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promotion: do not promote const-fn calls in const when that may fail without the entire const failing

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This commit is contained in:
Ralf Jung
2024-02-24 17:16:05 +01:00
parent 40dcd796d0
commit 7183fa09bb
11 changed files with 198 additions and 252 deletions
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102
compiler/rustc_mir_transform/src/promote_consts.rs
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@@ -13,6 +13,7 @@
//! move analysis runs after promotion on broken MIR.
use either::{Left, Right};
use rustc_data_structures::fx::FxHashSet;
use rustc_hir as hir;
use rustc_middle::mir;
use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
@@ -175,6 +176,12 @@ fn collect_temps_and_candidates<'tcx>(
struct Validator<'a, 'tcx> {
ccx: &'a ConstCx<'a, 'tcx>,
temps: &'a mut IndexSlice<Local, TempState>,
/// For backwards compatibility, we are promoting function calls in `const`/`static`
/// initializers. But we want to avoid evaluating code that might panic and that otherwise would
/// not have been evaluated, so we only promote such calls in basic blocks that are guaranteed
/// to execute. In other words, we only promote such calls in basic blocks that are definitely
/// not dead code. Here we cache the result of computing that set of basic blocks.
promotion_safe_blocks: Option<FxHashSet<BasicBlock>>,
}
impl<'a, 'tcx> std::ops::Deref for Validator<'a, 'tcx> {
@@ -260,7 +267,9 @@ impl<'tcx> Validator<'_, 'tcx> {
self.validate_rvalue(rhs)
}
Right(terminator) => match &terminator.kind {
TerminatorKind::Call { func, args, .. } => self.validate_call(func, args),
TerminatorKind::Call { func, args, .. } => {
self.validate_call(func, args, loc.block)
}
TerminatorKind::Yield { .. } => Err(Unpromotable),
kind => {
span_bug!(terminator.source_info.span, "{:?} not promotable", kind);
@@ -588,29 +597,79 @@ impl<'tcx> Validator<'_, 'tcx> {
Ok(())
}
/// Computes the sets of blocks of this MIR that are definitely going to be executed
/// if the function returns successfully. That makes it safe to promote calls in them
/// that might fail.
fn promotion_safe_blocks(body: &mir::Body<'tcx>) -> FxHashSet<BasicBlock> {
let mut safe_blocks = FxHashSet::default();
let mut safe_block = START_BLOCK;
loop {
safe_blocks.insert(safe_block);
// Let's see if we can find another safe block.
safe_block = match body.basic_blocks[safe_block].terminator().kind {
TerminatorKind::Goto { target } => target,
TerminatorKind::Call { target: Some(target), .. }
| TerminatorKind::Drop { target, .. } => {
// This calls a function or the destructor. `target` does not get executed if
// the callee loops or panics. But in both cases the const already fails to
// evaluate, so we are fine considering `target` a safe block for promotion.
target
}
TerminatorKind::Assert { target, .. } => {
// Similar to above, we only consider successful execution.
target
}
_ => {
// No next safe block.
break;
}
};
}
safe_blocks
}
/// Returns whether the block is "safe" for promotion, which means it cannot be dead code.
/// We use this to avoid promoting operations that can fail in dead code.
fn is_promotion_safe_block(&mut self, block: BasicBlock) -> bool {
let body = self.body;
let safe_blocks =
self.promotion_safe_blocks.get_or_insert_with(|| Self::promotion_safe_blocks(body));
safe_blocks.contains(&block)
}
fn validate_call(
&mut self,
callee: &Operand<'tcx>,
args: &[Spanned<Operand<'tcx>>],
block: BasicBlock,
) -> Result<(), Unpromotable> {
let fn_ty = callee.ty(self.body, self.tcx);
// Validate the operands. If they fail, there's no question -- we cannot promote.
self.validate_operand(callee)?;
for arg in args {
self.validate_operand(&arg.node)?;
}
// Inside const/static items, we promote all (eligible) function calls.
// Everywhere else, we require `#[rustc_promotable]` on the callee.
let promote_all_const_fn = matches!(
self.const_kind,
Some(hir::ConstContext::Static(_) | hir::ConstContext::Const { inline: false })
);
if !promote_all_const_fn {
if let ty::FnDef(def_id, _) = *fn_ty.kind() {
// Never promote runtime `const fn` calls of
// functions without `#[rustc_promotable]`.
if !self.tcx.is_promotable_const_fn(def_id) {
return Err(Unpromotable);
}
// Functions marked `#[rustc_promotable]` are explicitly allowed to be promoted, so we can
// accept them at this point.
let fn_ty = callee.ty(self.body, self.tcx);
if let ty::FnDef(def_id, _) = *fn_ty.kind() {
if self.tcx.is_promotable_const_fn(def_id) {
return Ok(());
}
}
// Ideally, we'd stop here and reject the rest.
// But for backward compatibility, we have to accept some promotion in const/static
// initializers. Inline consts are explicitly excluded, they are more recent so we have no
// backwards compatibility reason to allow more promotion inside of them.
let promote_all_fn = matches!(
self.const_kind,
Some(hir::ConstContext::Static(_) | hir::ConstContext::Const { inline: false })
);
if !promote_all_fn {
return Err(Unpromotable);
}
// Make sure the callee is a `const fn`.
let is_const_fn = match *fn_ty.kind() {
ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id),
_ => false,
@@ -618,12 +677,13 @@ impl<'tcx> Validator<'_, 'tcx> {
if !is_const_fn {
return Err(Unpromotable);
}
self.validate_operand(callee)?;
for arg in args {
self.validate_operand(&arg.node)?;
// The problem is, this may promote calls to functions that panic.
// We don't want to introduce compilation errors if there's a panic in a call in dead code.
// So we ensure that this is not dead code.
if !self.is_promotion_safe_block(block) {
return Err(Unpromotable);
}
// This passed all checks, so let's accept.
Ok(())
}
}
@@ -634,7 +694,7 @@ fn validate_candidates(
temps: &mut IndexSlice<Local, TempState>,
candidates: &[Candidate],
) -> Vec<Candidate> {
let mut validator = Validator { ccx, temps };
let mut validator = Validator { ccx, temps, promotion_safe_blocks: None };
candidates
.iter()