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Auto merge of #129283 - saethlin:unreachable-allocas, r=scottmcm

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Don't alloca for unused locals

We already have a concept of mono-unreachable basic blocks; this is primarily useful for ensuring that we do not compile code under an `if false`. But since we never gave locals the same analysis, a large local only used under an `if false` will still have stack space allocated for it.

There are 3 places we traverse MIR during monomorphization: Inside the collector, `non_ssa_locals`, and the walk to generate code. Unfortunately, https://github.com/rust-lang/rust/pull/129283#issuecomment-2297925578 indicates that we cannot afford the expense of tracking reachable locals during the collector's traversal, so we do need at least two mono-reachable traversals. And of course caching is of no help here because the benchmarks that regress are incr-unchanged; they don't do any codegen.

This fixes the second problem in https://github.com/rust-lang/rust/issues/129282, and brings us anther step toward `const if` at home.
This commit is contained in:
bors 2024-09-21 13:48:14 +00:00
commit 2836482241
9 changed files with 151 additions and 54 deletions
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25
compiler/rustc_codegen_ssa/src/mir/analyze.rs
View File

@ -15,6 +15,7 @@
pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
fx: &FunctionCx<'a, 'tcx, Bx>,
traversal_order: &[mir::BasicBlock],
) -> BitSet<mir::Local> {
let mir = fx.mir;
let dominators = mir.basic_blocks.dominators();
@ -24,13 +25,7 @@ pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
.map(|decl| {
let ty = fx.monomorphize(decl.ty);
let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
if layout.is_zst() {
LocalKind::ZST
} else if fx.cx.is_backend_immediate(layout) || fx.cx.is_backend_scalar_pair(layout) {
LocalKind::Unused
} else {
LocalKind::Memory
}
if layout.is_zst() { LocalKind::ZST } else { LocalKind::Unused }
})
.collect();
@ -44,7 +39,8 @@ pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
// If there exists a local definition that dominates all uses of that local,
// the definition should be visited first. Traverse blocks in an order that
// is a topological sort of dominance partial order.
for (bb, data) in traversal::reverse_postorder(mir) {
for bb in traversal_order.iter().copied() {
let data = &mir.basic_blocks[bb];
analyzer.visit_basic_block_data(bb, data);
}
@ -77,11 +73,22 @@ struct LocalAnalyzer<'a, 'b, 'tcx, Bx: BuilderMethods<'b, 'tcx>> {
impl<'a, 'b, 'tcx, Bx: BuilderMethods<'b, 'tcx>> LocalAnalyzer<'a, 'b, 'tcx, Bx> {
fn define(&mut self, local: mir::Local, location: DefLocation) {
let fx = self.fx;
let kind = &mut self.locals[local];
let decl = &fx.mir.local_decls[local];
match *kind {
LocalKind::ZST => {}
LocalKind::Memory => {}
LocalKind::Unused => *kind = LocalKind::SSA(location),
LocalKind::Unused => {
let ty = fx.monomorphize(decl.ty);
let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
*kind =
if fx.cx.is_backend_immediate(layout) || fx.cx.is_backend_scalar_pair(layout) {
LocalKind::SSA(location)
} else {
LocalKind::Memory
};
}
LocalKind::SSA(_) => *kind = LocalKind::Memory,
}
}

26
compiler/rustc_codegen_ssa/src/mir/mod.rs
View File

@ -218,7 +218,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
fx.per_local_var_debug_info = fx.compute_per_local_var_debug_info(&mut start_bx);
let memory_locals = analyze::non_ssa_locals(&fx);
let traversal_order = traversal::mono_reachable_reverse_postorder(mir, cx.tcx(), instance);
let memory_locals = analyze::non_ssa_locals(&fx, &traversal_order);
// Allocate variable and temp allocas
let local_values = {
@ -277,17 +278,20 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
// So drop the builder of `start_llbb` to avoid having two at the same time.
drop(start_bx);
let reachable_blocks = traversal::mono_reachable_as_bitset(mir, cx.tcx(), instance);
let mut unreached_blocks = BitSet::new_filled(mir.basic_blocks.len());
// Codegen the body of each reachable block using our reverse postorder list.
for bb in traversal_order {
fx.codegen_block(bb);
unreached_blocks.remove(bb);
}
// Codegen the body of each block using reverse postorder
for (bb, _) in traversal::reverse_postorder(mir) {
if reachable_blocks.contains(bb) {
fx.codegen_block(bb);
} else {
// We want to skip this block, because it's not reachable. But we still create
// the block so terminators in other blocks can reference it.
fx.codegen_block_as_unreachable(bb);
}
// FIXME: These empty unreachable blocks are *mostly* a waste. They are occasionally
// targets for a SwitchInt terminator, but the reimplementation of the mono-reachable
// simplification in SwitchInt lowering sometimes misses cases that
// mono_reachable_reverse_postorder manages to figure out.
// The solution is to do something like post-mono GVN. But for now we have this hack.
for bb in unreached_blocks.iter() {
fx.codegen_block_as_unreachable(bb);
}
}

