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Lighten up const_prop_lint, reusing const_prop
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@Dajamante
Dajamante committed Jul 25, 2022
1 parent 2f320a2 commit 68b433a
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Showing 2 changed files with 19 additions and 222 deletions.
12 changes: 6 additions & 6 deletions compiler/rustc_mir_transform/src/const_prop.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -155,18 +155,18 @@ impl<'tcx> MirPass<'tcx> for ConstProp {
}
}

struct ConstPropMachine<'mir, 'tcx> {
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.
written_only_inside_own_block_locals: FxHashSet<Local>,
pub written_only_inside_own_block_locals: FxHashSet<Local>,
/// Locals that need to be cleared after every block terminates.
only_propagate_inside_block_locals: BitSet<Local>,
can_const_prop: IndexVec<Local, ConstPropMode>,
pub only_propagate_inside_block_locals: BitSet<Local>,
pub can_const_prop: IndexVec<Local, ConstPropMode>,
}

impl ConstPropMachine<'_, '_> {
fn new(
pub fn new(
only_propagate_inside_block_locals: BitSet<Local>,
can_const_prop: IndexVec<Local, ConstPropMode>,
) -> Self {
Expand Down Expand Up @@ -816,7 +816,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {

/// The mode that `ConstProp` is allowed to run in for a given `Local`.
#[derive(Clone, Copy, Debug, PartialEq)]
enum ConstPropMode {
pub enum ConstPropMode {
/// The `Local` can be propagated into and reads of this `Local` can also be propagated.
FullConstProp,
/// The `Local` can only be propagated into and from its own block.
Expand Down
229 changes: 13 additions & 216 deletions compiler/rustc_mir_transform/src/const_prop_lint.rs
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Original file line number Diff line number Diff line change
@@ -1,19 +1,23 @@
//! Propagates constants for early reporting of statically known
//! assertion failures

use std::cell::Cell;

use rustc_ast::Mutability;
use rustc_data_structures::fx::FxHashSet;
use crate::const_prop::ConstPropMachine;
use crate::const_prop::ConstPropMode;
use crate::MirLint;
use rustc_const_eval::const_eval::ConstEvalErr;
use rustc_const_eval::interpret::{
self, InterpCx, InterpResult, LocalState, LocalValue, MemoryKind, OpTy, Scalar,
ScalarMaybeUninit, 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::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::{
AssertKind, BasicBlock, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind,
Location, Operand, Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement,
StatementKind, Terminator, TerminatorKind, UnOp, RETURN_PLACE,
AssertKind, BinOp, Body, Constant, ConstantKind, Local, LocalDecl, LocalKind, Location,
Operand, Place, Rvalue, SourceInfo, SourceScope, SourceScopeData, Statement, StatementKind,
Terminator, TerminatorKind, UnOp, RETURN_PLACE,
};
use rustc_middle::ty::layout::{LayoutError, LayoutOf, LayoutOfHelpers, TyAndLayout};
use rustc_middle::ty::subst::{InternalSubsts, Subst};
Expand All @@ -22,42 +26,15 @@ use rustc_middle::ty::{
TypeVisitable,
};
use rustc_session::lint;
use rustc_span::{def_id::DefId, Span};
use rustc_span::Span;
use rustc_target::abi::{HasDataLayout, Size, TargetDataLayout};
use rustc_target::spec::abi::Abi as CallAbi;
use rustc_trait_selection::traits;

use crate::MirLint;
use rustc_const_eval::const_eval::ConstEvalErr;
use rustc_const_eval::interpret::{
self, compile_time_machine, AllocId, ConstAllocation, Frame, ImmTy, InterpCx, InterpResult,
LocalState, LocalValue, MemoryKind, OpTy, PlaceTy, Pointer, Scalar, ScalarMaybeUninit,
StackPopCleanup, StackPopUnwind,
};
use std::cell::Cell;

/// The maximum number of bytes that we'll allocate space for a local or the return value.
/// Needed for #66397, because otherwise we eval into large places and that can cause OOM or just
/// Severely regress performance.
const MAX_ALLOC_LIMIT: u64 = 1024;

/// Macro for machine-specific `InterpError` without allocation.
/// (These will never be shown to the user, but they help diagnose ICEs.)
macro_rules! throw_machine_stop_str {
($($tt:tt)*) => {{
// We make a new local type for it. The type itself does not carry any information,
// but its vtable (for the `MachineStopType` trait) does.
struct Zst;
// Printing this type shows the desired string.
impl std::fmt::Display for Zst {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, $($tt)*)
}
}
impl rustc_middle::mir::interpret::MachineStopType for Zst {}
throw_machine_stop!(Zst)
}};
}

pub struct ConstProp;

impl<'tcx> MirLint<'tcx> for ConstProp {
Expand Down Expand Up @@ -151,172 +128,6 @@ impl<'tcx> MirLint<'tcx> for ConstProp {
}
}

