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Refactor the way interpreters handle special functions #121273

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c4ecc2b
Refactor the way interpreters handle special functions
WaffleLapkin Feb 18, 2024
e5f7a74
Adapt to the interpreter refactor in miri
WaffleLapkin Feb 18, 2024
99db695
Remove an unused parameter
WaffleLapkin Feb 18, 2024
586d704
Fix infinite recursion in ctfe `align_offset`
WaffleLapkin Feb 18, 2024
dc9b611
Move `eval_body` call inside a `match`
WaffleLapkin Feb 18, 2024
3f17b34
Fixup doc comment
WaffleLapkin Feb 19, 2024
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172 changes: 112 additions & 60 deletions compiler/rustc_const_eval/src/const_eval/machine.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@ use std::fmt;
use std::hash::Hash;
use std::ops::ControlFlow;

use either::Either;
use rustc_ast::Mutability;
use rustc_data_structures::fx::FxIndexMap;
use rustc_data_structures::fx::IndexEntry;
Expand All @@ -14,6 +15,7 @@ use rustc_middle::mir::AssertMessage;
use rustc_middle::query::TyCtxtAt;
use rustc_middle::ty;
use rustc_middle::ty::layout::{FnAbiOf, TyAndLayout};
use rustc_middle::ty::Ty;
use rustc_session::lint::builtin::WRITES_THROUGH_IMMUTABLE_POINTER;
use rustc_span::symbol::{sym, Symbol};
use rustc_span::Span;
Expand Down Expand Up @@ -191,6 +193,16 @@ impl interpret::MayLeak for ! {
}
}

#[derive(Debug, Copy, Clone)]
pub enum ExtraFnVal<'tcx> {
/// `#[rustc_const_panic_str]` or `#[lang = "begin_panic"]`
BeginPanic,
/// `#[lang = "panic_fmt"]`
PanicFmt(ty::Instance<'tcx>),
/// `#[lang = "align_offset"]`
AlignOffset(ty::Instance<'tcx>),
}

impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {
fn location_triple_for_span(&self, span: Span) -> (Symbol, u32, u32) {
let topmost = span.ctxt().outer_expn().expansion_cause().unwrap_or(span);
Expand All @@ -212,56 +224,29 @@ impl<'mir, 'tcx: 'mir> CompileTimeEvalContext<'mir, 'tcx> {

/// "Intercept" a function call, because we have something special to do for it.
/// All `#[rustc_do_not_const_check]` functions should be hooked here.
/// If this returns `Some` function, which may be `instance` or a different function with
/// compatible arguments, then evaluation should continue with that function.
/// If this returns `None`, the function call has been handled and the function has returned.
fn hook_special_const_fn(
&mut self,
instance: ty::Instance<'tcx>,
args: &[FnArg<'tcx>],
dest: &PlaceTy<'tcx>,
ret: Option<mir::BasicBlock>,
) -> InterpResult<'tcx, Option<ty::Instance<'tcx>>> {
///
/// If this returns `Some`, the function should be executed via [`call_extra_fn`].
/// If this returns `None`, the function should be executed as normal.
///
/// [`call_extra_fn`]: interpret::Machine::call_extra_fn
fn hook_special_const_fn(&mut self, instance: ty::Instance<'tcx>) -> Option<ExtraFnVal<'tcx>> {
let def_id = instance.def_id();

if self.tcx.has_attr(def_id, sym::rustc_const_panic_str)
|| Some(def_id) == self.tcx.lang_items().begin_panic_fn()
{
let args = self.copy_fn_args(args)?;
// &str or &&str
assert!(args.len() == 1);
return Some(ExtraFnVal::BeginPanic);
}

let mut msg_place = self.deref_pointer(&args[0])?;
while msg_place.layout.ty.is_ref() {
msg_place = self.deref_pointer(&msg_place)?;
}
if Some(def_id) == self.tcx.lang_items().panic_fmt() {
return Some(ExtraFnVal::PanicFmt(instance));
}

