Lower intrinsics calls: forget, size_of, unreachable, wrapping_*

This allows constant propagation to evaluate `size_of` and `wrapping_*`,
and unreachable propagation to propagate a call to `unreachable`.

The lowering is performed as a MIR optimization, rather than during MIR
building to preserve the special status of intrinsics with respect to
unsafety checks and promotion.

compiler/rustc_mir/src/transform/lower_intrinsics.rs

@@ -0,0 +1,108 @@
+//! Lowers intrinsic calls
+
+use crate::transform::MirPass;
+use rustc_middle::mir::*;
+use rustc_middle::ty::subst::SubstsRef;
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::symbol::{sym, Symbol};
+use rustc_target::spec::abi::Abi;
+
+pub struct LowerIntrinsics;
+
+impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        for block in body.basic_blocks_mut() {
+            let terminator = block.terminator.as_mut().unwrap();
+            if let TerminatorKind::Call {
+                func: Operand::Constant(box Constant { literal: ty::Const { ty: func_ty, .. }, .. }),
+                args,
+                destination,
+                ..
+            } = &mut terminator.kind
+            {
+                let (intrinsic_name, substs) = match resolve_rust_intrinsic(tcx, func_ty) {
+                    None => continue,
+                    Some(it) => it,
+                };
+                match intrinsic_name {
+                    sym::unreachable => {
+                        terminator.kind = TerminatorKind::Unreachable;
+                    }
+                    sym::forget => {
+                        if let Some((destination, target)) = *destination {
+                            block.statements.push(Statement {
+                                source_info: terminator.source_info,
+                                kind: StatementKind::Assign(box (
+                                    destination,
+                                    Rvalue::Use(Operand::Constant(box Constant {
+                                        span: terminator.source_info.span,
+                                        user_ty: None,
+                                        literal: ty::Const::zero_sized(tcx, tcx.types.unit),
+                                    })),
+                                )),
+                            });
+                            terminator.kind = TerminatorKind::Goto { target };
+                        }
+                    }
+                    sym::wrapping_add | sym::wrapping_sub | sym::wrapping_mul => {
+                        if let Some((destination, target)) = *destination {
+                            let lhs;
+                            let rhs;
+                            {
+                                let mut args = args.drain(..);
+                                lhs = args.next().unwrap();
+                                rhs = args.next().unwrap();
+                            }
+                            let bin_op = match intrinsic_name {
+                                sym::wrapping_add => BinOp::Add,
+                                sym::wrapping_sub => BinOp::Sub,
+                                sym::wrapping_mul => BinOp::Mul,
+                                _ => bug!("unexpected intrinsic"),
+                            };
+                            block.statements.push(Statement {
+                                source_info: terminator.source_info,
+                                kind: StatementKind::Assign(box (
+                                    destination,
+                                    Rvalue::BinaryOp(bin_op, lhs, rhs),
+                                )),
+                            });
+                            terminator.kind = TerminatorKind::Goto { target };
+                        }
+                    }
+                    sym::add_with_overflow | sym::sub_with_overflow | sym::mul_with_overflow => {
+                        // The checked binary operations are not suitable target for lowering here,
+                        // since their semantics depend on the value of overflow-checks flag used
+                        // during codegen. Issue #35310.
+                    }
+                    sym::size_of => {
+                        if let Some((destination, target)) = *destination {
+                            let tp_ty = substs.type_at(0);
+                            block.statements.push(Statement {
+                                source_info: terminator.source_info,
+                                kind: StatementKind::Assign(box (
+                                    destination,
+                                    Rvalue::NullaryOp(NullOp::SizeOf, tp_ty),
+                                )),
+                            });
+                            terminator.kind = TerminatorKind::Goto { target };
+                        }
+                    }
+                    _ => {}
+                }
+            }
+        }
+    }
+}
+
+fn resolve_rust_intrinsic(
+    tcx: TyCtxt<'tcx>,
+    func_ty: Ty<'tcx>,
+) -> Option<(Symbol, SubstsRef<'tcx>)> {
+    if let ty::FnDef(def_id, substs) = *func_ty.kind() {
+        let fn_sig = func_ty.fn_sig(tcx);
+        if fn_sig.abi() == Abi::RustIntrinsic {
+            return Some((tcx.item_name(def_id), substs));
+        }
+    }
+    None
+}

compiler/rustc_mir/src/transform/mod.rs

@@ -32,6 +32,7 @@ pub mod function_item_references;
 pub mod generator;
 pub mod inline;
 pub mod instcombine;
+pub mod lower_intrinsics;
 pub mod match_branches;
 pub mod multiple_return_terminators;
 pub mod no_landing_pads;
@@ -390,6 +391,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 
     // The main optimizations that we do on MIR.
     let optimizations: &[&dyn MirPass<'tcx>] = &[
+        &lower_intrinsics::LowerIntrinsics,
         &remove_unneeded_drops::RemoveUnneededDrops,
         &match_branches::MatchBranchSimplification,
         // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)