2
compiler/rustc_middle/src/mir/basic_blocks.rs
View File

@ -71,7 +71,7 @@ pub fn predecessors(&self) -> &Predecessors {
#[inline]
pub fn reverse_postorder(&self) -> &[BasicBlock] {
self.cache.reverse_postorder.get_or_init(|| {
let mut rpo: Vec<_> = Postorder::new(&self.basic_blocks, START_BLOCK).collect();
let mut rpo: Vec<_> = Postorder::new(&self.basic_blocks, START_BLOCK, ()).collect();
rpo.reverse();
rpo
})

13
compiler/rustc_middle/src/mir/mod.rs
View File

@ -1424,6 +1424,19 @@ pub fn visitable(&self, index: usize) -> &dyn MirVisitable<'tcx> {
pub fn is_empty_unreachable(&self) -> bool {
self.statements.is_empty() && matches!(self.terminator().kind, TerminatorKind::Unreachable)
}
/// Like [`Terminator::successors`] but tries to use information available from the [`Instance`]
/// to skip successors like the `false` side of an `if const {`.
///
/// This is used to implement [`traversal::mono_reachable`] and
/// [`traversal::mono_reachable_reverse_postorder`].
pub fn mono_successors(&self, tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) -> Successors<'_> {
if let Some((bits, targets)) = Body::try_const_mono_switchint(tcx, instance, self) {
targets.successors_for_value(bits)
} else {
self.terminator().successors()
}
}
}
///////////////////////////////////////////////////////////////////////////

11
compiler/rustc_middle/src/mir/terminator.rs
View File

@ -413,6 +413,17 @@ mod helper {
use super::*;
pub type Successors<'a> = impl DoubleEndedIterator<Item = BasicBlock> + 'a;
pub type SuccessorsMut<'a> = impl DoubleEndedIterator<Item = &'a mut BasicBlock> + 'a;
impl SwitchTargets {
/// Like [`SwitchTargets::target_for_value`], but returning the same type as
/// [`Terminator::successors`].
#[inline]
pub fn successors_for_value(&self, value: u128) -> Successors<'_> {
let target = self.target_for_value(value);
(&[]).into_iter().copied().chain(Some(target))
}
}
impl<'tcx> TerminatorKind<'tcx> {
#[inline]
pub fn successors(&self) -> Successors<'_> {

85
compiler/rustc_middle/src/mir/traversal.rs
View File

@ -104,36 +104,46 @@ fn size_hint(&self) -> (usize, Option<usize>) {
/// ```
///
/// A Postorder traversal of this graph is `D B C A` or `D C B A`
pub struct Postorder<'a, 'tcx> {
pub struct Postorder<'a, 'tcx, C> {
basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
visited: BitSet<BasicBlock>,
visit_stack: Vec<(BasicBlock, Successors<'a>)>,
root_is_start_block: bool,
extra: C,
}
impl<'a, 'tcx> Postorder<'a, 'tcx> {
impl<'a, 'tcx, C> Postorder<'a, 'tcx, C>
where
C: Customization<'tcx>,
{
pub fn new(
basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
root: BasicBlock,
) -> Postorder<'a, 'tcx> {
extra: C,
) -> Postorder<'a, 'tcx, C> {
let mut po = Postorder {
basic_blocks,
visited: BitSet::new_empty(basic_blocks.len()),
visit_stack: Vec::new(),
root_is_start_block: root == START_BLOCK,
extra,
};
let data = &po.basic_blocks[root];
if let Some(ref term) = data.terminator {
po.visited.insert(root);
po.visit_stack.push((root, term.successors()));
po.traverse_successor();
}
po.visit(root);
po.traverse_successor();
po
}
fn visit(&mut self, bb: BasicBlock) {
if !self.visited.insert(bb) {
return;
}
let data = &self.basic_blocks[bb];
let successors = C::successors(data, self.extra);
self.visit_stack.push((bb, successors));
}
fn traverse_successor(&mut self) {
// This is quite a complex loop due to 1. the borrow checker not liking it much
// and 2. what exactly is going on is not clear
@ -183,16 +193,15 @@ fn traverse_successor(&mut self) {
// since we've already visited `E`, that child isn't added to the stack. The last
// two iterations yield `B` and finally `A` for a final traversal of [E, D, C, B, A]
while let Some(bb) = self.visit_stack.last_mut().and_then(|(_, iter)| iter.next_back()) {
if self.visited.insert(bb) {
if let Some(term) = &self.basic_blocks[bb].terminator {
self.visit_stack.push((bb, term.successors()));
}
}
self.visit(bb);
}
}
}
impl<'tcx> Iterator for Postorder<'_, 'tcx> {
impl<'tcx, C> Iterator for Postorder<'_, 'tcx, C>
where
C: Customization<'tcx>,
{
type Item = BasicBlock;
fn next(&mut self) -> Option<BasicBlock> {
@ -232,6 +241,40 @@ pub fn postorder<'a, 'tcx>(
reverse_postorder(body).rev()
}
/// Lets us plug in some additional logic and data into a Postorder traversal. Or not.
pub trait Customization<'tcx>: Copy {
fn successors<'a>(_: &'a BasicBlockData<'tcx>, _: Self) -> Successors<'a>;
}
impl<'tcx> Customization<'tcx> for () {
fn successors<'a>(data: &'a BasicBlockData<'tcx>, _: ()) -> Successors<'a> {
data.terminator().successors()
}
}
impl<'tcx> Customization<'tcx> for (TyCtxt<'tcx>, Instance<'tcx>) {
fn successors<'a>(
data: &'a BasicBlockData<'tcx>,
(tcx, instance): (TyCtxt<'tcx>, Instance<'tcx>),
) -> Successors<'a> {
data.mono_successors(tcx, instance)
}
}
pub fn mono_reachable_reverse_postorder<'a, 'tcx>(
body: &'a Body<'tcx>,
tcx: TyCtxt<'tcx>,
instance: Instance<'tcx>,
) -> Vec<BasicBlock> {
let mut iter = Postorder::new(&body.basic_blocks, START_BLOCK, (tcx, instance));
let mut items = Vec::with_capacity(body.basic_blocks.len());
while let Some(block) = iter.next() {
items.push(block);
}
items.reverse();
items
}
/// Returns an iterator over all basic blocks reachable from the `START_BLOCK` in no particular
/// order.
///
@ -358,14 +401,8 @@ fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
let data = &self.body[idx];
if let Some((bits, targets)) =
Body::try_const_mono_switchint(self.tcx, self.instance, data)
{
let target = targets.target_for_value(bits);
self.add_work([target]);
} else {
self.add_work(data.terminator().successors());
}
let targets = data.mono_successors(self.tcx, self.instance);
self.add_work(targets);
return Some((idx, data));
}

