rust/compiler/rustc_mir_transform/src/instsimplify.rs

//! Performs various peephole optimizations.

use rustc_abi::ExternAbi;
use rustc_ast::attr;
use rustc_hir::LangItem;
use rustc_middle::bug;
use rustc_middle::mir::*;
use rustc_middle::ty::layout::ValidityRequirement;
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,
    AfterSimplifyCfg,
}

impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
    fn name(&self) -> &'static str {
        match self {
            InstSimplify::BeforeInline => "InstSimplify-before-inline",
            InstSimplify::AfterSimplifyCfg => "InstSimplify-after-simplifycfg",
        }
    }

    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>) {
        let ctx = InstSimplifyContext {
            tcx,
            local_decls: &body.local_decls,
            typing_env: body.typing_env(tcx),
        };
        let preserve_ub_checks =
            attr::contains_name(tcx.hir().krate_attrs(), sym::rustc_preserve_ub_checks);
        for block in body.basic_blocks.as_mut() {
            for statement in block.statements.iter_mut() {
                match statement.kind {
                    StatementKind::Assign(box (_place, ref mut rvalue)) => {
                        if !preserve_ub_checks {
                            ctx.simplify_ub_check(rvalue);
                        }
                        ctx.simplify_bool_cmp(rvalue);
                        ctx.simplify_ref_deref(rvalue);
                        ctx.simplify_ptr_aggregate(rvalue);
                        ctx.simplify_cast(rvalue);
                        ctx.simplify_repeated_aggregate(rvalue);
                        ctx.simplify_repeat_once(rvalue);
                    }
                    _ => {}
                }
            }

            ctx.simplify_primitive_clone(block.terminator.as_mut().unwrap(), &mut block.statements);
            ctx.simplify_intrinsic_assert(block.terminator.as_mut().unwrap());
            ctx.simplify_nounwind_call(block.terminator.as_mut().unwrap());
            simplify_duplicate_switch_targets(block.terminator.as_mut().unwrap());
        }
    }

    fn is_required(&self) -> bool {
        false
    }
}

struct InstSimplifyContext<'a, 'tcx> {
    tcx: TyCtxt<'tcx>,
    local_decls: &'a LocalDecls<'tcx>,
    typing_env: ty::TypingEnv<'tcx>,
}

impl<'tcx> InstSimplifyContext<'_, 'tcx> {
    /// Transform aggregates like [0, 0, 0, 0, 0] into [0; 5].
    /// GVN can also do this optimization, but GVN is only run at mir-opt-level 2 so having this in
    /// InstSimplify helps unoptimized builds.
    fn simplify_repeated_aggregate(&self, rvalue: &mut Rvalue<'tcx>) {
        let Rvalue::Aggregate(box AggregateKind::Array(_), fields) = rvalue else {
            return;
        };
        if fields.len() < 5 {
            return;
        }
        let first = &fields[rustc_abi::FieldIdx::ZERO];
        let Operand::Constant(first) = first else {
            return;
        };
        let Ok(first_val) = first.const_.eval(self.tcx, self.typing_env, first.span) else {
            return;
        };
        if fields.iter().all(|field| {
            let Operand::Constant(field) = field else {
                return false;
            };
            let field = field.const_.eval(self.tcx, self.typing_env, field.span);
            field == Ok(first_val)
        }) {
            let len = ty::Const::from_target_usize(self.tcx, fields.len().try_into().unwrap());
            *rvalue = Rvalue::Repeat(Operand::Constant(first.clone()), len);
        }
    }

    /// Transform boolean comparisons into logical operations.
    fn simplify_bool_cmp(&self, rvalue: &mut Rvalue<'tcx>) {
        match rvalue {
            Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), box (a, b)) => {
                let new = match (op, self.try_eval_bool(a), self.try_eval_bool(b)) {
                    // Transform "Eq(a, true)" ==> "a"
                    (BinOp::Eq, _, Some(true)) => Some(Rvalue::Use(a.clone())),

                    // Transform "Ne(a, false)" ==> "a"
                    (BinOp::Ne, _, Some(false)) => Some(Rvalue::Use(a.clone())),

