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Auto merge of #108872 - cjgillot:simp-const-prop, r=oli-obk

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Strengthen state tracking in const-prop

Some/many of the changes are replicated between both the const-prop lint and the const-prop optimization.

Behaviour changes:
- const-prop opt does not give a span to propagated values. This was useless as that span's primary purpose is to diagnose evaluation failure in codegen.
- we remove the `OnlyPropagateInto` mode. It was only used for function arguments, which are better modeled by a write before entry.
- the tracking of assignments and discriminants make clearer that we do nothing in `NoPropagation` mode or on indirect places.
This commit is contained in:
bors 2023-03-12 23:27:52 +00:00
commit b05bb29008
14 changed files with 254 additions and 335 deletions
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269
compiler/rustc_mir_transform/src/const_prop.rs
View file

@ -1,12 +1,9 @@
//! Propagates constants for early reporting of statically known
//! assertion failures
use std::cell::Cell;
use either::Right;
use rustc_const_eval::const_eval::CheckAlignment;
use rustc_data_structures::fx::FxHashSet;
use rustc_hir::def::DefKind;
use rustc_index::bit_set::BitSet;
use rustc_index::vec::IndexVec;
@ -17,7 +14,7 @@ use rustc_middle::mir::*;
use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
use rustc_middle::ty::InternalSubsts;
use rustc_middle::ty::{self, ConstKind, Instance, ParamEnv, Ty, TyCtxt, TypeVisitableExt};
use rustc_span::{def_id::DefId, Span};
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;
@ -25,8 +22,8 @@ use rustc_trait_selection::traits;
use crate::MirPass;
use rustc_const_eval::interpret::{
self, compile_time_machine, AllocId, ConstAllocation, ConstValue, CtfeValidationMode, Frame,
ImmTy, Immediate, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, PlaceTy,
Pointer, Scalar, StackPopCleanup, StackPopUnwind,
ImmTy, Immediate, InterpCx, InterpResult, LocalValue, MemoryKind, OpTy, PlaceTy, Pointer,
Scalar, StackPopCleanup, StackPopUnwind,
};
/// The maximum number of bytes that we'll allocate space for a local or the return value.
@ -154,24 +151,12 @@ impl<'tcx> MirPass<'tcx> for ConstProp {
pub struct ConstPropMachine<'mir, 'tcx> {
/// The virtual call stack.
stack: Vec<Frame<'mir, 'tcx>>,
/// `OnlyInsideOwnBlock` locals that were written in the current block get erased at the end.
pub written_only_inside_own_block_locals: FxHashSet<Local>,
/// Locals that need to be cleared after every block terminates.
pub only_propagate_inside_block_locals: BitSet<Local>,
pub can_const_prop: IndexVec<Local, ConstPropMode>,
}
impl ConstPropMachine<'_, '_> {
pub fn new(
only_propagate_inside_block_locals: BitSet<Local>,
can_const_prop: IndexVec<Local, ConstPropMode>,
) -> Self {
Self {
stack: Vec::new(),
written_only_inside_own_block_locals: Default::default(),
only_propagate_inside_block_locals,
can_const_prop,
}
pub fn new(can_const_prop: IndexVec<Local, ConstPropMode>) -> Self {
Self { stack: Vec::new(), can_const_prop }
}
}
@ -257,16 +242,14 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
frame: usize,
local: Local,
) -> InterpResult<'tcx, &'a mut interpret::Operand<Self::Provenance>> {
if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
}
if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
trace!(
"mutating local {:?} which is restricted to its block. \
Will remove it from const-prop after block is finished.",
local
);
ecx.machine.written_only_inside_own_block_locals.insert(local);
assert_eq!(frame, 0);
match ecx.machine.can_const_prop[local] {
ConstPropMode::NoPropagation => {
throw_machine_stop_str!(
"tried to write to a local that is marked as not propagatable"
)
}
ConstPropMode::OnlyInsideOwnBlock | ConstPropMode::FullConstProp => {}
}
ecx.machine.stack[frame].locals[local].access_mut()
}
@ -328,9 +311,6 @@ struct ConstPropagator<'mir, 'tcx> {
tcx: TyCtxt<'tcx>,
param_env: ParamEnv<'tcx>,
local_decls: &'mir IndexVec<Local, LocalDecl<'tcx>>,
// Because we have `MutVisitor` we can't obtain the `SourceInfo` from a `Location`. So we store
// the last known `SourceInfo` here and just keep revisiting it.
