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introduce local-scope to prevent StorageLive/StorageDead in statics #42023

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Merged
merged 2 commits into from
May 23, 2017
Merged

introduce local-scope to prevent StorageLive/StorageDead in statics #42023

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19bf544
introduce local-scope to prevent storagelive/storagedead in statics
nikomatsakis May 16, 2017
8a4e593
add arielb1 example
nikomatsakis May 22, 2017
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4 changes: 2 additions & 2 deletions src/librustc_mir/build/expr/as_operand.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -27,8 +27,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
-> BlockAnd<Operand<'tcx>>
where M: Mirror<'tcx, Output = Expr<'tcx>>
{
let topmost_scope = self.topmost_scope(); // FIXME(#6393)
self.as_operand(block, Some(topmost_scope), expr)
let local_scope = self.local_scope();
self.as_operand(block, local_scope, expr)
}

/// Compile `expr` into a value that can be used as an operand.
Expand Down
6 changes: 3 additions & 3 deletions src/librustc_mir/build/expr/as_rvalue.rs
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Expand Up @@ -33,8 +33,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
-> BlockAnd<Rvalue<'tcx>>
where M: Mirror<'tcx, Output = Expr<'tcx>>
{
let topmost_scope = self.topmost_scope(); // FIXME(#6393)
self.as_rvalue(block, Some(topmost_scope), expr)
let local_scope = self.local_scope();
self.as_rvalue(block, local_scope, expr)
}

/// Compile `expr`, yielding an rvalue.
Expand All @@ -51,7 +51,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
scope: Option<CodeExtent>,
expr: Expr<'tcx>)
-> BlockAnd<Rvalue<'tcx>> {
debug!("expr_as_rvalue(block={:?}, expr={:?})", block, expr);
debug!("expr_as_rvalue(block={:?}, scope={:?}, expr={:?})", block, scope, expr);

let this = self;
let expr_span = expr.span;
Expand Down
3 changes: 2 additions & 1 deletion src/librustc_mir/build/expr/as_temp.rs
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Expand Up @@ -35,7 +35,8 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
temp_lifetime: Option<CodeExtent>,
expr: Expr<'tcx>)
-> BlockAnd<Lvalue<'tcx>> {
debug!("expr_as_temp(block={:?}, expr={:?})", block, expr);
debug!("expr_as_temp(block={:?}, temp_lifetime={:?}, expr={:?})",
block, temp_lifetime, expr);
let this = self;

if let ExprKind::Scope { extent, value } = expr.kind {
Expand Down
8 changes: 4 additions & 4 deletions src/librustc_mir/build/mod.rs
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Expand Up @@ -392,9 +392,9 @@ fn construct_fn<'a, 'gcx, 'tcx, A>(hir: Cx<'a, 'gcx, 'tcx>,
mir
}

pub fn construct_const<'a, 'gcx, 'tcx>(hir: Cx<'a, 'gcx, 'tcx>,
body_id: hir::BodyId)
-> Mir<'tcx> {
fn construct_const<'a, 'gcx, 'tcx>(hir: Cx<'a, 'gcx, 'tcx>,
body_id: hir::BodyId)
-> Mir<'tcx> {
let tcx = hir.tcx();
let ast_expr = &tcx.hir.body(body_id).value;
let ty = hir.tables().expr_ty_adjusted(ast_expr);
Expand All @@ -415,7 +415,7 @@ pub fn construct_const<'a, 'gcx, 'tcx>(hir: Cx<'a, 'gcx, 'tcx>,
builder.finish(vec![], ty)
}