                    // Transform "Eq(true, b)" ==> "b"
                    (BinOp::Eq, Some(true), _) => Some(Rvalue::Use(b.clone())),

                    // Transform "Ne(false, b)" ==> "b"
                    (BinOp::Ne, Some(false), _) => Some(Rvalue::Use(b.clone())),

                    // Transform "Eq(false, b)" ==> "Not(b)"
                    (BinOp::Eq, Some(false), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),

                    // Transform "Ne(true, b)" ==> "Not(b)"
                    (BinOp::Ne, Some(true), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),

                    // Transform "Eq(a, false)" ==> "Not(a)"
                    (BinOp::Eq, _, Some(false)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),

                    // Transform "Ne(a, true)" ==> "Not(a)"
                    (BinOp::Ne, _, Some(true)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),

                    _ => None,
                };

                if let Some(new) = new {
                    *rvalue = new;
                }
            }

            _ => {}
        }
    }

    fn try_eval_bool(&self, a: &Operand<'_>) -> Option<bool> {
        let a = a.constant()?;
        if a.const_.ty().is_bool() { a.const_.try_to_bool() } else { None }
    }

    /// Transform `&(*a)` ==> `a`.
    fn simplify_ref_deref(&self, rvalue: &mut Rvalue<'tcx>) {
        if let Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) = rvalue {
            if let Some((base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
                if rvalue.ty(self.local_decls, self.tcx) != base.ty(self.local_decls, self.tcx).ty {
                    return;
                }

                *rvalue = Rvalue::Use(Operand::Copy(Place {
                    local: base.local,
                    projection: self.tcx.mk_place_elems(base.projection),
                }));
            }
        }
    }

    /// Transform `Aggregate(RawPtr, [p, ()])` ==> `Cast(PtrToPtr, p)`.
    fn simplify_ptr_aggregate(&self, rvalue: &mut Rvalue<'tcx>) {
        if let Rvalue::Aggregate(box AggregateKind::RawPtr(pointee_ty, mutability), fields) = rvalue
        {
            let meta_ty = fields.raw[1].ty(self.local_decls, self.tcx);
            if meta_ty.is_unit() {
                // The mutable borrows we're holding prevent printing `rvalue` here
                let mut fields = std::mem::take(fields);
                let _meta = fields.pop().unwrap();
                let data = fields.pop().unwrap();
                let ptr_ty = Ty::new_ptr(self.tcx, *pointee_ty, *mutability);
                *rvalue = Rvalue::Cast(CastKind::PtrToPtr, data, ptr_ty);
            }
        }
    }

    fn simplify_ub_check(&self, rvalue: &mut Rvalue<'tcx>) {
        if let Rvalue::NullaryOp(NullOp::UbChecks, _) = *rvalue {
            let const_ = Const::from_bool(self.tcx, self.tcx.sess.ub_checks());
            let constant = ConstOperand { span: DUMMY_SP, const_, user_ty: None };
            *rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
        }
    }

    fn simplify_cast(&self, rvalue: &mut Rvalue<'tcx>) {
        if let Rvalue::Cast(kind, operand, cast_ty) = rvalue {
            let operand_ty = operand.ty(self.local_decls, self.tcx);
            if operand_ty == *cast_ty {
                *rvalue = Rvalue::Use(operand.clone());
            } else if *kind == CastKind::Transmute {
                // Transmuting an integer to another integer is just a signedness cast
                if let (ty::Int(int), ty::Uint(uint)) | (ty::Uint(uint), ty::Int(int)) =
                    (operand_ty.kind(), cast_ty.kind())
                    && int.bit_width() == uint.bit_width()
                {
                    // The width check isn't strictly necessary, as different widths
                    // are UB and thus we'd be allowed to turn it into a cast anyway.
                    // But let's keep the UB around for codegen to exploit later.
                    // (If `CastKind::Transmute` ever becomes *not* UB for mismatched sizes,
                    // then the width check is necessary for big-endian correctness.)
                    *kind = CastKind::IntToInt;
                    return;
                }
            }
        }
    }