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.
written_only_inside_own_block_locals: FxHashSet<Local>,
/// Locals that need to be cleared after every block terminates.
only_propagate_inside_block_locals: BitSet<Local>,
can_const_prop: IndexVec<Local, ConstPropMode>,
}

impl ConstPropMachine<'_, '_> {
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,
}
}
}

impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
compile_time_machine!(<'mir, 'tcx>);
const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)

type MemoryKind = !;

fn load_mir(
_ecx: &InterpCx<'mir, 'tcx, Self>,
_instance: ty::InstanceDef<'tcx>,
) -> InterpResult<'tcx, &'tcx Body<'tcx>> {
throw_machine_stop_str!("calling functions isn't supported in ConstProp")
}

fn find_mir_or_eval_fn(
_ecx: &mut InterpCx<'mir, 'tcx, Self>,
_instance: ty::Instance<'tcx>,
_abi: CallAbi,
_args: &[OpTy<'tcx>],
_destination: &PlaceTy<'tcx>,
_target: Option<BasicBlock>,
_unwind: StackPopUnwind,
) -> InterpResult<'tcx, Option<(&'mir Body<'tcx>, ty::Instance<'tcx>)>> {
Ok(None)
}

fn call_intrinsic(
_ecx: &mut InterpCx<'mir, 'tcx, Self>,
_instance: ty::Instance<'tcx>,
_args: &[OpTy<'tcx>],
_destination: &PlaceTy<'tcx>,
_target: Option<BasicBlock>,
_unwind: StackPopUnwind,
) -> InterpResult<'tcx> {
throw_machine_stop_str!("calling intrinsics isn't supported in ConstProp")
}

fn assert_panic(
_ecx: &mut InterpCx<'mir, 'tcx, Self>,
_msg: &rustc_middle::mir::AssertMessage<'tcx>,
_unwind: Option<rustc_middle::mir::BasicBlock>,
) -> InterpResult<'tcx> {
bug!("panics terminators are not evaluated in ConstProp")
}

fn binary_ptr_op(
_ecx: &InterpCx<'mir, 'tcx, Self>,
_bin_op: BinOp,
_left: &ImmTy<'tcx>,
_right: &ImmTy<'tcx>,
) -> InterpResult<'tcx, (Scalar, bool, Ty<'tcx>)> {
// We can't do this because aliasing of memory can differ between const eval and llvm
throw_machine_stop_str!("pointer arithmetic or comparisons aren't supported in ConstProp")
}

fn access_local<'a>(
frame: &'a Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>,
local: Local,
) -> InterpResult<'tcx, &'a interpret::Operand<Self::Provenance>> {
let l = &frame.locals[local];

if matches!(
l.value,
LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit))
) {
// For us "uninit" means "we don't know its value, might be initiailized or not".
// So stop here.
throw_machine_stop_str!("tried to access a local with unknown value")
}

l.access()
}

fn access_local_mut<'a>(
ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
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);
}
ecx.machine.stack[frame].locals[local].access_mut()
}

fn before_access_global(
_tcx: TyCtxt<'tcx>,
_machine: &Self,
_alloc_id: AllocId,
alloc: ConstAllocation<'tcx, Self::Provenance, Self::AllocExtra>,
_static_def_id: Option<DefId>,
is_write: bool,
) -> InterpResult<'tcx> {
if is_write {
throw_machine_stop_str!("can't write to global");
}
// If the static allocation is mutable, then we can't const prop it as its content
// might be different at runtime.
if alloc.inner().mutability == Mutability::Mut {
throw_machine_stop_str!("can't access mutable globals in ConstProp");
}

Ok(())
}

#[inline(always)]
fn expose_ptr(
_ecx: &mut InterpCx<'mir, 'tcx, Self>,
_ptr: Pointer<AllocId>,
) -> InterpResult<'tcx> {
throw_machine_stop_str!("exposing pointers isn't supported in ConstProp")
}

#[inline(always)]
fn init_frame_extra(
_ecx: &mut InterpCx<'mir, 'tcx, Self>,
frame: Frame<'mir, 'tcx>,
) -> InterpResult<'tcx, Frame<'mir, 'tcx>> {
Ok(frame)
}

#[inline(always)]
fn stack<'a>(
ecx: &'a InterpCx<'mir, 'tcx, Self>,
) -> &'a [Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>] {
&ecx.machine.stack
}

#[inline(always)]
fn stack_mut<'a>(
ecx: &'a mut InterpCx<'mir, 'tcx, Self>,
) -> &'a mut Vec<Frame<'mir, 'tcx, Self::Provenance, Self::FrameExtra>> {
&mut ecx.machine.stack
}
}

/// Finds optimization opportunities on the MIR.
struct ConstPropagator<'mir, 'tcx> {
ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
Expand Down Expand Up @@ -711,20 +522,6 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
}
}

/// The mode that `ConstProp` is allowed to run in for a given `Local`.
#[derive(Clone, Copy, Debug, PartialEq)]
enum ConstPropMode {
/// The `Local` can be propagated into and reads of this `Local` can also be propagated.
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,
}

struct CanConstProp {
can_const_prop: IndexVec<Local, ConstPropMode>,
// False at the beginning. Once set, no more assignments are allowed to that local.
Expand Down

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