source_info: Option<SourceInfo>,
}
impl<'tcx> LayoutOfHelpers<'tcx> for ConstPropagator<'_, 'tcx> {
@ -374,17 +354,11 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
let param_env = tcx.param_env_reveal_all_normalized(def_id);
let can_const_prop = CanConstProp::check(tcx, param_env, body);
let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
for (l, mode) in can_const_prop.iter_enumerated() {
if *mode == ConstPropMode::OnlyInsideOwnBlock {
only_propagate_inside_block_locals.insert(l);
}
}
let mut ecx = InterpCx::new(
tcx,
tcx.def_span(def_id),
param_env,
ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
ConstPropMachine::new(can_const_prop),
);
let ret_layout = ecx
@ -411,13 +385,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
)
.expect("failed to push initial stack frame");
ConstPropagator {
ecx,
tcx,
param_env,
local_decls: &dummy_body.local_decls,
source_info: None,
}
ConstPropagator { ecx, tcx, param_env, local_decls: &dummy_body.local_decls }
}
fn get_const(&self, place: Place<'tcx>) -> Option<OpTy<'tcx>> {
@ -446,10 +414,8 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
/// Remove `local` from the pool of `Locals`. Allows writing to them,
/// but not reading from them anymore.
fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
ecx.frame_mut().locals[local] = LocalState {
value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
layout: Cell::new(None),
};
ecx.frame_mut().locals[local].value =
LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit));
}
/// Returns the value, if any, of evaluating `c`.
@ -492,11 +458,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
scalar,
)) = *value
{
*operand = self.operand_from_scalar(
scalar,
value.layout.ty,
self.source_info.unwrap().span,
);
*operand = self.operand_from_scalar(scalar, value.layout.ty);
}
}
}
@ -504,7 +466,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
}
}
fn const_prop(&mut self, rvalue: &Rvalue<'tcx>, place: Place<'tcx>) -> Option<()> {
fn check_rvalue(&mut self, rvalue: &Rvalue<'tcx>) -> Option<()> {
// Perform any special handling for specific Rvalue types.
// Generally, checks here fall into one of two categories:
// 1. Additional checking to provide useful lints to the user
@ -561,7 +523,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
return None;
}
self.eval_rvalue_with_identities(rvalue, place)
Some(())
}
// Attempt to use algebraic identities to eliminate constant expressions
@ -621,20 +583,24 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
}
/// Creates a new `Operand::Constant` from a `Scalar` value
fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>, span: Span) -> Operand<'tcx> {
fn operand_from_scalar(&self, scalar: Scalar, ty: Ty<'tcx>) -> Operand<'tcx> {
Operand::Constant(Box::new(Constant {
span,
span: DUMMY_SP,
user_ty: None,
literal: ConstantKind::from_scalar(self.tcx, scalar, ty),
}))
}
fn replace_with_const(
&mut self,
rval: &mut Rvalue<'tcx>,
value: &OpTy<'tcx>,
source_info: SourceInfo,
) {
fn replace_with_const(&mut self, place: Place<'tcx>, rval: &mut Rvalue<'tcx>) {
// This will return None if the above `const_prop` invocation only "wrote" a
// type whose creation requires no write. E.g. a generator whose initial state
// consists solely of uninitialized memory (so it doesn't capture any locals).
let Some(ref value) = self.get_const(place) else { return };
if !self.should_const_prop(value) {
return;
}
trace!("replacing {:?}={:?} with {:?}", place, rval, value);
if let Rvalue::Use(Operand::Constant(c)) = rval {
match c.literal {
ConstantKind::Ty(c) if matches!(c.kind(), ConstKind::Unevaluated(..)) => {}
@ -664,11 +630,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
if let Some(Right(imm)) = imm {
match *imm {
interpret::Immediate::Scalar(scalar) => {
*rval = Rvalue::Use(self.operand_from_scalar(
scalar,
value.layout.ty,
source_info.span,
));
*rval = Rvalue::Use(self.operand_from_scalar(scalar, value.layout.ty));
}
Immediate::ScalarPair(..) => {
// Found a value represented as a pair. For now only do const-prop if the type
@ -701,7 +663,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
let const_val = ConstValue::ByRef { alloc, offset: Size::ZERO };
let literal = ConstantKind::Val(const_val, ty);
*rval = Rvalue::Use(Operand::Constant(Box::new(Constant {
span: source_info.span,
span: DUMMY_SP,
user_ty: None,
literal,
})));
@ -730,6 +692,19 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
_ => false,
}
}
fn ensure_not_propagated(&mut self, local: Local) {
if cfg!(debug_assertions) {
assert!(
self.get_const(local.into()).is_none()
|| self
.layout_of(self.local_decls[local].ty)
.map_or(true, |layout| layout.is_zst()),
"failed to remove values for `{local:?}`, value={:?}",
self.get_const(local.into()),
)
}
}
}
/// The mode that `ConstProp` is allowed to run in for a given `Local`.
