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Omit non-needs_drop drop_in_place in vtables

Browse source 
This replaces the drop_in_place reference with null in vtables. On
librustc_driver.so, this drops about ~17k dynamic relocations from the
output, since many vtables can now be placed in read-only memory, rather
than having a relocated pointer included.

This makes a tradeoff by adding a null check at vtable call sites.
That's hard to avoid without changing the vtable format (e.g., to use a
pc-relative relocation instead of an absolute address, and avoid the
dynamic relocation that way). But it seems likely that the check is
cheap at runtime.
This commit is contained in:
Mark Rousskov 2024-03-17 17:42:37 -04:00
parent b0f8618938
commit 4c002fce9d
7 changed files with 203 additions and 116 deletions
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16
compiler/rustc_codegen_cranelift/src/abi/mod.rs
View file

@ -593,6 +593,7 @@ pub(crate) fn codegen_drop<'tcx>(
fx: &mut FunctionCx<'_, '_, 'tcx>,
source_info: mir::SourceInfo,
drop_place: CPlace<'tcx>,
target: BasicBlock,
) {
let ty = drop_place.layout().ty;
let drop_instance = Instance::resolve_drop_in_place(fx.tcx, ty).polymorphize(fx.tcx);
@ -620,6 +621,12 @@ pub(crate) fn codegen_drop<'tcx>(
let ptr = ptr.get_addr(fx);
let drop_fn = crate::vtable::drop_fn_of_obj(fx, vtable);
let is_null = fx.bcx.ins().icmp_imm(IntCC::Equal, drop_fn, 0);
let target_block = fx.get_block(target);
let continued = fx.bcx.create_block();
fx.bcx.ins().brif(is_null, target_block, &[], continued, &[]);
fx.bcx.switch_to_block(continued);
// FIXME(eddyb) perhaps move some of this logic into
// `Instance::resolve_drop_in_place`?
let virtual_drop = Instance {
@ -659,6 +666,12 @@ pub(crate) fn codegen_drop<'tcx>(
let (data, vtable) = drop_place.to_cvalue(fx).dyn_star_force_data_on_stack(fx);
let drop_fn = crate::vtable::drop_fn_of_obj(fx, vtable);
let is_null = fx.bcx.ins().icmp_imm(IntCC::Equal, drop_fn, 0);
let target_block = fx.get_block(target);
let continued = fx.bcx.create_block();
fx.bcx.ins().brif(is_null, target_block, &[], continued, &[]);
fx.bcx.switch_to_block(continued);
let virtual_drop = Instance {
def: ty::InstanceDef::Virtual(drop_instance.def_id(), 0),
args: drop_instance.args,
@ -697,4 +710,7 @@ pub(crate) fn codegen_drop<'tcx>(
}
}
}
let target_block = fx.get_block(target);
fx.bcx.ins().jump(target_block, &[]);
}

5
compiler/rustc_codegen_cranelift/src/base.rs
View file

@ -548,10 +548,7 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
}
TerminatorKind::Drop { place, target, unwind: _, replace: _ } => {
let drop_place = codegen_place(fx, *place);
crate::abi::codegen_drop(fx, source_info, drop_place);
let target_block = fx.get_block(*target);
fx.bcx.ins().jump(target_block, &[]);
crate::abi::codegen_drop(fx, source_info, drop_place, *target);
}
};
}

