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Micro-optimize InstSimplify's simplify_primitive_clone #139644

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Merged
merged 1 commit into from
Apr 14, 2025
+8 −17
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Micro-optimize InstSimplify's simplify_primitive_clone #139644

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25 changes: 8 additions & 17 deletions compiler/rustc_mir_transform/src/instsimplify.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,6 @@ use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, layout};
use rustc_span::{DUMMY_SP, Symbol, sym};

use crate::simplify::simplify_duplicate_switch_targets;
use crate::take_array;

pub(super) enum InstSimplify {
BeforeInline,
Expand Down Expand Up @@ -229,39 +228,31 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
terminator: &mut Terminator<'tcx>,
statements: &mut Vec<Statement<'tcx>>,
) {
let TerminatorKind::Call { func, args, destination, target, .. } = &mut terminator.kind
let TerminatorKind::Call {
func, args, destination, target: Some(destination_block), ..
} = &terminator.kind
else {
return;
};

// It's definitely not a clone if there are multiple arguments
let [arg] = &args[..] else { return };

let Some(destination_block) = *target else { return };

// Only bother looking more if it's easy to know what we're calling
let Some((fn_def_id, fn_args)) = func.const_fn_def() else { return };

// Clone needs one arg, so we can cheaply rule out other stuff
if fn_args.len() != 1 {
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No need for second check that the fn being called has only 1 arg

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I can maybe leave a debug_assert here as a sanity check?

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It's fine, we'll ICE otherwise or at least fail MIR validation.

return;
}
let Some((fn_def_id, ..)) = func.const_fn_def() else { return };

// These types are easily available from locals, so check that before
// doing DefId lookups to figure out what we're actually calling.
let arg_ty = arg.node.ty(self.local_decls, self.tcx);

let ty::Ref(_region, inner_ty, Mutability::Not) = *arg_ty.kind() else { return };

if !inner_ty.is_trivially_pure_clone_copy() {
return;
}

if !self.tcx.is_lang_item(fn_def_id, LangItem::CloneFn) {
if !self.tcx.is_lang_item(fn_def_id, LangItem::CloneFn)
|| !inner_ty.is_trivially_pure_clone_copy()
{
return;
}

let Ok([arg]) = take_array(args) else { return };
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No need to call take_array(args), since we're overriding terminator.kind later on.
Actually, I think it might also leave args in an invalid state if we early-return on the next line, but haven't been able to exercise that code path.
Allows taking an immutable ref to &terminator.kind at the beginning of the function.

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@yotamofek yotamofek Apr 10, 2025 •
edited
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Also, this can never return Err, we already checked that args's len is 1

let Some(arg_place) = arg.node.place() else { return };

statements.push(Statement {
Expand All @@ -273,7 +264,7 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
)),
))),
});
terminator.kind = TerminatorKind::Goto { target: destination_block };
terminator.kind = TerminatorKind::Goto { target: *destination_block };
}

fn simplify_nounwind_call(&self, terminator: &mut Terminator<'tcx>) {
Expand Down
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