Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
420 lines (375 sloc) 16 KB
//! An analysis to determine which locals require allocas and
//! which do not.
use rustc_data_structures::bit_set::BitSet;
use rustc_data_structures::graph::dominators::Dominators;
use rustc_data_structures::indexed_vec::{Idx, IndexVec};
use rustc::mir::{self, Location, TerminatorKind};
use rustc::mir::visit::{Visitor, PlaceContext, MutatingUseContext, NonMutatingUseContext};
use rustc::mir::traversal;
use rustc::ty;
use rustc::ty::layout::{LayoutOf, HasTyCtxt};
use syntax_pos::DUMMY_SP;
use super::FunctionCx;
use crate::traits::*;
pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
fx: &FunctionCx<'a, 'tcx, Bx>,
) -> BitSet<mir::Local> {
let mir = fx.mir;
let mut analyzer = LocalAnalyzer::new(fx);
analyzer.visit_body(mir);
for (index, (ty, span)) in mir.local_decls.iter()
.map(|l| (l.ty, l.source_info.span))
.enumerate()
{
let ty = fx.monomorphize(&ty);
debug!("local {} has type {:?}", index, ty);
let layout = fx.cx.spanned_layout_of(ty, span);
if fx.cx.is_backend_immediate(layout) {
// These sorts of types are immediates that we can store
// in an Value without an alloca.
} else if fx.cx.is_backend_scalar_pair(layout) {
// We allow pairs and uses of any of their 2 fields.
} else {
// These sorts of types require an alloca. Note that
// is_llvm_immediate() may *still* be true, particularly
// for newtypes, but we currently force some types
// (e.g., structs) into an alloca unconditionally, just so
// that we don't have to deal with having two pathways
// (gep vs extractvalue etc).
analyzer.not_ssa(mir::Local::new(index));
}
}
analyzer.non_ssa_locals
}
struct LocalAnalyzer<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> {
fx: &'mir FunctionCx<'a, 'tcx, Bx>,
dominators: Dominators<mir::BasicBlock>,
non_ssa_locals: BitSet<mir::Local>,
// The location of the first visited direct assignment to each
// local, or an invalid location (out of bounds `block` index).
first_assignment: IndexVec<mir::Local, Location>,
}
impl<Bx: BuilderMethods<'a, 'tcx>> LocalAnalyzer<'mir, 'a, 'tcx, Bx> {
fn new(fx: &'mir FunctionCx<'a, 'tcx, Bx>) -> Self {
let invalid_location =
mir::BasicBlock::new(fx.mir.basic_blocks().len()).start_location();
let mut analyzer = LocalAnalyzer {
fx,
dominators: fx.mir.dominators(),
non_ssa_locals: BitSet::new_empty(fx.mir.local_decls.len()),
first_assignment: IndexVec::from_elem(invalid_location, &fx.mir.local_decls)
};
// Arguments get assigned to by means of the function being called
for arg in fx.mir.args_iter() {
analyzer.first_assignment[arg] = mir::START_BLOCK.start_location();
}
analyzer
}
fn first_assignment(&self, local: mir::Local) -> Option<Location> {
let location = self.first_assignment[local];
if location.block.index() < self.fx.mir.basic_blocks().len() {
Some(location)
} else {
None
}
}
fn not_ssa(&mut self, local: mir::Local) {
debug!("marking {:?} as non-SSA", local);
self.non_ssa_locals.insert(local);
}
fn assign(&mut self, local: mir::Local, location: Location) {
if self.first_assignment(local).is_some() {
self.not_ssa(local);
} else {
self.first_assignment[local] = location;
}
}
fn process_place(
&mut self,
place_ref: &mir::PlaceRef<'_, 'tcx>,
context: PlaceContext,
location: Location,
) {
let cx = self.fx.cx;
if let [proj_base @ .., elem] = place_ref.projection {
// Allow uses of projections that are ZSTs or from scalar fields.
let is_consume = match context {
PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => true,
_ => false
};
if is_consume {
let base_ty =
mir::Place::ty_from(place_ref.base, proj_base, self.fx.mir, cx.tcx());
let base_ty = self.fx.monomorphize(&base_ty);
// ZSTs don't require any actual memory access.