@ -739,8 +714,6 @@ pub enum ConstPropMode {
FullConstProp,
/// The `Local` can only be propagated into and from its own block.
OnlyInsideOwnBlock,
/// The `Local` can be propagated into but reads cannot be propagated.
OnlyPropagateInto,
/// The `Local` cannot be part of propagation at all. Any statement
/// referencing it either for reading or writing will not get propagated.
NoPropagation,
@ -750,8 +723,6 @@ pub struct CanConstProp {
can_const_prop: IndexVec<Local, ConstPropMode>,
// False at the beginning. Once set, no more assignments are allowed to that local.
found_assignment: BitSet<Local>,
// Cache of locals' information
local_kinds: IndexVec<Local, LocalKind>,
}
impl CanConstProp {
@ -764,10 +735,6 @@ impl CanConstProp {
let mut cpv = CanConstProp {
can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
found_assignment: BitSet::new_empty(body.local_decls.len()),
local_kinds: IndexVec::from_fn_n(
|local| body.local_kind(local),
body.local_decls.len(),
),
};
for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
let ty = body.local_decls[local].ty;
@ -780,24 +747,10 @@ impl CanConstProp {
continue;
}
}
// Cannot use args at all
// Cannot use locals because if x < y { y - x } else { x - y } would
// lint for x != y
// FIXME(oli-obk): lint variables until they are used in a condition
// FIXME(oli-obk): lint if return value is constant
if cpv.local_kinds[local] == LocalKind::Arg {
*val = ConstPropMode::OnlyPropagateInto;
trace!(
"local {:?} can't be const propagated because it's a function argument",
local
);
} else if cpv.local_kinds[local] == LocalKind::Var {
*val = ConstPropMode::OnlyInsideOwnBlock;
trace!(
"local {:?} will only be propagated inside its block, because it's a user variable",
local
);
}
}
// Consider that arguments are assigned on entry.
for arg in body.args_iter() {
cpv.found_assignment.insert(arg);
}
cpv.visit_body(&body);
cpv.can_const_prop
@ -827,7 +780,6 @@ impl Visitor<'_> for CanConstProp {
// states as applicable.
ConstPropMode::OnlyInsideOwnBlock => {}
ConstPropMode::NoPropagation => {}
ConstPropMode::OnlyPropagateInto => {}
other @ ConstPropMode::FullConstProp => {
trace!(
"local {:?} can't be propagated because of multiple assignments. Previous state: {:?}",
@ -892,42 +844,23 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
self.eval_constant(constant);
}
fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
trace!("visit_statement: {:?}", statement);
let source_info = statement.source_info;
self.source_info = Some(source_info);
match statement.kind {
StatementKind::Assign(box (place, ref mut rval)) => {
let can_const_prop = self.ecx.machine.can_const_prop[place.local];
if let Some(()) = self.const_prop(rval, place) {
// This will return None if the above `const_prop` invocation only "wrote" a
// type whose creation requires no write. E.g. a generator whose initial state
// consists solely of uninitialized memory (so it doesn't capture any locals).
if let Some(ref value) = self.get_const(place) && self.should_const_prop(value) {
trace!("replacing {:?} with {:?}", rval, value);
self.replace_with_const(rval, value, source_info);
if can_const_prop == ConstPropMode::FullConstProp
|| can_const_prop == ConstPropMode::OnlyInsideOwnBlock
{
trace!("propagated into {:?}", place);
}
}
match can_const_prop {
ConstPropMode::OnlyInsideOwnBlock => {
trace!(
"found local restricted to its block. \
Will remove it from const-prop after block is finished. Local: {:?}",
place.local
);
}
ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
trace!("can't propagate into {:?}", place);
if place.local != RETURN_PLACE {
Self::remove_const(&mut self.ecx, place.local);
}
}
ConstPropMode::FullConstProp => {}
}
fn visit_assign(
&mut self,
place: &mut Place<'tcx>,
rvalue: &mut Rvalue<'tcx>,
location: Location,
) {
self.super_assign(place, rvalue, location);
let Some(()) = self.check_rvalue(rvalue) else { return };
match self.ecx.machine.can_const_prop[place.local] {
// Do nothing if the place is indirect.
