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|
//! This pass lowers calls to core::slice::len to just Len op.
//! It should run before inlining!
use crate::MirPass;
use rustc_hir::def_id::DefId;
use rustc_index::vec::IndexVec;
use rustc_middle::mir::*;
use rustc_middle::ty::{self, TyCtxt};
pub struct LowerSliceLenCalls;
impl<'tcx> MirPass<'tcx> for LowerSliceLenCalls {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() > 0
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
lower_slice_len_calls(tcx, body)
}
}
pub fn lower_slice_len_calls<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let language_items = tcx.lang_items();
let Some(slice_len_fn_item_def_id) = language_items.slice_len_fn() else {
// there is no language item to compare to :)
return;
};
// The one successor remains unchanged, so no need to invalidate
let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
for block in basic_blocks {
// lower `<[_]>::len` calls
lower_slice_len_call(tcx, block, &body.local_decls, slice_len_fn_item_def_id);
}
}
struct SliceLenPatchInformation<'tcx> {
add_statement: Statement<'tcx>,
new_terminator_kind: TerminatorKind<'tcx>,
}
fn lower_slice_len_call<'tcx>(
tcx: TyCtxt<'tcx>,
block: &mut BasicBlockData<'tcx>,
local_decls: &IndexVec<Local, LocalDecl<'tcx>>,
slice_len_fn_item_def_id: DefId,
) {
let mut patch_found: Option<SliceLenPatchInformation<'_>> = None;
let terminator = block.terminator();
match &terminator.kind {
TerminatorKind::Call {
func,
args,
destination,
target: Some(bb),
cleanup: None,
from_hir_call: true,
..
} => {
// some heuristics for fast rejection
if args.len() != 1 {
return;
}
let Some(arg) = args[0].place() else { return };
let func_ty = func.ty(local_decls, tcx);
match func_ty.kind() {
ty::FnDef(fn_def_id, _) if fn_def_id == &slice_len_fn_item_def_id => {
// perform modifications
// from something like `_5 = core::slice::<impl [u8]>::len(move _6) -> bb1`
// into:
// ```
// _5 = Len(*_6)
// goto bb1
// ```
// make new RValue for Len
let deref_arg = tcx.mk_place_deref(arg);
let r_value = Rvalue::Len(deref_arg);
let len_statement_kind =
StatementKind::Assign(Box::new((*destination, r_value)));
let add_statement =
Statement { kind: len_statement_kind, source_info: terminator.source_info };
// modify terminator into simple Goto
let new_terminator_kind = TerminatorKind::Goto { target: *bb };
let patch = SliceLenPatchInformation { add_statement, new_terminator_kind };
patch_found = Some(patch);
}
_ => {}
}
}
_ => {}
}
if let Some(SliceLenPatchInformation { add_statement, new_terminator_kind }) = patch_found {
block.statements.push(add_statement);
block.terminator_mut().kind = new_terminator_kind;
}
}
|