27
compiler/rustc_codegen_ssa/src/meth.rs
View file

@ -15,12 +15,13 @@ pub fn from_index(index: usize) -> Self {
VirtualIndex(index as u64)
}
pub fn get_fn<Bx: BuilderMethods<'a, 'tcx>>(
fn get_fn_inner<Bx: BuilderMethods<'a, 'tcx>>(
self,
bx: &mut Bx,
llvtable: Bx::Value,
ty: Ty<'tcx>,
fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
nonnull: bool,
) -> Bx::Value {
// Load the function pointer from the object.
debug!("get_fn({llvtable:?}, {ty:?}, {self:?})");
@ -41,13 +42,35 @@ pub fn get_fn<Bx: BuilderMethods<'a, 'tcx>>(
} else {
let gep = bx.inbounds_ptradd(llvtable, bx.const_usize(vtable_byte_offset));
let ptr = bx.load(llty, gep, ptr_align);
bx.nonnull_metadata(ptr);
// VTable loads are invariant.
bx.set_invariant_load(ptr);
if nonnull {
bx.nonnull_metadata(ptr);
}
ptr
}
}
pub fn get_optional_fn<Bx: BuilderMethods<'a, 'tcx>>(
self,
bx: &mut Bx,
llvtable: Bx::Value,
ty: Ty<'tcx>,
fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
) -> Bx::Value {
self.get_fn_inner(bx, llvtable, ty, fn_abi, false)
}
pub fn get_fn<Bx: BuilderMethods<'a, 'tcx>>(
self,
bx: &mut Bx,
llvtable: Bx::Value,
ty: Ty<'tcx>,
fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
) -> Bx::Value {
self.get_fn_inner(bx, llvtable, ty, fn_abi, true)
}
pub fn get_usize<Bx: BuilderMethods<'a, 'tcx>>(
self,
bx: &mut Bx,