pub fn construct_error<'a, 'gcx, 'tcx>(hir: Cx<'a, 'gcx, 'tcx>,
fn construct_error<'a, 'gcx, 'tcx>(hir: Cx<'a, 'gcx, 'tcx>,
body_id: hir::BodyId)
-> Mir<'tcx> {
let span = hir.tcx().hir.span(hir.tcx().hir.body_owner(body_id));
Expand Down
36 changes: 36 additions & 0 deletions src/librustc_mir/build/scope.rs
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Expand Up @@ -93,6 +93,7 @@ use rustc::middle::const_val::ConstVal;
use rustc::ty::subst::{Kind, Subst};
use rustc::ty::{Ty, TyCtxt};
use rustc::mir::*;
use rustc::mir::transform::MirSource;
use syntax_pos::Span;
use rustc_data_structures::indexed_vec::Idx;
use rustc_data_structures::fx::FxHashMap;
Expand Down Expand Up @@ -428,6 +429,41 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
self.scopes.last().expect("topmost_scope: no scopes present").extent
}

/// Returns the scope that we should use as the lifetime of an
/// operand. Basically, an operand must live until it is consumed.
/// This is similar to, but not quite the same as, the temporary
/// scope (which can be larger or smaller).
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This is wrong in the general case - why don't we rely on temporary_scope of None?

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@nikomatsakis nikomatsakis May 16, 2017 •
edited
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I was just trying to preserve the existing logic. To be honest, I don't quite understand the reasoning here -- it does seem to me that top-most scope is not necessarily correct. @arielb1 can maybe elaborate more -- it was introduced in arielb1@1425eed as part of the PR to fix #38669, though I didn't find the PR itself (presumably I could trace to find the bors commit...)

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In an expression like foo(bar(), baz()), we want the operands holding the result of bar() and baz() to be destroyed right after foo is called, and if calling baz panics then bar is destroyed first - at least that is the ordering that makes the most sense to me.

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To be clear, when you say "destroyed" here, you don't mean that we will run any destructors, you just mean that we'll reclaim the storage, right? (Because I would expect the values to have been moved to the callee in any case...)

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More concretely, if we have foo(Box::new(2), panic!()), because we evaluate arguments from left to right, we allocate the Box and then free it (because foo can also panic, we actually emit a drop flag there - but a "sufficiently smart" drop unswitching pass could eliminate that). We also emit a drop after the call on the "normal" path, but that drop is a no-op because the value is already consumed.

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We might want to add the above to the comment.

///
/// Consider:
///
/// let x = foo(bar(X, Y));
///
/// We wish to pop the storage for X and Y after `bar()` is
/// called, not after the whole `let` is completed.
///
/// As another example, if the second argument diverges:
///
/// foo(Box::new(2), panic!())
///
/// We would allocate the box but then free it on the unwinding
/// path; we would also emit a free on the 'success' path from
/// panic, but that will turn out to be removed as dead-code.
///
/// When building statics/constants, returns `None` since
/// intermediate values do not have to be dropped in that case.
pub fn local_scope(&self) -> Option<CodeExtent> {
match self.hir.src {
MirSource::Const(_) |
MirSource::Static(..) =>
// No need to free storage in this context.
None,
MirSource::Fn(_) =>
Some(self.topmost_scope()),
MirSource::Promoted(..) =>
bug!(),
}
}

// Scheduling drops
// ================
/// Indicates that `lvalue` should be dropped on exit from
Expand Down
8 changes: 7 additions & 1 deletion src/librustc_mir/hair/cx/mod.rs
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Expand Up @@ -36,8 +36,14 @@ pub struct Cx<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
tcx: TyCtxt<'a, 'gcx, 'tcx>,
infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
pub region_maps: Rc<RegionMaps>,

/// This is `Constness::Const` if we are compiling a `static`,
/// `const`, or the body of a `const fn`.
constness: hir::Constness,

/// What are we compiling?
pub src: MirSource,

/// True if this constant/function needs overflow checks.
check_overflow: bool,
}
Expand Down Expand Up @@ -74,7 +80,7 @@ impl<'a, 'gcx, 'tcx> Cx<'a, 'gcx, 'tcx> {
// Constants and const fn's always need overflow checks.
check_overflow |= constness == hir::Constness::Const;

Cx { tcx, infcx, region_maps, constness, check_overflow }
Cx { tcx, infcx, region_maps, constness, src, check_overflow }
}
}

Expand Down
100 changes: 100 additions & 0 deletions src/test/mir-opt/storage_live_dead_in_statics.rs
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@@ -0,0 +1,100 @@
// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// Check that when we compile the static `XXX` into MIR, we do not
// generate `StorageStart` or `StorageEnd` statements.

