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Auto merge of #96285 - flip1995:pk-vfe, r=nagisa

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Introduce `-Zvirtual-function-elimination` codegen flag

Fixes #68262

This PR adds a codegen flag `-Zvirtual-function-elimination` to enable the VFE optimization in LLVM. To make this work, additonal  information has to be added to vtables ([`!vcall_visibility` metadata](https://llvm.org/docs/TypeMetadata.html#vcall-visibility-metadata) and a `typeid` of the trait). Furthermore, instead of just `load`ing functions, the [`llvm.type.checked.load` intrinsic](https://llvm.org/docs/LangRef.html#llvm-type-checked-load-intrinsic) has to be used to map functions to vtables.

For technical details of the changes, see the commit messages.

I also tested this flag on https://github.com/tock/tock on different boards to verify that this fixes the issue https://github.com/tock/tock/issues/2594. This flag is able to improve the size of the resulting binary by about 8k-9k bytes by removing the unused debug print functions.

[Rendered documentation update](https://github.com/flip1995/rust/blob/pk-vfe/src/doc/rustc/src/codegen-options/index.md#virtual-function-elimination)
This commit is contained in:
bors 2022-06-14 21:37:11 +00:00
commit 2d1e075079
20 changed files with 431 additions and 32 deletions
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52
compiler/rustc_codegen_ssa/src/meth.rs
View file

@ -1,6 +1,8 @@
use crate::traits::*;
use rustc_middle::ty::{self, Ty};
use rustc_middle::ty::{self, subst::GenericArgKind, ExistentialPredicate, Ty, TyCtxt};
use rustc_session::config::Lto;
use rustc_symbol_mangling::typeid_for_trait_ref;
use rustc_target::abi::call::FnAbi;
#[derive(Copy, Clone, Debug)]
@ -15,20 +17,32 @@ impl<'a, 'tcx> VirtualIndex {
self,
bx: &mut Bx,
llvtable: Bx::Value,
ty: Ty<'tcx>,
fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
) -> Bx::Value {
// Load the data pointer from the object.
debug!("get_fn({:?}, {:?})", llvtable, self);
debug!("get_fn({llvtable:?}, {ty:?}, {self:?})");
let llty = bx.fn_ptr_backend_type(fn_abi);
let llvtable = bx.pointercast(llvtable, bx.type_ptr_to(llty));
let ptr_align = bx.tcx().data_layout.pointer_align.abi;
let gep = bx.inbounds_gep(llty, llvtable, &[bx.const_usize(self.0)]);
let ptr = bx.load(llty, gep, ptr_align);
bx.nonnull_metadata(ptr);
// Vtable loads are invariant.
bx.set_invariant_load(ptr);
ptr
if bx.cx().sess().opts.debugging_opts.virtual_function_elimination
&& bx.cx().sess().lto() == Lto::Fat
{
let typeid =
bx.typeid_metadata(typeid_for_trait_ref(bx.tcx(), get_trait_ref(bx.tcx(), ty)));
let vtable_byte_offset = self.0 * bx.data_layout().pointer_size.bytes();
let type_checked_load = bx.type_checked_load(llvtable, vtable_byte_offset, typeid);
let func = bx.extract_value(type_checked_load, 0);
bx.pointercast(func, llty)
} else {
let ptr_align = bx.tcx().data_layout.pointer_align.abi;
let gep = bx.inbounds_gep(llty, llvtable, &[bx.const_usize(self.0)]);
let ptr = bx.load(llty, gep, ptr_align);
bx.nonnull_metadata(ptr);
// Vtable loads are invariant.
bx.set_invariant_load(ptr);
ptr
}
}
pub fn get_usize<Bx: BuilderMethods<'a, 'tcx>>(
@ -50,6 +64,24 @@ impl<'a, 'tcx> VirtualIndex {
}
}
fn get_trait_ref<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ty::PolyExistentialTraitRef<'tcx> {
for arg in ty.peel_refs().walk() {
if let GenericArgKind::Type(ty) = arg.unpack() {
if let ty::Dynamic(trait_refs, _) = ty.kind() {
return trait_refs[0].map_bound(|trait_ref| match trait_ref {
ExistentialPredicate::Trait(tr) => tr,
ExistentialPredicate::Projection(proj) => proj.trait_ref(tcx),
ExistentialPredicate::AutoTrait(_) => {
bug!("auto traits don't have functions")
}
});
}
}
}
bug!("expected a `dyn Trait` ty, found {ty:?}")
}
/// Creates a dynamic vtable for the given type and vtable origin.
/// This is used only for objects.
///

18
compiler/rustc_codegen_ssa/src/mir/block.rs
View file

@ -401,7 +401,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
args = &args[..1];
(
meth::VirtualIndex::from_index(ty::COMMON_VTABLE_ENTRIES_DROPINPLACE)
.get_fn(&mut bx, vtable, &fn_abi),
.get_fn(&mut bx, vtable, ty, &fn_abi),
fn_abi,
)
}
@ -819,9 +819,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
// the data pointer as the first argument
match op.val {
Pair(data_ptr, meta) => {
llfn = Some(
meth::VirtualIndex::from_index(idx).get_fn(&mut bx, meta, &fn_abi),
);
llfn = Some(meth::VirtualIndex::from_index(idx).get_fn(
&mut bx,
meta,
op.layout.ty,
&fn_abi,
));
llargs.push(data_ptr);
continue 'make_args;
}
@ -829,7 +832,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
}
} else if let Ref(data_ptr, Some(meta), _) = op.val {
// by-value dynamic dispatch
llfn = Some(meth::VirtualIndex::from_index(idx).get_fn(&mut bx, meta, &fn_abi));
llfn = Some(meth::VirtualIndex::from_index(idx).get_fn(
&mut bx,
meta,
op.layout.ty,
&fn_abi,
));
llargs.push(data_ptr);
continue;
} else {

8
compiler/rustc_codegen_ssa/src/traits/intrinsic.rs
View file

@ -22,6 +22,14 @@ pub trait IntrinsicCallMethods<'tcx>: BackendTypes {
fn expect(&mut self, cond: Self::Value, expected: bool) -> Self::Value;
/// Trait method used to test whether a given pointer is associated with a type identifier.
fn type_test(&mut self, pointer: Self::Value, typeid: Self::Value) -> Self::Value;
/// Trait method used to load a function while testing if it is associated with a type
/// identifier.
fn type_checked_load(
&mut self,
llvtable: Self::Value,
vtable_byte_offset: u64,
typeid: Self::Value,
) -> Self::Value;
/// Trait method used to inject `va_start` on the "spoofed" `VaListImpl` in
/// Rust defined C-variadic functions.
fn va_start(&mut self, val: Self::Value) -> Self::Value;