200
compiler/rustc_codegen_ssa/src/mir/block.rs
View file

@ -500,6 +500,7 @@ fn codegen_drop_terminator(
&mut self,
helper: TerminatorCodegenHelper<'tcx>,
bx: &mut Bx,
source_info: &mir::SourceInfo,
location: mir::Place<'tcx>,
target: mir::BasicBlock,
unwind: mir::UnwindAction,
@ -523,90 +524,106 @@ fn codegen_drop_terminator(
args1 = [place.val.llval];
&args1[..]
};
let (drop_fn, fn_abi, drop_instance) =
match ty.kind() {
// FIXME(eddyb) perhaps move some of this logic into
// `Instance::resolve_drop_in_place`?
ty::Dynamic(_, _, ty::Dyn) => {
// IN THIS ARM, WE HAVE:
// ty = *mut (dyn Trait)
// which is: exists<T> ( *mut T, Vtable<T: Trait> )
// args[0] args[1]
//
// args = ( Data, Vtable )
// |
// v
// /-------\
// | ... |
// \-------/
//
let virtual_drop = Instance {
def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0), // idx 0: the drop function
args: drop_fn.args,
};
debug!("ty = {:?}", ty);
debug!("drop_fn = {:?}", drop_fn);
debug!("args = {:?}", args);
let fn_abi = bx.fn_abi_of_instance(virtual_drop, ty::List::empty());
let vtable = args[1];
// Truncate vtable off of args list
args = &args[..1];
(
meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_DROPINPLACE)
.get_fn(bx, vtable, ty, fn_abi),
fn_abi,
virtual_drop,
)
}
ty::Dynamic(_, _, ty::DynStar) => {
// IN THIS ARM, WE HAVE:
// ty = *mut (dyn* Trait)
// which is: *mut exists<T: sizeof(T) == sizeof(usize)> (T, Vtable<T: Trait>)
//
// args = [ * ]
// |
// v
// ( Data, Vtable )
// |
// v
// /-------\
// | ... |
// \-------/
//
//
// WE CAN CONVERT THIS INTO THE ABOVE LOGIC BY DOING
//
// data = &(*args[0]).0 // gives a pointer to Data above (really the same pointer)
// vtable = (*args[0]).1 // loads the vtable out
// (data, vtable) // an equivalent Rust `*mut dyn Trait`
//
// SO THEN WE CAN USE THE ABOVE CODE.
let virtual_drop = Instance {
def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0), // idx 0: the drop function
args: drop_fn.args,
};
debug!("ty = {:?}", ty);
debug!("drop_fn = {:?}", drop_fn);
debug!("args = {:?}", args);
let fn_abi = bx.fn_abi_of_instance(virtual_drop, ty::List::empty());
let meta_ptr = place.project_field(bx, 1);
let meta = bx.load_operand(meta_ptr);
// Truncate vtable off of args list
args = &args[..1];
debug!("args' = {:?}", args);
(
meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_DROPINPLACE)
.get_fn(bx, meta.immediate(), ty, fn_abi),
fn_abi,
virtual_drop,
)
}
_ => (
bx.get_fn_addr(drop_fn),
bx.fn_abi_of_instance(drop_fn, ty::List::empty()),
drop_fn,
),
};
let (maybe_null, drop_fn, fn_abi, drop_instance) = match ty.kind() {
// FIXME(eddyb) perhaps move some of this logic into
// `Instance::resolve_drop_in_place`?
ty::Dynamic(_, _, ty::Dyn) => {
// IN THIS ARM, WE HAVE:
// ty = *mut (dyn Trait)
// which is: exists<T> ( *mut T, Vtable<T: Trait> )
// args[0] args[1]
//
// args = ( Data, Vtable )
// |
// v
// /-------\
// | ... |
// \-------/
//
let virtual_drop = Instance {
def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0), // idx 0: the drop function
args: drop_fn.args,
};
debug!("ty = {:?}", ty);
debug!("drop_fn = {:?}", drop_fn);
debug!("args = {:?}", args);
let fn_abi = bx.fn_abi_of_instance(virtual_drop, ty::List::empty());
let vtable = args[1];
// Truncate vtable off of args list
args = &args[..1];
(
true,
meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_DROPINPLACE)
.get_optional_fn(bx, vtable, ty, fn_abi),
fn_abi,
virtual_drop,
)
}
ty::Dynamic(_, _, ty::DynStar) => {
// IN THIS ARM, WE HAVE:
// ty = *mut (dyn* Trait)
// which is: *mut exists<T: sizeof(T) == sizeof(usize)> (T, Vtable<T: Trait>)
//
// args = [ * ]
// |
// v
// ( Data, Vtable )
// |
// v
// /-------\
// | ... |
// \-------/
//
//
// WE CAN CONVERT THIS INTO THE ABOVE LOGIC BY DOING
//
// data = &(*args[0]).0 // gives a pointer to Data above (really the same pointer)
// vtable = (*args[0]).1 // loads the vtable out
// (data, vtable) // an equivalent Rust `*mut dyn Trait`
//
// SO THEN WE CAN USE THE ABOVE CODE.
let virtual_drop = Instance {
def: ty::InstanceDef::Virtual(drop_fn.def_id(), 0), // idx 0: the drop function
args: drop_fn.args,
};
debug!("ty = {:?}", ty);
debug!("drop_fn = {:?}", drop_fn);
debug!("args = {:?}", args);
let fn_abi = bx.fn_abi_of_instance(virtual_drop, ty::List::empty());
let meta_ptr = place.project_field(bx, 1);
let meta = bx.load_operand(meta_ptr);
// Truncate vtable off of args list
args = &args[..1];
debug!("args' = {:?}", args);
(
true,
meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_DROPINPLACE)
.get_optional_fn(bx, meta.immediate(), ty, fn_abi),
fn_abi,
virtual_drop,
)
}
_ => (
false,
bx.get_fn_addr(drop_fn),
bx.fn_abi_of_instance(drop_fn, ty::List::empty()),
drop_fn,
),
};
// We generate a null check for the drop_fn. This saves a bunch of relocations being
// generated for no-op drops.
if maybe_null {
let is_not_null = bx.append_sibling_block("is_not_null");
let llty = bx.fn_ptr_backend_type(fn_abi);
let null = bx.const_null(llty);
let non_null =
bx.icmp(base::bin_op_to_icmp_predicate(mir::BinOp::Ne, false), drop_fn, null);
bx.cond_br(non_null, is_not_null, helper.llbb_with_cleanup(self, target));
bx.switch_to_block(is_not_null);
self.set_debug_loc(bx, *source_info);
}
helper.do_call(
self,
bx,
@ -617,7 +634,7 @@ fn codegen_drop_terminator(
unwind,
&[],
Some(drop_instance),
mergeable_succ,
!maybe_null && mergeable_succ,
)
}
@ -1346,9 +1363,16 @@ fn codegen_terminator(
MergingSucc::False
}
mir::TerminatorKind::Drop { place, target, unwind, replace: _ } => {
self.codegen_drop_terminator(helper, bx, place, target, unwind, mergeable_succ())
}
mir::TerminatorKind::Drop { place, target, unwind, replace: _ } => self
.codegen_drop_terminator(
helper,
bx,
&terminator.source_info,
place,
target,
unwind,
mergeable_succ(),
),
mir::TerminatorKind::Assert { ref cond, expected, ref msg, target, unwind } => self
.codegen_assert_terminator(