let msg = Symbol::intern(self.read_str(&msg_place)?);
let span = self.find_closest_untracked_caller_location();
let (file, line, col) = self.location_triple_for_span(span);
return Err(ConstEvalErrKind::Panic { msg, file, line, col }.into());
} else if Some(def_id) == self.tcx.lang_items().panic_fmt() {
// For panic_fmt, call const_panic_fmt instead.
let const_def_id = self.tcx.require_lang_item(LangItem::ConstPanicFmt, None);
let new_instance = ty::Instance::resolve(
*self.tcx,
ty::ParamEnv::reveal_all(),
const_def_id,
instance.args,
)
.unwrap()
.unwrap();

return Ok(Some(new_instance));
} else if Some(def_id) == self.tcx.lang_items().align_offset_fn() {
let args = self.copy_fn_args(args)?;
// For align_offset, we replace the function call if the pointer has no address.
match self.align_offset(instance, &args, dest, ret)? {
ControlFlow::Continue(()) => return Ok(Some(instance)),
ControlFlow::Break(()) => return Ok(None),
}
if Some(def_id) == self.tcx.lang_items().align_offset_fn() {
return Some(ExtraFnVal::AlignOffset(instance));
}
Ok(Some(instance))

None
}

/// `align_offset(ptr, target_align)` needs special handling in const eval, because the pointer
Expand Down Expand Up @@ -371,6 +356,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
compile_time_machine!(<'mir, 'tcx>);

type MemoryKind = MemoryKind;
type ExtraFnVal = ExtraFnVal<'tcx>;

const PANIC_ON_ALLOC_FAIL: bool = false; // will be raised as a proper error

Expand Down Expand Up @@ -399,7 +385,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
.delayed_bug("This is likely a const item that is missing from its impl");
throw_inval!(AlreadyReported(guar.into()));
} else {
// `find_mir_or_eval_fn` checks that this is a const fn before even calling us,
// `find_mir_or_extra_fn` checks that this is a const fn before even calling us,
// so this should be unreachable.
let path = ecx.tcx.def_path_str(def);
bug!("trying to call extern function `{path}` at compile-time");
Expand All @@ -409,22 +395,16 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
}
}

fn find_mir_or_eval_fn(
fn find_mir_or_extra_fn(
ecx: &mut InterpCx<'mir, 'tcx, Self>,
orig_instance: ty::Instance<'tcx>,
_abi: CallAbi,
args: &[FnArg<'tcx>],
dest: &PlaceTy<'tcx>,
ret: Option<mir::BasicBlock>,
_unwind: mir::UnwindAction, // unwinding is not supported in consts
) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>> {
debug!("find_mir_or_eval_fn: {:?}", orig_instance);
instance: ty::Instance<'tcx>,
) -> InterpResult<'tcx, Either<&'mir mir::Body<'tcx>, Self::ExtraFnVal>> {
debug!("find_mir_or_extra_fn: {:?}", instance);

// Replace some functions.
let Some(instance) = ecx.hook_special_const_fn(orig_instance, args, dest, ret)? else {
// Call has already been handled.
return Ok(None);
};
if let Some(extra) = ecx.hook_special_const_fn(instance) {
return Ok(Either::Right(extra));
}

// Only check non-glue functions
if let ty::InstanceDef::Item(def) = instance.def {
Expand All @@ -442,10 +422,82 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
}
}

// This is a const fn. Call it.
// In case of replacement, we return the *original* instance to make backtraces work out
// (and we hope this does not confuse the FnAbi checks too much).
Ok(Some((ecx.load_mir(instance.def, None)?, orig_instance)))
// This is a const fn. Return its mir to be called.
ecx.load_mir(instance.def, None).map(Either::Left)
}