let elem_ty = base_ty
.projection_ty(cx.tcx(), elem)
.ty;
let elem_ty = self.fx.monomorphize(&elem_ty);
let span = if let mir::PlaceBase::Local(index) = place_ref.base {
self.fx.mir.local_decls[*index].source_info.span
} else {
DUMMY_SP
};
if cx.spanned_layout_of(elem_ty, span).is_zst() {
return;
}
if let mir::ProjectionElem::Field(..) = elem {
let layout = cx.spanned_layout_of(base_ty.ty, span);
if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) {
// Recurse with the same context, instead of `Projection`,
// potentially stopping at non-operand projections,
// which would trigger `not_ssa` on locals.
self.process_place(
&mir::PlaceRef {
base: place_ref.base,
projection: proj_base,
},
context,
location,
);
return;
}
}
}
// A deref projection only reads the pointer, never needs the place.
if let mir::ProjectionElem::Deref = elem {
self.process_place(
&mir::PlaceRef {
base: place_ref.base,
projection: proj_base,
},
PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
location
);
return;
}
}
// FIXME this is super_place code, is repeated here to avoid cloning place or changing
// visit_place API
let mut context = context;
if !place_ref.projection.is_empty() {
context = if context.is_mutating_use() {
PlaceContext::MutatingUse(MutatingUseContext::Projection)
} else {
PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
};
}
self.visit_place_base(place_ref.base, context, location);
self.visit_projection(place_ref.base, place_ref.projection, context, location);
}
}
impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
for LocalAnalyzer<'mir, 'a, 'tcx, Bx>
{
fn visit_assign(&mut self,
place: &mir::Place<'tcx>,
rvalue: &mir::Rvalue<'tcx>,
location: Location) {
debug!("visit_assign(place={:?}, rvalue={:?})", place, rvalue);
if let mir::Place {
base: mir::PlaceBase::Local(index),
projection: box [],
} = *place {
self.assign(index, location);
let decl_span = self.fx.mir.local_decls[index].source_info.span;
if !self.fx.rvalue_creates_operand(rvalue, decl_span) {
self.not_ssa(index);
}
} else {
self.visit_place(
place,
PlaceContext::MutatingUse(MutatingUseContext::Store),
location
);
}
self.visit_rvalue(rvalue, location);
}
fn visit_terminator_kind(&mut self,
kind: &mir::TerminatorKind<'tcx>,
location: Location) {
let check = match *kind {
mir::TerminatorKind::Call {
func: mir::Operand::Constant(ref c),
ref args, ..
} => match c.literal.ty.sty {
ty::FnDef(did, _) => Some((did, args)),
_ => None,
},
_ => None,
};
if let Some((def_id, args)) = check {
if Some(def_id) == self.fx.cx.tcx().lang_items().box_free_fn() {
// box_free(x) shares with `drop x` the property that it
// is not guaranteed to be statically dominated by the
// definition of x, so x must always be in an alloca.
if let mir::Operand::Move(ref place) = args[0] {
self.visit_place(
place,
PlaceContext::MutatingUse(MutatingUseContext::Drop),
location
);
}
}
}
self.super_terminator_kind(kind, location);
}
fn visit_place(&mut self,
place: &mir::Place<'tcx>,
context: PlaceContext,
location: Location) {
debug!("visit_place(place={:?}, context={:?})", place, context);
self.process_place(&place.as_ref(), context, location);
}
fn visit_local(&mut self,
&local: &mir::Local,
context: PlaceContext,
location: Location) {
match context {
PlaceContext::MutatingUse(MutatingUseContext::Call) => {
self.assign(local, location);
}
PlaceContext::NonUse(_) |
PlaceContext::MutatingUse(MutatingUseContext::Retag) => {}
PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::Move) => {
// Reads from uninitialized variables (e.g., in dead code, after
// optimizations) require locals to be in (uninitialized) memory.