12
compiler/rustc_middle/src/ty/vtable.rs
View file

@ -84,10 +84,14 @@ pub(super) fn vtable_allocation_provider<'tcx>(
let idx: u64 = u64::try_from(idx).unwrap();
let scalar = match entry {
VtblEntry::MetadataDropInPlace => {
let instance = ty::Instance::resolve_drop_in_place(tcx, ty);
let fn_alloc_id = tcx.reserve_and_set_fn_alloc(instance);
let fn_ptr = Pointer::from(fn_alloc_id);
Scalar::from_pointer(fn_ptr, &tcx)
if ty.needs_drop(tcx, ty::ParamEnv::reveal_all()) {
let instance = ty::Instance::resolve_drop_in_place(tcx, ty);
let fn_alloc_id = tcx.reserve_and_set_fn_alloc(instance);
let fn_ptr = Pointer::from(fn_alloc_id);
Scalar::from_pointer(fn_ptr, &tcx)
} else {
Scalar::from_maybe_pointer(Pointer::null(), &tcx)
}
}
VtblEntry::MetadataSize => Scalar::from_uint(size, ptr_size),
VtblEntry::MetadataAlign => Scalar::from_uint(align, ptr_size),

43
tests/codegen-units/partitioning/vtable-through-const.rs
View file

@ -9,19 +9,25 @@
#![feature(start)]
mod mod1 {
struct NeedsDrop;
impl Drop for NeedsDrop {
fn drop(&mut self) {}
}
pub trait Trait1 {
fn do_something(&self) {}
fn do_something_else(&self) {}
}
impl Trait1 for u32 {}
impl Trait1 for NeedsDrop {}
pub trait Trait1Gen<T> {
fn do_something(&self, x: T) -> T;
fn do_something_else(&self, x: T) -> T;
}
impl<T> Trait1Gen<T> for u32 {
impl<T> Trait1Gen<T> for NeedsDrop {
fn do_something(&self, x: T) -> T { x }
fn do_something_else(&self, x: T) -> T { x }
}
@ -30,8 +36,8 @@ fn do_something_else(&self, x: T) -> T { x }
fn id<T>(x: T) -> T { x }
// These are referenced, so they produce mono-items (see start())
pub const TRAIT1_REF: &'static Trait1 = &0u32 as &Trait1;
pub const TRAIT1_GEN_REF: &'static Trait1Gen<u8> = &0u32 as &Trait1Gen<u8>;
pub const TRAIT1_REF: &'static Trait1 = &NeedsDrop as &Trait1;
pub const TRAIT1_GEN_REF: &'static Trait1Gen<u8> = &NeedsDrop as &Trait1Gen<u8>;
pub const ID_CHAR: fn(char) -> char = id::<char>;
@ -41,47 +47,48 @@ fn do_something(&self) {}
fn do_something_else(&self) {}
}
//~ MONO_ITEM fn <u32 as mod1::Trait2>::do_something @@ vtable_through_const-mod1.volatile[Internal]
//~ MONO_ITEM fn <u32 as mod1::Trait2>::do_something_else @@ vtable_through_const-mod1.volatile[Internal]
impl Trait2 for u32 {}
//~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait2>::do_something @@ vtable_through_const-mod1.volatile[Internal]
//~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait2>::do_something_else @@ vtable_through_const-mod1.volatile[Internal]
impl Trait2 for NeedsDrop {}
pub trait Trait2Gen<T> {
fn do_something(&self, x: T) -> T;
fn do_something_else(&self, x: T) -> T;
}
impl<T> Trait2Gen<T> for u32 {
impl<T> Trait2Gen<T> for NeedsDrop {
fn do_something(&self, x: T) -> T { x }
fn do_something_else(&self, x: T) -> T { x }
}
// These are not referenced, so they do not produce mono-items
pub const TRAIT2_REF: &'static Trait2 = &0u32 as &Trait2;
pub const TRAIT2_GEN_REF: &'static Trait2Gen<u8> = &0u32 as &Trait2Gen<u8>;
pub const TRAIT2_REF: &'static Trait2 = &NeedsDrop as &Trait2;