#[inline(always)]
fn call_extra_fn(
ecx: &mut InterpCx<'mir, 'tcx, Self>,
fn_val: Self::ExtraFnVal,
abis: (CallAbi, &rustc_target::abi::call::FnAbi<'tcx, Ty<'tcx>>),
args: &[FnArg<'tcx>],
destination: &PlaceTy<'tcx, Self::Provenance>,
target: Option<mir::BasicBlock>,
unwind: mir::UnwindAction,
) -> InterpResult<'tcx> {
match fn_val {
ExtraFnVal::BeginPanic => {
let args = ecx.copy_fn_args(args)?;
// &str or &&str
assert!(args.len() == 1);

let mut msg_place = ecx.deref_pointer(&args[0])?;
while msg_place.layout.ty.is_ref() {
msg_place = ecx.deref_pointer(&msg_place)?;
}

let msg = Symbol::intern(ecx.read_str(&msg_place)?);
let span = ecx.find_closest_untracked_caller_location();
let (file, line, col) = ecx.location_triple_for_span(span);
return Err(ConstEvalErrKind::Panic { msg, file, line, col }.into());
}
ExtraFnVal::PanicFmt(instance) => {
// For panic_fmt, call const_panic_fmt instead.
let const_def_id = ecx.tcx.require_lang_item(LangItem::ConstPanicFmt, None);
let new_instance = ty::Instance::resolve(
*ecx.tcx,
ty::ParamEnv::reveal_all(),
const_def_id,
instance.args,
)
.unwrap()
.unwrap();

ecx.eval_fn_call(
FnVal::Instance(new_instance),
abis,
args,
true,
destination,
target,
unwind,
)
}
ExtraFnVal::AlignOffset(instance) => {
let args2 = ecx.copy_fn_args(args)?;
// For align_offset, we replace the function call if the pointer has no address.
match ecx.align_offset(instance, &args2, destination, target)? {
ControlFlow::Continue(()) => {
// Can't use `eval_fn_call` here because `eval_fn_call` tries to call
// const eval extra fn which ends up here, so calling `eval_fn_call`
// would cause infinite recursion and stack overflow.
let body = ecx.load_mir(instance.def, None)?;
ecx.eval_body(
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Instead of having to introduce and expose a new very low-level function here, would it work to have find_mir_or_extra_fn make the choice of whether to call the real function or the interpreter shim? Fundamentally the problem seems to be that we first said we want an ExtraFn but then later we decided we want MIR anyway.

OTOH, Miri also has that situation, I wonder how this is handled there.

instance,
body,
abis,
args,
false,
destination,
target,
unwind,
)
}
ControlFlow::Break(()) => Ok(()),
}
}
}
}

fn panic_nounwind(ecx: &mut InterpCx<'mir, 'tcx, Self>, msg: &str) -> InterpResult<'tcx> {
Expand Down
11 changes: 6 additions & 5 deletions compiler/rustc_const_eval/src/interpret/intern.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -24,12 +24,12 @@ use super::{AllocId, Allocation, InterpCx, MPlaceTy, Machine, MemoryKind, PlaceT
use crate::const_eval;
use crate::errors::{DanglingPtrInFinal, MutablePtrInFinal};

pub trait CompileTimeMachine<'mir, 'tcx: 'mir, T> = Machine<
pub trait CompileTimeMachine<'mir, 'tcx: 'mir, T, U = const_eval::ExtraFnVal<'tcx>> = Machine<
'mir,
'tcx,
MemoryKind = T,
Provenance = CtfeProvenance,
ExtraFnVal = !,
ExtraFnVal = U,
FrameExtra = (),
AllocExtra = (),
MemoryMap = FxIndexMap<AllocId, (MemoryKind<T>, Allocation)>,
Expand All @@ -42,7 +42,7 @@ pub trait CompileTimeMachine<'mir, 'tcx: 'mir, T> = Machine<
/// already mutable (as a sanity check).
///
/// Returns an iterator over all relocations referred to by this allocation.
fn intern_shallow<'rt, 'mir, 'tcx, T, M: CompileTimeMachine<'mir, 'tcx, T>>(
fn intern_shallow<'rt, 'mir, 'tcx, T, U, M: CompileTimeMachine<'mir, 'tcx, T, U>>(
ecx: &'rt mut InterpCx<'mir, 'tcx, M>,
alloc_id: AllocId,
mutability: Mutability,
Expand Down Expand Up @@ -236,7 +236,8 @@ pub fn intern_const_alloc_for_constprop<
'mir,
'tcx: 'mir,
T,
M: CompileTimeMachine<'mir, 'tcx, T>,
U,
M: CompileTimeMachine<'mir, 'tcx, T, U>,
>(
ecx: &mut InterpCx<'mir, 'tcx, M>,
alloc_id: AllocId,
Expand All @@ -255,7 +256,7 @@ pub fn intern_const_alloc_for_constprop<
Ok(())
}