_ if place.is_indirect() => {}
ConstPropMode::NoPropagation => self.ensure_not_propagated(place.local),
ConstPropMode::OnlyInsideOwnBlock | ConstPropMode::FullConstProp => {
if let Some(()) = self.eval_rvalue_with_identities(rvalue, *place) {
self.replace_with_const(*place, rvalue);
} else {
// Const prop failed, so erase the destination, ensuring that whatever happens
// from here on, does not know about the previous value.
@ -947,8 +880,22 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
Self::remove_const(&mut self.ecx, place.local);
}
}
}
}
fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
trace!("visit_statement: {:?}", statement);
// We want to evaluate operands before any change to the assigned-to value,
// so we recurse first.
self.super_statement(statement, location);
match statement.kind {
StatementKind::SetDiscriminant { ref place, .. } => {
match self.ecx.machine.can_const_prop[place.local] {
// Do nothing if the place is indirect.
_ if place.is_indirect() => {}
ConstPropMode::NoPropagation => self.ensure_not_propagated(place.local),
ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
if self.ecx.statement(statement).is_ok() {
trace!("propped discriminant into {:?}", place);
@ -956,28 +903,22 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
Self::remove_const(&mut self.ecx, place.local);
}
}
ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
Self::remove_const(&mut self.ecx, place.local);
}
}
}
StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
StatementKind::StorageLive(local) => {
let frame = self.ecx.frame_mut();
frame.locals[local].value = if let StatementKind::StorageLive(_) = statement.kind {
LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit))
} else {
LocalValue::Dead
};
frame.locals[local].value =
LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit));
}
StatementKind::StorageDead(local) => {
let frame = self.ecx.frame_mut();
frame.locals[local].value = LocalValue::Dead;
}
_ => {}
}
self.super_statement(statement, location);
}
fn visit_terminator(&mut self, terminator: &mut Terminator<'tcx>, location: Location) {
let source_info = terminator.source_info;
self.source_info = Some(source_info);
self.super_terminator(terminator, location);
match &mut terminator.kind {
@ -987,11 +928,7 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
&& self.should_const_prop(value)
{
trace!("assertion on {:?} should be {:?}", value, expected);
*cond = self.operand_from_scalar(
value_const,
self.tcx.types.bool,
source_info.span,
);
*cond = self.operand_from_scalar(value_const, self.tcx.types.bool);
}
}
TerminatorKind::SwitchInt { ref mut discr, .. } => {
@ -1026,23 +963,17 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
// We remove all Locals which are restricted in propagation to their containing blocks and
// which were modified in the current block.
// Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
for &local in locals.iter() {
Self::remove_const(&mut self.ecx, local);
}
locals.clear();
// Put it back so we reuse the heap of the storage
self.ecx.machine.written_only_inside_own_block_locals = locals;
if cfg!(debug_assertions) {
// Ensure we are correctly erasing locals with the non-debug-assert logic.
for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
assert!(
self.get_const(local.into()).is_none()
|| self
.layout_of(self.local_decls[local].ty)
.map_or(true, |layout| layout.is_zst())
)
let can_const_prop = std::mem::take(&mut self.ecx.machine.can_const_prop);
for (local, &mode) in can_const_prop.iter_enumerated() {
match mode {
ConstPropMode::FullConstProp => {}
ConstPropMode::NoPropagation => self.ensure_not_propagated(local),
ConstPropMode::OnlyInsideOwnBlock => {
Self::remove_const(&mut self.ecx, local);
self.ensure_not_propagated(local);
}
}
}
self.ecx.machine.can_const_prop = can_const_prop;
}
}

213
compiler/rustc_mir_transform/src/const_prop_lint.rs
View file

@ -1,24 +1,17 @@
//! Propagates constants for early reporting of statically known
//! assertion failures
use std::cell::Cell;
use either::{Left, Right};
use rustc_const_eval::interpret::Immediate;
use rustc_const_eval::interpret::{
self, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, Scalar, StackPopCleanup,
self, InterpCx, InterpResult, LocalValue, MemoryKind, OpTy, Scalar, StackPopCleanup,
};
use rustc_hir::def::DefKind;
use rustc_hir::HirId;
use rustc_index::bit_set::BitSet;
use rustc_index::vec::IndexVec;
use rustc_middle::mir::visit::Visitor;
use rustc_middle::mir::{
AssertKind, BinOp, Body, Constant, Local, LocalDecl, Location, Operand, Place, Rvalue,
SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind, Terminator, TerminatorKind,
UnOp, RETURN_PLACE,
};
use rustc_middle::mir::*;
use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
use rustc_middle::ty::InternalSubsts;
use rustc_middle::ty::{
@ -185,17 +178,11 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
let param_env = tcx.param_env_reveal_all_normalized(def_id);
let can_const_prop = CanConstProp::check(tcx, param_env, body);
let mut only_propagate_inside_block_locals = BitSet::new_empty(can_const_prop.len());
for (l, mode) in can_const_prop.iter_enumerated() {
if *mode == ConstPropMode::OnlyInsideOwnBlock {
only_propagate_inside_block_locals.insert(l);
}
}
let mut ecx = InterpCx::new(
tcx,
tcx.def_span(def_id),
param_env,
ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
ConstPropMachine::new(can_const_prop),
);
let ret_layout = ecx
@ -258,10 +245,8 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
/// Remove `local` from the pool of `Locals`. Allows writing to them,
/// but not reading from them anymore.
