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Remove slow HashSet during miri stack frame creation #49274

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merged 6 commits into from
Mar 24, 2018
Merged

Remove slow HashSet during miri stack frame creation #49274

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9839e5f
Remove slow HashSet during miri stack frame creation
oli-obk Mar 22, 2018
9fa14e4
Skip checking for Storage* statements in constants/statics
oli-obk Mar 22, 2018
bf8e4f2
Vec<_> -> IndexVec<Local, _>
oli-obk Mar 23, 2018
b18b776
Replace uses of `Hash(Map|Set)` with `FxHash(Map|Set)` in miri
oli-obk Mar 23, 2018
4ea4dd2
Don't allocate a local array at all if there are no locals
oli-obk Mar 23, 2018
f9019ae
Simplify local accessors
oli-obk Mar 23, 2018
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49 changes: 19 additions & 30 deletions src/librustc_mir/interpret/eval_context.rs
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
use std::collections::HashSet;
use std::fmt::Write;

use rustc::hir::def_id::DefId;
use rustc::hir::def::Def;
use rustc::hir::map::definitions::DefPathData;
use rustc::middle::const_val::{ConstVal, ErrKind};
use rustc::mir;
Expand Down Expand Up @@ -383,40 +383,29 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M
) -> EvalResult<'tcx> {
::log_settings::settings().indentation += 1;

/// Return the set of locals that have a storage annotation anywhere
fn collect_storage_annotations<'mir, 'tcx>(mir: &'mir mir::Mir<'tcx>) -> HashSet<mir::Local> {
use rustc::mir::StatementKind::*;

let mut set = HashSet::new();
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Would be a good idea to check for Hash{Map,Set} used by miri code: if it needs to be a hash map/set, it should be FxHash{Map,Set} instead.

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Done, found a few in interpret::memory

for block in mir.basic_blocks() {
for stmt in block.statements.iter() {
match stmt.kind {
StorageLive(local) |
StorageDead(local) => {
set.insert(local);
}
_ => {}
}
}
}
set
}

// Subtract 1 because `local_decls` includes the ReturnMemoryPointer, but we don't store a local
// `Value` for that.
let num_locals = mir.local_decls.len() - 1;

let locals = {
let annotated_locals = collect_storage_annotations(mir);
let mut locals = vec![None; num_locals];
for i in 0..num_locals {
let local = mir::Local::new(i + 1);
if !annotated_locals.contains(&local) {
locals[i] = Some(Value::ByVal(PrimVal::Undef));
let mut locals = vec![Some(Value::ByVal(PrimVal::Undef)); num_locals];
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Please use IndexVec here.

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The reason we didn't use that is theat we don't actually use the return pointer local at all.

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@oli-obk oli-obk Mar 23, 2018 •
edited
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Fixed, the waste of one space is massively outweighted by the many .index() - 1 calls we had.

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Are you sure? Now you have to allocate something on the heap even if it's never used.

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For truly trivial constants (const FOO: usize = 42;) that is true, but even const FOO: usize = 40 + 2; will create that heap alloc.

I added a fast path for trivial constants

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👍

match self.tcx.describe_def(instance.def_id()) {
// statics and constants don't have `Storage*` statements, no need to look for them
Some(Def::Static(..)) | Some(Def::Const(..)) | Some(Def::AssociatedConst(..)) => {},
_ => {
trace!("push_stack_frame: {:?}: num_bbs: {}", span, mir.basic_blocks().len());
for block in mir.basic_blocks() {
for stmt in block.statements.iter() {
use rustc::mir::StatementKind::{StorageDead, StorageLive};
match stmt.kind {
StorageLive(local) | StorageDead(local) => if local.index() > 0 {
locals[local.index() - 1] = None;
},
_ => {}
}
}
}
}
locals
};
},
}

self.stack.push(Frame {
mir,
Expand Down