2
src/tools/clippy/clippy_utils/src/mir/mod.rs
View File

@ -30,7 +30,7 @@ pub fn visit_local_usage(locals: &[Local], mir: &Body<'_>, location: Location) -
locals.len()
];
traversal::Postorder::new(&mir.basic_blocks, location.block)
traversal::Postorder::new(&mir.basic_blocks, location.block, ())
.collect::<Vec<_>>()
.into_iter()
.rev()

14
tests/codegen/constant-branch.rs
View File

@ -7,18 +7,19 @@
// CHECK-LABEL: @if_bool
#[no_mangle]
pub fn if_bool() {
// CHECK: br label %{{.+}}
// CHECK-NOT: br i1
// CHECK-NOT: switch
_ = if true { 0 } else { 1 };
// CHECK: br label %{{.+}}
_ = if false { 0 } else { 1 };
}
// CHECK-LABEL: @if_constant_int_eq
#[no_mangle]
pub fn if_constant_int_eq() {
// CHECK-NOT: br i1
// CHECK-NOT: switch
let val = 0;
// CHECK: br label %{{.+}}
_ = if val == 0 { 0 } else { 1 };
// CHECK: br label %{{.+}}
@ -28,23 +29,20 @@ pub fn if_constant_int_eq() {
// CHECK-LABEL: @if_constant_match
#[no_mangle]
pub fn if_constant_match() {
// CHECK: br label %{{.+}}
// CHECK-NOT: br i1
// CHECK-NOT: switch
_ = match 1 {
1 => 2,
2 => 3,
_ => 4,
};
// CHECK: br label %{{.+}}
_ = match 1 {
2 => 3,
_ => 4,
};
// CHECK: br label %[[MINUS1:.+]]
_ = match -1 {
// CHECK: [[MINUS1]]:
// CHECK: store i32 1
-1 => 1,
_ => 0,
}

27
tests/codegen/no-alloca-inside-if-false.rs Normal file
View File

@ -0,0 +1,27 @@
//@ compile-flags: -Cno-prepopulate-passes -Copt-level=0 -Cpanic=abort
// Check that there's an alloca for the reference and the vector, but nothing else.
// We use panic=abort because unwinding panics give hint::black_box a cleanup block, which has
// another alloca.
#![crate_type = "lib"]
#[inline(never)]
fn test<const SIZE: usize>() {
// CHECK-LABEL: no_alloca_inside_if_false::test
// CHECK: start:
// CHECK-NEXT: alloca [{{12|24}} x i8]
// CHECK-NOT: alloca
if const { SIZE < 4096 } {
let arr = [0u8; SIZE];
std::hint::black_box(&arr);
} else {
let vec = vec![0u8; SIZE];
std::hint::black_box(&vec);
}
}
// CHECK-LABEL: @main
#[no_mangle]
pub fn main() {
test::<8192>();
}