// N.B., there can be uninitialized reads of a local visited after
// an assignment to that local, if they happen on disjoint paths.
let ssa_read = match self.first_assignment(local) {
Some(assignment_location) => {
assignment_location.dominates(location, &self.dominators)
}
None => false
};
if !ssa_read {
self.not_ssa(local);
}
}
PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect) |
PlaceContext::MutatingUse(MutatingUseContext::Store) |
PlaceContext::MutatingUse(MutatingUseContext::AsmOutput) |
PlaceContext::MutatingUse(MutatingUseContext::Borrow) |
PlaceContext::MutatingUse(MutatingUseContext::Projection) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow) |
PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection) => {
self.not_ssa(local);
}
PlaceContext::MutatingUse(MutatingUseContext::Drop) => {
let ty = self.fx.mir.local_decls[local].ty;
let ty = self.fx.monomorphize(&ty);
// Only need the place if we're actually dropping it.
if self.fx.cx.type_needs_drop(ty) {
self.not_ssa(local);
}
}
}
}
}
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum CleanupKind {
NotCleanup,
Funclet,
Internal { funclet: mir::BasicBlock }
}
impl CleanupKind {
pub fn funclet_bb(self, for_bb: mir::BasicBlock) -> Option<mir::BasicBlock> {
match self {
CleanupKind::NotCleanup => None,
CleanupKind::Funclet => Some(for_bb),
CleanupKind::Internal { funclet } => Some(funclet),
}
}
}
pub fn cleanup_kinds(mir: &mir::Body<'_>) -> IndexVec<mir::BasicBlock, CleanupKind> {
fn discover_masters<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
mir: &mir::Body<'tcx>) {
for (bb, data) in mir.basic_blocks().iter_enumerated() {
match data.terminator().kind {
TerminatorKind::Goto { .. } |
TerminatorKind::Resume |
TerminatorKind::Abort |
TerminatorKind::Return |
TerminatorKind::GeneratorDrop |
TerminatorKind::Unreachable |
TerminatorKind::SwitchInt { .. } |
TerminatorKind::Yield { .. } |
TerminatorKind::FalseEdges { .. } |
TerminatorKind::FalseUnwind { .. } => {
/* nothing to do */
}
TerminatorKind::Call { cleanup: unwind, .. } |
TerminatorKind::Assert { cleanup: unwind, .. } |
TerminatorKind::DropAndReplace { unwind, .. } |
TerminatorKind::Drop { unwind, .. } => {
if let Some(unwind) = unwind {
debug!("cleanup_kinds: {:?}/{:?} registering {:?} as funclet",
bb, data, unwind);
result[unwind] = CleanupKind::Funclet;
}
}
}
}
}
fn propagate<'tcx>(result: &mut IndexVec<mir::BasicBlock, CleanupKind>,
mir: &mir::Body<'tcx>) {
let mut funclet_succs = IndexVec::from_elem(None, mir.basic_blocks());
let mut set_successor = |funclet: mir::BasicBlock, succ| {
match funclet_succs[funclet] {
ref mut s @ None => {
debug!("set_successor: updating successor of {:?} to {:?}",
funclet, succ);
*s = Some(succ);
},
Some(s) => if s != succ {
span_bug!(mir.span, "funclet {:?} has 2 parents - {:?} and {:?}",
funclet, s, succ);
}
}
};
for (bb, data) in traversal::reverse_postorder(mir) {
let funclet = match result[bb] {
CleanupKind::NotCleanup => continue,
CleanupKind::Funclet => bb,
CleanupKind::Internal { funclet } => funclet,
};
debug!("cleanup_kinds: {:?}/{:?}/{:?} propagating funclet {:?}",
bb, data, result[bb], funclet);
for &succ in data.terminator().successors() {
let kind = result[succ];
debug!("cleanup_kinds: propagating {:?} to {:?}/{:?}",
funclet, succ, kind);
match kind {
CleanupKind::NotCleanup => {
result[succ] = CleanupKind::Internal { funclet };
}
CleanupKind::Funclet => {
if funclet != succ {
set_successor(funclet, succ);
}
}
CleanupKind::Internal { funclet: succ_funclet } => {
if funclet != succ_funclet {
// `succ` has 2 different funclet going into it, so it must
// be a funclet by itself.
debug!("promoting {:?} to a funclet and updating {:?}", succ,
succ_funclet);
result[succ] = CleanupKind::Funclet;
set_successor(succ_funclet, succ);
set_successor(funclet, succ);
}
}
}
}
}
}
let mut result = IndexVec::from_elem(CleanupKind::NotCleanup, mir.basic_blocks());
discover_masters(&mut result, mir);
propagate(&mut result, mir);
debug!("cleanup_kinds: result={:?}", result);
result
}
You can’t perform that action at this time.