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Auto merge of #110728 - cjgillot:no-false-optes, r=oli-obk

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Do not bother optimizing impossible functions.

This is currently checked by `ConstProp`, but I see no reason to restrict it to ConstProp only.
This commit is contained in:
bors 2023-04-27 04:29:49 +00:00
commit 8b8110e146
2 changed files with 46 additions and 38 deletions
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37
compiler/rustc_mir_transform/src/const_prop.rs
View File

@ -18,7 +18,6 @@
use rustc_span::{def_id::DefId, Span, DUMMY_SP};
use rustc_target::abi::{self, Align, HasDataLayout, Size, TargetDataLayout};
use rustc_target::spec::abi::Abi as CallAbi;
use rustc_trait_selection::traits;
use crate::MirPass;
use rustc_const_eval::interpret::{
@ -84,42 +83,6 @@ fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
return;
}
// Check if it's even possible to satisfy the 'where' clauses
// for this item.
// This branch will never be taken for any normal function.
// However, it's possible to `#!feature(trivial_bounds)]` to write
// a function with impossible to satisfy clauses, e.g.:
// `fn foo() where String: Copy {}`
//
// We don't usually need to worry about this kind of case,
// since we would get a compilation error if the user tried
// to call it. However, since we can do const propagation
// even without any calls to the function, we need to make
// sure that it even makes sense to try to evaluate the body.
// If there are unsatisfiable where clauses, then all bets are
// off, and we just give up.
//
// We manually filter the predicates, skipping anything that's not
// "global". We are in a potentially generic context
// (e.g. we are evaluating a function without substituting generic
// parameters, so this filtering serves two purposes:
//
// 1. We skip evaluating any predicates that we would
// never be able prove are unsatisfiable (e.g. `<T as Foo>`
// 2. We avoid trying to normalize predicates involving generic
// parameters (e.g. `<T as Foo>::MyItem`). This can confuse
// the normalization code (leading to cycle errors), since
// it's usually never invoked in this way.
let predicates = tcx
.predicates_of(def_id.to_def_id())
.predicates
.iter()
.filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
if traits::impossible_predicates(tcx, traits::elaborate(tcx, predicates).collect()) {
trace!("ConstProp skipped for {:?}: found unsatisfiable predicates", def_id);
return;
}
trace!("ConstProp starting for {:?}", def_id);
let dummy_body = &Body::new(

47
compiler/rustc_mir_transform/src/lib.rs
View File

@ -30,11 +30,12 @@
use rustc_middle::mir::{
traversal, AnalysisPhase, Body, ClearCrossCrate, ConstQualifs, Constant, LocalDecl, MirPass,
MirPhase, Operand, Place, ProjectionElem, Promoted, RuntimePhase, Rvalue, SourceInfo,
Statement, StatementKind, TerminatorKind,
Statement, StatementKind, TerminatorKind, START_BLOCK,
};
use rustc_middle::ty::query::Providers;
use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
use rustc_span::sym;
use rustc_trait_selection::traits;
#[macro_use]
mod pass_manager;
@ -389,6 +390,50 @@ fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &
body.tainted_by_errors = Some(error_reported);
}
// Check if it's even possible to satisfy the 'where' clauses
// for this item.
//
// This branch will never be taken for any normal function.
// However, it's possible to `#!feature(trivial_bounds)]` to write
// a function with impossible to satisfy clauses, e.g.:
// `fn foo() where String: Copy {}`
//
// We don't usually need to worry about this kind of case,
// since we would get a compilation error if the user tried
// to call it. However, since we optimize even without any
// calls to the function, we need to make sure that it even
// makes sense to try to evaluate the body.
//
// If there are unsatisfiable where clauses, then all bets are
// off, and we just give up.
//
// We manually filter the predicates, skipping anything that's not
// "global". We are in a potentially generic context
// (e.g. we are evaluating a function without substituting generic
// parameters, so this filtering serves two purposes:
//
// 1. We skip evaluating any predicates that we would
// never be able prove are unsatisfiable (e.g. `<T as Foo>`
// 2. We avoid trying to normalize predicates involving generic
// parameters (e.g. `<T as Foo>::MyItem`). This can confuse
// the normalization code (leading to cycle errors), since
// it's usually never invoked in this way.
let predicates = tcx
.predicates_of(body.source.def_id())
.predicates
.iter()
.filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
if traits::impossible_predicates(tcx, traits::elaborate(tcx, predicates).collect()) {
trace!("found unsatisfiable predicates for {:?}", body.source);
// Clear the body to only contain a single `unreachable` statement.
let bbs = body.basic_blocks.as_mut();
bbs.raw.truncate(1);
bbs[START_BLOCK].statements.clear();
bbs[START_BLOCK].terminator_mut().kind = TerminatorKind::Unreachable;
body.var_debug_info.clear();
body.local_decls.raw.truncate(body.arg_count + 1);
}
run_analysis_to_runtime_passes(tcx, &mut body);
tcx.alloc_steal_mir(body)