fn remove_const(ecx: &mut InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>, local: Local) {
ecx.frame_mut().locals[local] = LocalState {
value: LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit)),
layout: Cell::new(None),
};
ecx.frame_mut().locals[local].value =
LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit));
}
fn lint_root(&self, source_info: SourceInfo) -> Option<HirId> {
@ -420,12 +405,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
Some(())
}
fn const_prop(
&mut self,
rvalue: &Rvalue<'tcx>,
source_info: SourceInfo,
place: Place<'tcx>,
) -> Option<()> {
fn check_rvalue(&mut self, rvalue: &Rvalue<'tcx>, source_info: SourceInfo) -> Option<()> {
// Perform any special handling for specific Rvalue types.
// Generally, checks here fall into one of two categories:
// 1. Additional checking to provide useful lints to the user
@ -501,7 +481,20 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
return None;
}
self.use_ecx(source_info, |this| this.ecx.eval_rvalue_into_place(rvalue, place))
Some(())
}
fn ensure_not_propagated(&mut self, local: Local) {
if cfg!(debug_assertions) {
assert!(
self.get_const(local.into()).is_none()
|| self
.layout_of(self.local_decls[local].ty)
.map_or(true, |layout| layout.is_zst()),
"failed to remove values for `{local:?}`, value={:?}",
self.get_const(local.into()),
)
}
}
}
@ -522,82 +515,78 @@ impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
self.eval_constant(constant, self.source_info.unwrap());
}
fn visit_assign(&mut self, place: &Place<'tcx>, rvalue: &Rvalue<'tcx>, location: Location) {
self.super_assign(place, rvalue, location);
let source_info = self.source_info.unwrap();
let Some(()) = self.check_rvalue(rvalue, source_info) else { return };
match self.ecx.machine.can_const_prop[place.local] {
// Do nothing if the place is indirect.
_ if place.is_indirect() => {}
ConstPropMode::NoPropagation => self.ensure_not_propagated(place.local),
ConstPropMode::OnlyInsideOwnBlock | ConstPropMode::FullConstProp => {
if self
.use_ecx(source_info, |this| this.ecx.eval_rvalue_into_place(rvalue, *place))
.is_none()
{
// Const prop failed, so erase the destination, ensuring that whatever happens
// from here on, does not know about the previous value.
// This is important in case we have
// ```rust
// let mut x = 42;
// x = SOME_MUTABLE_STATIC;
// // x must now be uninit
// ```
// FIXME: we overzealously erase the entire local, because that's easier to
// implement.
trace!(
"propagation into {:?} failed.
Nuking the entire site from orbit, it's the only way to be sure",
place,
);
Self::remove_const(&mut self.ecx, place.local);
}
}
}
}
fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
trace!("visit_statement: {:?}", statement);
let source_info = statement.source_info;
self.source_info = Some(source_info);
if let StatementKind::Assign(box (place, ref rval)) = statement.kind {
let can_const_prop = self.ecx.machine.can_const_prop[place.local];
if let Some(()) = self.const_prop(rval, source_info, place) {
match can_const_prop {
ConstPropMode::OnlyInsideOwnBlock => {
trace!(
"found local restricted to its block. \
Will remove it from const-prop after block is finished. Local: {:?}",
place.local
);
}
ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
trace!("can't propagate into {:?}", place);
if place.local != RETURN_PLACE {
Self::remove_const(&mut self.ecx, place.local);
}
}
ConstPropMode::FullConstProp => {}
}
} else {
// Const prop failed, so erase the destination, ensuring that whatever happens
// from here on, does not know about the previous value.
