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Auto merge of #66294 - davidhewitt:const_fn_memoization, r=oli-obk
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Add memoization for const function evaluations

When a const function is being evaluated, as long as all its arguments are zero-sized-types (or it has no arguments) then we can trivially memoize the evaluation result using the existing query mechanism.

With thanks to @oli-obk for mentoring me through this at RustFest Barcelona.

r? @oli-obk
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bors committed Nov 28, 2019
2 parents 96ad8e5 + a28fbd4 commit 2539b5f
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Showing 10 changed files with 101 additions and 132 deletions.
5 changes: 4 additions & 1 deletion src/librustc/mir/interpret/mod.rs
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Expand Up @@ -123,7 +123,10 @@ use rustc_data_structures::tiny_list::TinyList;
use rustc_macros::HashStable;
use byteorder::{WriteBytesExt, ReadBytesExt, LittleEndian, BigEndian};

/// Uniquely identifies a specific constant or static.
/// Uniquely identifies one of the following:
/// - A constant
/// - A static
/// - A const fn where all arguments (if any) are zero-sized types
#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, RustcEncodable, RustcDecodable)]
#[derive(HashStable, Lift)]
pub struct GlobalId<'tcx> {
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11 changes: 10 additions & 1 deletion src/librustc_mir/const_eval.rs
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Expand Up @@ -146,7 +146,16 @@ fn eval_body_using_ecx<'mir, 'tcx>(
let name = ty::tls::with(|tcx| tcx.def_path_str(cid.instance.def_id()));
let prom = cid.promoted.map_or(String::new(), |p| format!("::promoted[{:?}]", p));
trace!("eval_body_using_ecx: pushing stack frame for global: {}{}", name, prom);
assert!(body.arg_count == 0);

// Assert all args (if any) are zero-sized types; `eval_body_using_ecx` doesn't
// make sense if the body is expecting nontrivial arguments.
// (The alternative would be to use `eval_fn_call` with an args slice.)
for arg in body.args_iter() {
let decl = body.local_decls.get(arg).expect("arg missing from local_decls");
let layout = ecx.layout_of(decl.ty.subst(tcx, cid.instance.substs))?;
assert!(layout.is_zst())
};

ecx.push_stack_frame(
cid.instance,
body.span,
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33 changes: 32 additions & 1 deletion src/librustc_mir/interpret/terminator.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use syntax::source_map::Span;
use rustc_target::spec::abi::Abi;

use super::{
InterpResult, PointerArithmetic,
GlobalId, InterpResult, PointerArithmetic,
InterpCx, Machine, OpTy, ImmTy, PlaceTy, MPlaceTy, StackPopCleanup, FnVal,
};

Expand Down Expand Up @@ -284,6 +284,18 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
ty::InstanceDef::DropGlue(..) |
ty::InstanceDef::CloneShim(..) |
ty::InstanceDef::Item(_) => {
// If this function is a `const fn` then as an optimization we can query this
// evaluation immediately.
//
// For the moment we only do this for functions which take no arguments
// (or all arguments are ZSTs) so that we don't memoize too much.
if self.tcx.is_const_fn_raw(instance.def.def_id()) &&
args.iter().all(|a| a.layout.is_zst())
{
let gid = GlobalId { instance, promoted: None };
return self.eval_const_fn_call(gid, ret);
}

// We need MIR for this fn
let body = match M::find_fn(self, instance, args, ret, unwind)? {
Some(body) => body,
Expand Down Expand Up @@ -449,6 +461,25 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
}
}

/// Evaluate a const function where all arguments (if any) are zero-sized types.
/// The evaluation is memoized thanks to the query system.
fn eval_const_fn_call(
&mut self,
gid: GlobalId<'tcx>,
ret: Option<(PlaceTy<'tcx, M::PointerTag>, mir::BasicBlock)>,
) -> InterpResult<'tcx> {
trace!("eval_const_fn_call: {:?}", gid);

let place = self.const_eval_raw(gid)?;
let dest = ret.ok_or_else(|| err_ub!(Unreachable))?.0;

self.copy_op(place.into(), dest)?;

self.return_to_block(ret.map(|r| r.1))?;
self.dump_place(*dest);
return Ok(())
}

fn drop_in_place(
&mut self,
place: PlaceTy<'tcx, M::PointerTag>,
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14 changes: 14 additions & 0 deletions src/test/ui/consts/const-fn-zst-args.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
// build-pass