// ignore-tidy-linelength

static XXX: &'static Foo = &Foo {
tup: "hi",
data: &[
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
(0, 1), (0, 2), (0, 3),
]
};

#[derive(Debug)]
struct Foo {
tup: &'static str,
data: &'static [(u32, u32)]
}

fn main() {
println!("{:?}", XXX);
}

// END RUST SOURCE
// START rustc.node4.mir_map.0.mir
// bb0: {
// _7 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:29:9: 29:15
// _8 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:29:17: 29:23
// _9 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:29:25: 29:31
// _10 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:30:9: 30:15
// _11 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:30:17: 30:23
// _12 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:30:25: 30:31
// _13 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:31:9: 31:15
// _14 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:31:17: 31:23
// _15 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:31:25: 31:31
// _16 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:32:9: 32:15
// _17 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:32:17: 32:23
// _18 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:32:25: 32:31
// _19 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:33:9: 33:15
// _20 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:33:17: 33:23
// _21 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:33:25: 33:31
// _22 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:34:9: 34:15
// _23 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:34:17: 34:23
// _24 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:34:25: 34:31
// _25 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:35:9: 35:15
// _26 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:35:17: 35:23
// _27 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:35:25: 35:31
// _28 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:36:9: 36:15
// _29 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:36:17: 36:23
// _30 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:36:25: 36:31
// _31 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:37:9: 37:15
// _32 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:37:17: 37:23
// _33 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:37:25: 37:31
// _34 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:38:9: 38:15
// _35 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:38:17: 38:23
// _36 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:38:25: 38:31
// _37 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:39:9: 39:15
// _38 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:39:17: 39:23
// _39 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:39:25: 39:31
// _40 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:40:9: 40:15
// _41 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:40:17: 40:23
// _42 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:40:25: 40:31
// _43 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:41:9: 41:15
// _44 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:41:17: 41:23
// _45 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:41:25: 41:31
// _46 = (const 0u32, const 1u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:42:9: 42:15
// _47 = (const 0u32, const 2u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:42:17: 42:23
// _48 = (const 0u32, const 3u32); // scope 0 at src/test/mir-opt/basic_assignment.rs:42:25: 42:31
// _6 = [_7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, _30, _31, _32, _33, _34, _35, _36, _37, _38, _39, _40, _41, _42, _43, _44, _45, _46, _47, _48]; // scope 0 at src/test/mir-opt/basic_assignment.rs:28:12: 43:6
// _5 = &_6; // scope 0 at src/test/mir-opt/basic_assignment.rs:28:11: 43:6
// _4 = &(*_5); // scope 0 at src/test/mir-opt/basic_assignment.rs:28:11: 43:6
// _3 = _4 as &'static [(u32, u32)] (Unsize); // scope 0 at src/test/mir-opt/basic_assignment.rs:28:11: 43:6
// _2 = Foo { tup: const "hi", data: _3 }; // scope 0 at src/test/mir-opt/basic_assignment.rs:26:29: 44:2
// _1 = &_2; // scope 0 at src/test/mir-opt/basic_assignment.rs:26:28: 44:2
// _0 = &(*_1); // scope 0 at src/test/mir-opt/basic_assignment.rs:26:28: 44:2
// return; // scope 0 at src/test/mir-opt/basic_assignment.rs:26:1: 44:3
// }
// END rustc.node4.mir_map.0.mir