compiler/rustc_mir_transform/src/remove_zsts.rs

   1 //! Removes assignments to ZST places.
   2
   3 use crate::MirPass;
   4 use rustc_middle::mir::tcx::PlaceTy;
   5 use rustc_middle::mir::{Body, LocalDecls, Place, StatementKind};
   6 use rustc_middle::ty::{self, Ty, TyCtxt};
   7
   8 pub struct RemoveZsts;
   9
  10 impl<'tcx> MirPass<'tcx> for RemoveZsts {
  11     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
  12         sess.mir_opt_level() > 0
  13     }
  14
  15     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
  16         // Avoid query cycles (generators require optimized MIR for layout).
  17         if tcx.type_of(body.source.def_id()).is_generator() {
  18             return;
  19         }
  20         let param_env = tcx.param_env(body.source.def_id());
  21         let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
  22         for block in basic_blocks.iter_mut() {
  23             for statement in block.statements.iter_mut() {
  24                 if let StatementKind::Assign(box (place, _)) = statement.kind {
  25                     let place_ty = place.ty(local_decls, tcx).ty;
  26                     if !maybe_zst(place_ty) {
  27                         continue;
  28                     }
  29                     let layout = match tcx.layout_of(param_env.and(place_ty)) {
  30                         Ok(layout) => layout,
  31                         Err(_) => continue,
  32                     };
  33                     if !layout.is_zst() {
  34                         continue;
  35                     }
  36                     if involves_a_union(place, local_decls, tcx) {
  37                         continue;
  38                     }
  39                     if tcx.consider_optimizing(|| {
  40                         format!(
  41                             "RemoveZsts - Place: {:?} SourceInfo: {:?}",
  42                             place, statement.source_info
  43                         )
  44                     }) {
  45                         statement.make_nop();
  46                     }
  47                 }
  48             }
  49         }
  50     }
  51 }
  52
  53 /// A cheap, approximate check to avoid unnecessary `layout_of` calls.
  54 fn maybe_zst(ty: Ty<'_>) -> bool {
  55     match ty.kind() {
  56         // maybe ZST (could be more precise)
  57         ty::Adt(..) | ty::Array(..) | ty::Closure(..) | ty::Tuple(..) | ty::Opaque(..) => true,
  58         // definitely ZST
  59         ty::FnDef(..) | ty::Never => true,
  60         // unreachable or can't be ZST
  61         _ => false,
  62     }
  63 }
  64
  65 /// Miri lazily allocates memory for locals on assignment,
  66 /// so we must preserve writes to unions and union fields,
  67 /// or it will ICE on reads of those fields.
  68 fn involves_a_union<'tcx>(
  69     place: Place<'tcx>,
  70     local_decls: &LocalDecls<'tcx>,
  71     tcx: TyCtxt<'tcx>,
  72 ) -> bool {
  73     let mut place_ty = PlaceTy::from_ty(local_decls[place.local].ty);
  74     if place_ty.ty.is_union() {
  75         return true;
  76     }
  77     for elem in place.projection {
  78         place_ty = place_ty.projection_ty(tcx, elem);
  79         if place_ty.ty.is_union() {
  80             return true;
  81         }
  82     }
  83     return false;
  84 }