impl<'mir, 'tcx: 'mir, M: super::intern::CompileTimeMachine<'mir, 'tcx, !>>
impl<'mir, 'tcx: 'mir, M: super::intern::CompileTimeMachine<'mir, 'tcx, !, !>>
InterpCx<'mir, 'tcx, M>
{
/// A helper function that allocates memory for the layout given and gives you access to mutate
Expand Down
29 changes: 5 additions & 24 deletions compiler/rustc_const_eval/src/interpret/machine.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,12 +6,13 @@ use std::borrow::{Borrow, Cow};
use std::fmt::Debug;
use std::hash::Hash;

use either::Either;
use rustc_apfloat::{Float, FloatConvert};
use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
use rustc_middle::mir;
use rustc_middle::query::TyCtxtAt;
use rustc_middle::ty;
use rustc_middle::ty::layout::TyAndLayout;
use rustc_middle::ty::{self, Ty};
use rustc_span::def_id::DefId;
use rustc_span::Span;
use rustc_target::abi::{Align, Size};
Expand Down Expand Up @@ -191,22 +192,17 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
/// nor just jump to `ret`, but instead push their own stack frame.)
/// Passing `dest`and `ret` in the same `Option` proved very annoying when only one of them
/// was used.
fn find_mir_or_eval_fn(
fn find_mir_or_extra_fn(
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The documentation of this function needs an update

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I fixed it up, should be better now.

Athough it looks like there are other tests and comments mentioning __rust_maybe_catch_panic, which doesn't seem to exist anymore?... Possibly someone needs to go over them and update them too.

ecx: &mut InterpCx<'mir, 'tcx, Self>,
instance: ty::Instance<'tcx>,
abi: CallAbi,
args: &[FnArg<'tcx, Self::Provenance>],
destination: &PlaceTy<'tcx, Self::Provenance>,
target: Option<mir::BasicBlock>,
unwind: mir::UnwindAction,
) -> InterpResult<'tcx, Option<(&'mir mir::Body<'tcx>, ty::Instance<'tcx>)>>;
) -> InterpResult<'tcx, Either<&'mir mir::Body<'tcx>, Self::ExtraFnVal>>;

/// Execute `fn_val`. It is the hook's responsibility to advance the instruction
/// pointer as appropriate.
fn call_extra_fn(
ecx: &mut InterpCx<'mir, 'tcx, Self>,
fn_val: Self::ExtraFnVal,
abi: CallAbi,
abis: (CallAbi, &rustc_target::abi::call::FnAbi<'tcx, Ty<'tcx>>),
args: &[FnArg<'tcx, Self::Provenance>],
destination: &PlaceTy<'tcx, Self::Provenance>,
target: Option<mir::BasicBlock>,
Expand Down Expand Up @@ -555,8 +551,6 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
type Provenance = CtfeProvenance;
type ProvenanceExtra = bool; // the "immutable" flag

type ExtraFnVal = !;

type MemoryMap =
rustc_data_structures::fx::FxIndexMap<AllocId, (MemoryKind<Self::MemoryKind>, Allocation)>;
const GLOBAL_KIND: Option<Self::MemoryKind> = None; // no copying of globals from `tcx` to machine memory
Expand All @@ -578,19 +572,6 @@ pub macro compile_time_machine(<$mir: lifetime, $tcx: lifetime>) {
unreachable!("unwinding cannot happen during compile-time evaluation")
}

#[inline(always)]
fn call_extra_fn(
_ecx: &mut InterpCx<$mir, $tcx, Self>,
fn_val: !,
_abi: CallAbi,
_args: &[FnArg<$tcx>],
_destination: &PlaceTy<$tcx, Self::Provenance>,
_target: Option<mir::BasicBlock>,
_unwind: mir::UnwindAction,
) -> InterpResult<$tcx> {
match fn_val {}
}

#[inline(always)]
fn adjust_allocation<'b>(
_ecx: &InterpCx<$mir, $tcx, Self>,
Expand Down
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