pub const TRAIT2_GEN_REF: &'static Trait2Gen<u8> = &NeedsDrop as &Trait2Gen<u8>;
pub const ID_I64: fn(i64) -> i64 = id::<i64>;
}
//~ MONO_ITEM fn start
#[start]
fn start(_: isize, _: *const *const u8) -> isize {
//~ MONO_ITEM fn std::ptr::drop_in_place::<u32> - shim(None) @@ vtable_through_const[Internal]
//~ MONO_ITEM fn <mod1::NeedsDrop as std::ops::Drop>::drop @@ vtable_through_const-fallback.cgu[Internal]
//~ MONO_ITEM fn std::ptr::drop_in_place::<mod1::NeedsDrop> - shim(Some(mod1::NeedsDrop)) @@ vtable_through_const-fallback.cgu[External]
// Since Trait1::do_something() is instantiated via its default implementation,
// it is considered a generic and is instantiated here only because it is
// referenced in this module.
//~ MONO_ITEM fn <u32 as mod1::Trait1>::do_something_else @@ vtable_through_const-mod1.volatile[External]
//~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait1>::do_something_else @@ vtable_through_const-mod1.volatile[External]
// Although it is never used, Trait1::do_something_else() has to be
// instantiated locally here too, otherwise the <&u32 as &Trait1> vtable
// instantiated locally here too, otherwise the <&NeedsDrop as &Trait1> vtable
// could not be fully constructed.
//~ MONO_ITEM fn <u32 as mod1::Trait1>::do_something @@ vtable_through_const-mod1.volatile[External]
//~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait1>::do_something @@ vtable_through_const-mod1.volatile[External]
mod1::TRAIT1_REF.do_something();
// Same as above
//~ MONO_ITEM fn <u32 as mod1::Trait1Gen<u8>>::do_something @@ vtable_through_const-mod1.volatile[External]
//~ MONO_ITEM fn <u32 as mod1::Trait1Gen<u8>>::do_something_else @@ vtable_through_const-mod1.volatile[External]
//~ MONO_ITEM fn <u32 as mod1::Trait2Gen<u8>>::do_something @@ vtable_through_const-mod1.volatile[Internal]
//~ MONO_ITEM fn <u32 as mod1::Trait2Gen<u8>>::do_something_else @@ vtable_through_const-mod1.volatile[Internal]
//~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait1Gen<u8>>::do_something @@ vtable_through_const-mod1.volatile[External]
//~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait1Gen<u8>>::do_something_else @@ vtable_through_const-mod1.volatile[External]
//~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait2Gen<u8>>::do_something @@ vtable_through_const-mod1.volatile[Internal]
//~ MONO_ITEM fn <mod1::NeedsDrop as mod1::Trait2Gen<u8>>::do_something_else @@ vtable_through_const-mod1.volatile[Internal]
mod1::TRAIT1_GEN_REF.do_something(0u8);
//~ MONO_ITEM fn mod1::id::<char> @@ vtable_through_const-mod1.volatile[External]

16
tests/codegen/vtable-loads.rs Normal file
View file

@ -0,0 +1,16 @@
//@ compile-flags: -O
#![crate_type = "lib"]
// CHECK-LABEL: @loop_skips_vtable_load
#[no_mangle]
pub fn loop_skips_vtable_load(x: &dyn Fn()) {
// CHECK: load ptr, ptr %0{{.*}}, !invariant.load
// CHECK-NEXT: tail call void %1
// CHECK-NOT: load ptr
x();
for _ in 0..100 {
// CHECK: tail call void %1
x();
}
}