    /// Simplify `[x; 1]` to just `[x]`.
    fn simplify_repeat_once(&self, rvalue: &mut Rvalue<'tcx>) {
        if let Rvalue::Repeat(operand, count) = rvalue
            && let Some(1) = count.try_to_target_usize(self.tcx)
        {
            *rvalue = Rvalue::Aggregate(
                Box::new(AggregateKind::Array(operand.ty(self.local_decls, self.tcx))),
                [operand.clone()].into(),
            );
        }
    }

    fn simplify_primitive_clone(
        &self,
        terminator: &mut Terminator<'tcx>,
        statements: &mut Vec<Statement<'tcx>>,
    ) {
        let TerminatorKind::Call { func, args, destination, target, .. } = &mut 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 {
            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) {
            return;
        }

        let Ok([arg]) = take_array(args) else { return };
        let Some(arg_place) = arg.node.place() else { return };

        statements.push(Statement {
            source_info: terminator.source_info,
            kind: StatementKind::Assign(Box::new((
                *destination,
                Rvalue::Use(Operand::Copy(
                    arg_place.project_deeper(&[ProjectionElem::Deref], self.tcx),
                )),
            ))),
        });
        terminator.kind = TerminatorKind::Goto { target: destination_block };
    }

    fn simplify_nounwind_call(&self, terminator: &mut Terminator<'tcx>) {
        let TerminatorKind::Call { func, unwind, .. } = &mut terminator.kind else {
            return;
        };

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

        let body_ty = self.tcx.type_of(def_id).skip_binder();
        let body_abi = match body_ty.kind() {
            ty::FnDef(..) => body_ty.fn_sig(self.tcx).abi(),
            ty::Closure(..) => ExternAbi::RustCall,
            ty::Coroutine(..) => ExternAbi::Rust,
            _ => bug!("unexpected body ty: {:?}", body_ty),
        };

        if !layout::fn_can_unwind(self.tcx, Some(def_id), body_abi) {
            *unwind = UnwindAction::Unreachable;
        }
    }

    fn simplify_intrinsic_assert(&self, terminator: &mut Terminator<'tcx>) {
        let TerminatorKind::Call { func, target, .. } = &mut terminator.kind else {
            return;
        };
        let Some(target_block) = target else {
            return;
        };
        let func_ty = func.ty(self.local_decls, self.tcx);
        let Some((intrinsic_name, args)) = resolve_rust_intrinsic(self.tcx, func_ty) else {
            return;
        };
        // The intrinsics we are interested in have one generic parameter
        if args.is_empty() {
            return;
        }

        let known_is_valid =
            intrinsic_assert_panics(self.tcx, self.typing_env, args[0], intrinsic_name);
        match known_is_valid {
            // We don't know the layout or it's not validity assertion at all, don't touch it
            None => {}
            Some(true) => {
                // If we know the assert panics, indicate to later opts that the call diverges
                *target = None;
            }
            Some(false) => {
                // If we know the assert does not panic, turn the call into a Goto
                terminator.kind = TerminatorKind::Goto { target: *target_block };
            }
        }
    }
}

fn intrinsic_assert_panics<'tcx>(
    tcx: TyCtxt<'tcx>,
    typing_env: ty::TypingEnv<'tcx>,
    arg: ty::GenericArg<'tcx>,
    intrinsic_name: Symbol,
) -> Option<bool> {
    let requirement = ValidityRequirement::from_intrinsic(intrinsic_name)?;
    let ty = arg.expect_ty();
    Some(!tcx.check_validity_requirement((requirement, typing_env.as_query_input(ty))).ok()?)
}

fn resolve_rust_intrinsic<'tcx>(
    tcx: TyCtxt<'tcx>,
    func_ty: Ty<'tcx>,
) -> Option<(Symbol, GenericArgsRef<'tcx>)> {
    if let ty::FnDef(def_id, args) = *func_ty.kind() {
        let intrinsic = tcx.intrinsic(def_id)?;
        return Some((intrinsic.name, args));
    }
    None
}