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Move unstatisfaction check earlier.

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This commit is contained in:
Camille GILLOT 2023-04-25 17:11:46 +00:00
parent 0f857791ad
commit 0ee32fb3c7
1 changed files with 44 additions and 50 deletions
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94
compiler/rustc_mir_transform/src/lib.rs
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@ -390,6 +390,50 @@ fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &
body.tainted_by_errors = Some(error_reported); 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); run_analysis_to_runtime_passes(tcx, &mut body);
tcx.alloc_steal_mir(body) tcx.alloc_steal_mir(body)
@ -482,56 +526,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
WithMinOptLevel(1, x) WithMinOptLevel(1, x)
} }
// 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!("optimizations skipped for {:?}: found unsatisfiable predicates", 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);
pm::run_passes(
tcx,
body,
&[&reveal_all::RevealAll, &dump_mir::Marker("PreCodegen")],
Some(MirPhase::Runtime(RuntimePhase::Optimized)),
);
return;
}
// The main optimizations that we do on MIR. // The main optimizations that we do on MIR.
pm::run_passes( pm::run_passes(
tcx, tcx,