars) -- which is essentially useless. This
optimizes the code to avoid those query calls by directly obtaining the
size for these types, when possible.

It may be worth considering adding a `size_of` query in the future which
might be far faster, especially if specialized for "const" cases --
match arms being the most obvious example. It's possibly such a function
would benefit from *not* being a query as well, since it's trivially
evaluatable from the sty for many cases whereas a query needs to hash
the input and such.
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Mark-Simulacrum committed Sep 21, 2019
1 parent 06c6894 commit 06c6e75aae8918f2d7e5ab7089b36c7ac9ec73bb
Showing with 16 additions and 2 deletions.
  1. +16 −2 src/librustc/ty/sty.rs
@@ -13,7 +13,7 @@ use rustc_macros::HashStable;
use crate::ty::subst::{InternalSubsts, Subst, SubstsRef, GenericArg, GenericArgKind};
use crate::ty::{self, AdtDef, Discr, DefIdTree, TypeFlags, Ty, TyCtxt, TypeFoldable};
use crate::ty::{List, TyS, ParamEnvAnd, ParamEnv};
use crate::ty::layout::VariantIdx;
use crate::ty::layout::{Size, Integer, IntegerExt, VariantIdx};
use crate::util::captures::Captures;
use crate::mir::interpret::{Scalar, GlobalId};

@@ -24,6 +24,7 @@ use std::marker::PhantomData;
use std::ops::Range;
use rustc_target::spec::abi;
use syntax::ast::{self, Ident};
use syntax::attr::{SignedInt, UnsignedInt};
use syntax::symbol::{kw, InternedString};

use self::InferTy::*;
@@ -2298,8 +2299,21 @@ impl<'tcx> Const<'tcx> {
ty: Ty<'tcx>,
) -> Option<u128> {
assert_eq!(self.ty, ty);
// This is purely an optimization -- layout_of is a pretty expensive operation,
// but if we can determine the size without calling it, we don't need all that complexity
// (hashing, caching, etc.). As such, try to skip it.
let size = match ty.kind {
ty::Bool => Size::from_bytes(1),
ty::Char => Size::from_bytes(4),
ty::Int(ity) => {
Integer::from_attr(&tcx, SignedInt(ity)).size()
}
ty::Uint(uty) => {
Integer::from_attr(&tcx, UnsignedInt(uty)).size()
}
_ => tcx.layout_of(param_env.with_reveal_all().and(ty)).ok()?.size,
};
// if `ty` does not depend on generic parameters, use an empty param_env
let size = tcx.layout_of(param_env.with_reveal_all().and(ty)).ok()?.size;
self.eval(tcx, param_env).val.try_to_bits(size)
}

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