// Check that the evaluation of const-functions with
// zero-sized types as arguments compiles successfully

struct Zst {}

const fn foo(val: Zst) -> Zst { val }

const FOO: Zst = foo(Zst {});

fn main() {
const _: Zst = FOO;
}
5 changes: 5 additions & 0 deletions src/test/ui/consts/const-size_of-cycle.stderr
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Expand Up @@ -10,6 +10,11 @@ note: ...which requires const-evaluating + checking `Foo::bytes::{{constant}}#0`
LL | bytes: [u8; std::mem::size_of::<Foo>()]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires const-evaluating `Foo::bytes::{{constant}}#0`...
--> $DIR/const-size_of-cycle.rs:5:17
|
LL | bytes: [u8; std::mem::size_of::<Foo>()]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires const-evaluating `std::mem::size_of`...
--> $SRC_DIR/libcore/mem/mod.rs:LL:COL
|
LL | intrinsics::size_of::<T>()
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6 changes: 3 additions & 3 deletions src/test/ui/consts/uninhabited-const-issue-61744.rs
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Original file line number Diff line number Diff line change
@@ -1,11 +1,11 @@
// compile-fail

pub const unsafe fn fake_type<T>() -> T {
hint_unreachable() //~ ERROR any use of this value will cause an error
hint_unreachable()
}

pub const unsafe fn hint_unreachable() -> ! {
fake_type()
fake_type() //~ ERROR cycle detected when const-evaluating `hint_unreachable` [E0391]
}

trait Const {
Expand All @@ -15,5 +15,5 @@ trait Const {
impl <T> Const for T {}

pub fn main() -> () {
dbg!(i32::CONSTANT); //~ ERROR erroneous constant used
dbg!(i32::CONSTANT);
}
80 changes: 14 additions & 66 deletions src/test/ui/consts/uninhabited-const-issue-61744.stderr
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Original file line number Diff line number Diff line change
@@ -1,73 +1,21 @@
error: any use of this value will cause an error
error[E0391]: cycle detected when const-evaluating `hint_unreachable`
--> $DIR/uninhabited-const-issue-61744.rs:8:5
|
LL | fake_type()
| ^^^^^^^^^^^
|
note: ...which requires const-evaluating `fake_type`...
--> $DIR/uninhabited-const-issue-61744.rs:4:5
|
LL | hint_unreachable()
| ^^^^^^^^^^^^^^^^^^
| |
| reached the configured maximum number of stack frames
| inside call to `hint_unreachable` at $DIR/uninhabited-const-issue-61744.rs:4:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<!>` at $DIR/uninhabited-const-issue-61744.rs:8:5
| inside call to `fake_type::<i32>` at $DIR/uninhabited-const-issue-61744.rs:12:36
...
LL | const CONSTANT: i32 = unsafe { fake_type() };
| ---------------------------------------------
|
= note: `#[deny(const_err)]` on by default

error[E0080]: erroneous constant used
--> $DIR/uninhabited-const-issue-61744.rs:18:10
= note: ...which again requires const-evaluating `hint_unreachable`, completing the cycle
note: cycle used when const-evaluating `fake_type`
--> $DIR/uninhabited-const-issue-61744.rs:4:5
|
LL | dbg!(i32::CONSTANT);
| ^^^^^^^^^^^^^ referenced constant has errors
LL | hint_unreachable()
| ^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors
error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.
For more information about this error, try `rustc --explain E0391`.
2 changes: 1 addition & 1 deletion src/test/ui/infinite/infinite-recursion-const-fn.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
//https://github.com/rust-lang/rust/issues/31364

const fn a() -> usize { b() } //~ ERROR evaluation of constant value failed
const fn a() -> usize { b() } //~ ERROR cycle detected when const-evaluating `a` [E0391]
const fn b() -> usize { a() }
const ARR: [i32; a()] = [5; 6];

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69 changes: 12 additions & 57 deletions src/test/ui/infinite/infinite-recursion-const-fn.stderr
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Original file line number Diff line number Diff line change
@@ -1,66 +1,21 @@
error[E0080]: evaluation of constant value failed
error[E0391]: cycle detected when const-evaluating `a`
--> $DIR/infinite-recursion-const-fn.rs:3:25
|
LL | const fn a() -> usize { b() }
| ^^^
| |
| reached the configured maximum number of stack frames
| inside call to `b` at $DIR/infinite-recursion-const-fn.rs:3:25
|
note: ...which requires const-evaluating `b`...
--> $DIR/infinite-recursion-const-fn.rs:4:25
|
LL | const fn b() -> usize { a() }
| ---
| |
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| inside call to `a` at $DIR/infinite-recursion-const-fn.rs:4:25
| ^^^
= note: ...which again requires const-evaluating `a`, completing the cycle
note: cycle used when const-evaluating `ARR::{{constant}}#0`
--> $DIR/infinite-recursion-const-fn.rs:5:18
|
LL | const ARR: [i32; a()] = [5; 6];
| --- inside call to `a` at $DIR/infinite-recursion-const-fn.rs:5:18
| ^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.
For more information about this error, try `rustc --explain E0391`.
8 changes: 6 additions & 2 deletions src/test/ui/invalid_const_promotion.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,12 +17,16 @@ use std::env;
use std::process::{Command, Stdio};

// this will panic in debug mode and overflow in release mode
//
// NB we give bar an unused argument because otherwise memoization
// of the const fn kicks in, causing a different code path in the
// compiler to be executed (see PR #66294).
#[stable(feature = "rustc", since = "1.0.0")]
#[rustc_promotable]
const fn bar() -> usize { 0 - 1 }
const fn bar(_: bool) -> usize { 0 - 1 }

fn foo() {
let _: &'static _ = &bar();
let _: &'static _ = &bar(true);
}

#[cfg(unix)]
Expand Down

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