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This makes it possible to mutably borrow different fields of the MIR body without resorting to methods like `basic_blocks_local_decls_mut_and_var_debug_info`. To preserve validity of control flow graph caches in the presence of modifications, a new struct `BasicBlocks` wraps together basic blocks and control flow graph caches. The `BasicBlocks` dereferences to `IndexVec<BasicBlock, BasicBlockData>`. On the other hand a mutable access requires explicit `as_mut()` call.
87 lines
2.9 KiB
Rust
87 lines
2.9 KiB
Rust
//! Removes assignments to ZST places.
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use crate::MirPass;
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use rustc_middle::mir::tcx::PlaceTy;
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use rustc_middle::mir::{Body, LocalDecls, Place, StatementKind};
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use rustc_middle::ty::{self, Ty, TyCtxt};
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pub struct RemoveZsts;
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impl<'tcx> MirPass<'tcx> for RemoveZsts {
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fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
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sess.mir_opt_level() > 0
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}
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fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
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// Avoid query cycles (generators require optimized MIR for layout).
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if tcx.type_of(body.source.def_id()).is_generator() {
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return;
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}
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let param_env = tcx.param_env(body.source.def_id());
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let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
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let local_decls = &body.local_decls;
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for block in basic_blocks {
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for statement in block.statements.iter_mut() {
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if let StatementKind::Assign(box (place, _)) | StatementKind::Deinit(box place) =
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statement.kind
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{
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let place_ty = place.ty(local_decls, tcx).ty;
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if !maybe_zst(place_ty) {
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continue;
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}
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let Ok(layout) = tcx.layout_of(param_env.and(place_ty)) else {
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continue;
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};
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if !layout.is_zst() {
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continue;
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}
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if involves_a_union(place, local_decls, tcx) {
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continue;
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}
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if tcx.consider_optimizing(|| {
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format!(
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"RemoveZsts - Place: {:?} SourceInfo: {:?}",
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place, statement.source_info
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)
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}) {
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statement.make_nop();
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}
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}
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}
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}
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}
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}
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/// A cheap, approximate check to avoid unnecessary `layout_of` calls.
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fn maybe_zst(ty: Ty<'_>) -> bool {
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match ty.kind() {
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// maybe ZST (could be more precise)
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ty::Adt(..) | ty::Array(..) | ty::Closure(..) | ty::Tuple(..) | ty::Opaque(..) => true,
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// definitely ZST
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ty::FnDef(..) | ty::Never => true,
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// unreachable or can't be ZST
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_ => false,
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}
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}
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/// Miri lazily allocates memory for locals on assignment,
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/// so we must preserve writes to unions and union fields,
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/// or it will ICE on reads of those fields.
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fn involves_a_union<'tcx>(
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place: Place<'tcx>,
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local_decls: &LocalDecls<'tcx>,
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tcx: TyCtxt<'tcx>,
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) -> bool {
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let mut place_ty = PlaceTy::from_ty(local_decls[place.local].ty);
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if place_ty.ty.is_union() {
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return true;
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}
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for elem in place.projection {
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place_ty = place_ty.projection_ty(tcx, elem);
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if place_ty.ty.is_union() {
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return true;
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}
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}
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return false;
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}
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