// This is important in case we have
// ```rust
// let mut x = 42;
// x = SOME_MUTABLE_STATIC;
// // x must now be uninit
// ```
// FIXME: we overzealously erase the entire local, because that's easier to
// implement.
trace!(
"propagation into {:?} failed.
Nuking the entire site from orbit, it's the only way to be sure",
place,
);
Self::remove_const(&mut self.ecx, place.local);
}
} else {
match statement.kind {
StatementKind::SetDiscriminant { ref place, .. } => {
match self.ecx.machine.can_const_prop[place.local] {
ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
if self
.use_ecx(source_info, |this| this.ecx.statement(statement))
.is_some()
{
trace!("propped discriminant into {:?}", place);
} else {
Self::remove_const(&mut self.ecx, place.local);
}
}
ConstPropMode::OnlyPropagateInto | ConstPropMode::NoPropagation => {
Self::remove_const(&mut self.ecx, place.local);
}
}
}
StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
let frame = self.ecx.frame_mut();
frame.locals[local].value =
if let StatementKind::StorageLive(_) = statement.kind {
LocalValue::Live(interpret::Operand::Immediate(
interpret::Immediate::Uninit,
))
} else {
LocalValue::Dead
};
}
_ => {}
}
}
// We want to evaluate operands before any change to the assigned-to value,
// so we recurse first.
self.super_statement(statement, location);
match statement.kind {
StatementKind::SetDiscriminant { ref place, .. } => {
match self.ecx.machine.can_const_prop[place.local] {
// Do nothing if the place is indirect.
_ if place.is_indirect() => {}
ConstPropMode::NoPropagation => self.ensure_not_propagated(place.local),
ConstPropMode::FullConstProp | ConstPropMode::OnlyInsideOwnBlock => {
if self.use_ecx(source_info, |this| this.ecx.statement(statement)).is_some()
{
trace!("propped discriminant into {:?}", place);
} else {
Self::remove_const(&mut self.ecx, place.local);
}
}
}
}
StatementKind::StorageLive(local) => {
let frame = self.ecx.frame_mut();
frame.locals[local].value =
LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit));
}
StatementKind::StorageDead(local) => {
let frame = self.ecx.frame_mut();
frame.locals[local].value = LocalValue::Dead;
}
_ => {}
}
}
fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
@ -694,27 +683,25 @@ impl<'tcx> Visitor<'tcx> for ConstPropagator<'_, 'tcx> {
| TerminatorKind::Call { .. }
| TerminatorKind::InlineAsm { .. } => {}
}
}
fn visit_basic_block_data(&mut self, block: BasicBlock, data: &BasicBlockData<'tcx>) {
self.super_basic_block_data(block, data);
// We remove all Locals which are restricted in propagation to their containing blocks and
// which were modified in the current block.
// Take it out of the ecx so we can get a mutable reference to the ecx for `remove_const`.
let mut locals = std::mem::take(&mut self.ecx.machine.written_only_inside_own_block_locals);
for &local in locals.iter() {
Self::remove_const(&mut self.ecx, local);
}
locals.clear();
// Put it back so we reuse the heap of the storage
self.ecx.machine.written_only_inside_own_block_locals = locals;
if cfg!(debug_assertions) {
// Ensure we are correctly erasing locals with the non-debug-assert logic.
for local in self.ecx.machine.only_propagate_inside_block_locals.iter() {
assert!(
self.get_const(local.into()).is_none()
|| self
.layout_of(self.local_decls[local].ty)
.map_or(true, |layout| layout.is_zst())
)
let can_const_prop = std::mem::take(&mut self.ecx.machine.can_const_prop);
for (local, &mode) in can_const_prop.iter_enumerated() {
match mode {
ConstPropMode::FullConstProp => {}
ConstPropMode::NoPropagation => self.ensure_not_propagated(local),
ConstPropMode::OnlyInsideOwnBlock => {
Self::remove_const(&mut self.ecx, local);
self.ensure_not_propagated(local);
}
}
}
self.ecx.machine.can_const_prop = can_const_prop;
}
}