Optimize codegen of use values that are copy post monomorphization

Author: spastorino

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -1,9 +1,10 @@
 use std::iter;
 
+use rustc_hir as hir;
 use rustc_index::IndexVec;
 use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
-use rustc_middle::mir::{Local, UnwindTerminateReason, traversal};
+use rustc_middle::mir::{Body, Local, UnwindTerminateReason, traversal};
 use rustc_middle::ty::layout::{FnAbiOf, HasTyCtxt, HasTypingEnv, TyAndLayout};
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
 use rustc_middle::{bug, mir, span_bug};
@@ -171,18 +172,24 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     assert!(!instance.args.has_infer());
 
     let llfn = cx.get_fn(instance);
+    let tcx = cx.tcx();
 
-    let mir = cx.tcx().instance_mir(instance.def);
+    let mir = tcx.instance_mir(instance.def);
+    let mir = instance.instantiate_mir_and_normalize_erasing_regions(
+        tcx,
+        ty::TypingEnv::fully_monomorphized(),
+        ty::EarlyBinder::bind(mir.clone()),
+    );
 
     let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
     debug!("fn_abi: {:?}", fn_abi);
 
-    if cx.tcx().codegen_fn_attrs(instance.def_id()).flags.contains(CodegenFnAttrFlags::NAKED) {
+    if tcx.codegen_fn_attrs(instance.def_id()).flags.contains(CodegenFnAttrFlags::NAKED) {
         crate::mir::naked_asm::codegen_naked_asm::<Bx>(cx, &mir, instance);
         return;
     }
 
-    let debug_context = cx.create_function_debug_context(instance, fn_abi, llfn, mir);
+    let debug_context = cx.create_function_debug_context(instance, fn_abi, llfn, &mir);
 
     let start_llbb = Bx::append_block(cx, llfn, "start");
     let mut start_bx = Bx::build(cx, start_llbb);
@@ -194,7 +201,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 
     let cleanup_kinds =
-        base::wants_new_eh_instructions(cx.tcx().sess).then(|| analyze::cleanup_kinds(mir));
+        base::wants_new_eh_instructions(tcx.sess).then(|| analyze::cleanup_kinds(&mir));
 
     let cached_llbbs: IndexVec<mir::BasicBlock, CachedLlbb<Bx::BasicBlock>> =
         mir.basic_blocks
@@ -204,6 +211,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             })
             .collect();
 
+    let mir = tcx.arena.alloc(optimize_use_clone(tcx, mir));
+
     let mut fx = FunctionCx {
         instance,
         mir,
@@ -217,7 +226,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         cleanup_kinds,
         landing_pads: IndexVec::from_elem(None, &mir.basic_blocks),
         funclets: IndexVec::from_fn_n(|_| None, mir.basic_blocks.len()),
-        cold_blocks: find_cold_blocks(cx.tcx(), mir),
+        cold_blocks: find_cold_blocks(tcx, mir),
         locals: locals::Locals::empty(),
         debug_context,
         per_local_var_debug_info: None,
@@ -233,7 +242,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         fx.compute_per_local_var_debug_info(&mut start_bx).unzip();
     fx.per_local_var_debug_info = per_local_var_debug_info;
 
-    let traversal_order = traversal::mono_reachable_reverse_postorder(mir, cx.tcx(), instance);
+    let traversal_order = traversal::mono_reachable_reverse_postorder(mir, tcx, instance);
     let memory_locals = analyze::non_ssa_locals(&fx, &traversal_order);
 
     // Allocate variable and temp allocas
@@ -310,6 +319,71 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 }
 
+// FIXME: Move this function to mir::transform when post-mono MIR passes land.
+fn optimize_use_clone<'tcx>(tcx: TyCtxt<'tcx>, mut mir: Body<'tcx>) -> Body<'tcx> {
+    if tcx.features().ergonomic_clones() {
+        for bb in mir.basic_blocks.as_mut() {
+            let mir::TerminatorKind::Call {
+                func,
+                args,
+                destination,
+                target,
+                call_source: mir::CallSource::Use,
+                ..
+            } = &bb.terminator().kind
+            else {
+                continue;
+            };
+
+            // It's definitely not a clone if there are multiple arguments
+            let [arg] = &args[..] else { continue };
+
+            // 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(&mir.local_decls, tcx);
+
+            // monomorphize
+            // is already monomorphized
+            // let arg_ty = instance.instantiate_mir_and_normalize_erasing_regions(
+            //     cx.tcx(),
+            //     cx.typing_env(),
+            //     ty::EarlyBinder::bind(arg_ty),
+            // );
+
+            // Only bother looking more if it's easy to know what we're calling
+            let Some((fn_def_id, _)) = func.const_fn_def() else { continue };
+
+            let ty::Ref(_region, inner_ty, mir::Mutability::Not) = *arg_ty.kind() else { continue };
+
+            if !inner_ty.is_trivially_pure_clone_copy() {
+                continue;
+            }
+
+            if !tcx.is_lang_item(fn_def_id, hir::LangItem::CloneFn) {
+                continue;
+            }
+
+            let Some(arg_place) = arg.node.place() else { continue };
+
+            let destination_block = target.unwrap();
+
+            bb.statements.push(mir::Statement {
+                source_info: bb.terminator().source_info,
+                kind: mir::StatementKind::Assign(Box::new((
+                    *destination,
+                    mir::Rvalue::Use(mir::Operand::Copy(
+                        arg_place.project_deeper(&[mir::ProjectionElem::Deref], tcx),
+                    )),
+                ))),
+            });
+
+            bb.terminator_mut().kind = mir::TerminatorKind::Goto { target: destination_block };
+        }
+    }
+
+    mir
+}
+
 /// Produces, for each argument, a `Value` pointing at the
 /// argument's value. As arguments are places, these are always
 /// indirect.