mir: unused_generic_params query

This commit implements the `unused_generic_params` query, an initial
version of polymorphization which detects when an item does not use
generic parameters and is being needlessly monomorphized as a result.

Signed-off-by: David Wood <david@davidtw.co>

Author: davidtwco

src/librustc_codegen_llvm/callee.rs

@@ -13,7 +13,7 @@ use log::debug;
 use rustc_codegen_ssa::traits::*;
 
 use rustc_middle::ty::layout::{FnAbiExt, HasTyCtxt};
-use rustc_middle::ty::{Instance, TypeFoldable};
+use rustc_middle::ty::{self, Instance, TypeFoldable};
 
 /// Codegens a reference to a fn/method item, monomorphizing and
 /// inlining as it goes.
@@ -29,14 +29,18 @@ pub fn get_fn(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'tcx>) -> &'ll Value
 
     assert!(!instance.substs.needs_infer());
     assert!(!instance.substs.has_escaping_bound_vars());
-    assert!(!instance.substs.has_param_types_or_consts());
 
     if let Some(&llfn) = cx.instances.borrow().get(&instance) {
         return llfn;
     }
 
     let sym = tcx.symbol_name(instance).name;
-    debug!("get_fn({:?}: {:?}) => {}", instance, instance.monomorphic_ty(cx.tcx()), sym);
+    debug!(
+        "get_fn({:?}: {:?}) => {}",
+        instance,
+        instance.ty(cx.tcx(), ty::ParamEnv::reveal_all()),
+        sym
+    );
 
     let fn_abi = FnAbi::of_instance(cx, instance, &[]);
 

src/librustc_codegen_llvm/consts.rs

@@ -203,7 +203,7 @@ impl CodegenCx<'ll, 'tcx> {
             def_id
         );
 
-        let ty = instance.monomorphic_ty(self.tcx);
+        let ty = instance.ty(self.tcx, ty::ParamEnv::reveal_all());
         let sym = self.tcx.symbol_name(instance).name;
 
         debug!("get_static: sym={} instance={:?}", sym, instance);
@@ -361,7 +361,7 @@ impl StaticMethods for CodegenCx<'ll, 'tcx> {
             };
 
             let instance = Instance::mono(self.tcx, def_id);
-            let ty = instance.monomorphic_ty(self.tcx);
+            let ty = instance.ty(self.tcx, ty::ParamEnv::reveal_all());
             let llty = self.layout_of(ty).llvm_type(self);
             let g = if val_llty == llty {
                 g

src/librustc_codegen_llvm/debuginfo/metadata.rs

@@ -2481,7 +2481,7 @@ pub fn create_global_var_metadata(cx: &CodegenCx<'ll, '_>, def_id: DefId, global
     };
 
     let is_local_to_unit = is_node_local_to_unit(cx, def_id);
-    let variable_type = Instance::mono(cx.tcx, def_id).monomorphic_ty(cx.tcx);
+    let variable_type = Instance::mono(cx.tcx, def_id).ty(cx.tcx, ty::ParamEnv::reveal_all());
     let type_metadata = type_metadata(cx, variable_type, span);
     let var_name = tcx.item_name(def_id).as_str();
     let linkage_name = mangled_name_of_instance(cx, Instance::mono(tcx, def_id)).name;

src/librustc_codegen_llvm/intrinsic.rs

@@ -160,7 +160,7 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
         caller_instance: ty::Instance<'tcx>,
     ) {
         let tcx = self.tcx;
-        let callee_ty = instance.monomorphic_ty(tcx);
+        let callee_ty = instance.ty(tcx, ty::ParamEnv::reveal_all());
 
         let (def_id, substs) = match callee_ty.kind {
             ty::FnDef(def_id, substs) => (def_id, substs),

src/librustc_codegen_llvm/mono_item.rs

@@ -10,7 +10,7 @@ use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 pub use rustc_middle::mir::mono::MonoItem;
 use rustc_middle::mir::mono::{Linkage, Visibility};
 use rustc_middle::ty::layout::FnAbiExt;
-use rustc_middle::ty::{Instance, TypeFoldable};
+use rustc_middle::ty::{self, Instance, TypeFoldable};
 use rustc_target::abi::LayoutOf;
 
 impl PreDefineMethods<'tcx> for CodegenCx<'ll, 'tcx> {
@@ -22,7 +22,7 @@ impl PreDefineMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         symbol_name: &str,
     ) {
         let instance = Instance::mono(self.tcx, def_id);
-        let ty = instance.monomorphic_ty(self.tcx);
+        let ty = instance.ty(self.tcx, ty::ParamEnv::reveal_all());
         let llty = self.layout_of(ty).llvm_type(self);
 
         let g = self.define_global(symbol_name, llty).unwrap_or_else(|| {
@@ -47,7 +47,7 @@ impl PreDefineMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         visibility: Visibility,
         symbol_name: &str,
     ) {
-        assert!(!instance.substs.needs_infer() && !instance.substs.has_param_types_or_consts());
+        assert!(!instance.substs.needs_infer());
 
         let fn_abi = FnAbi::of_instance(self, instance, &[]);
         let lldecl = self.declare_fn(symbol_name, &fn_abi);

src/librustc_codegen_ssa/meth.rs

@@ -94,7 +94,8 @@ pub fn get_vtable<'tcx, Cx: CodegenMethods<'tcx>>(
                     def_id,
                     substs,
                 )
-                .unwrap(),
+                .unwrap()
+                .polymorphize(cx.tcx()),
             )
         })
     });

src/librustc_codegen_ssa/mir/block.rs

@@ -543,7 +543,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 Some(
                     ty::Instance::resolve(bx.tcx(), ty::ParamEnv::reveal_all(), def_id, substs)
                         .unwrap()
-                        .unwrap(),
+                        .unwrap()
+                        .polymorphize(bx.tcx()),
                 ),
                 None,
             ),

src/librustc_codegen_ssa/mir/rvalue.rs

@@ -190,17 +190,15 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                 if bx.cx().tcx().has_attr(def_id, sym::rustc_args_required_const) {
                                     bug!("reifying a fn ptr that requires const arguments");
                                 }
-                                OperandValue::Immediate(
-                                    bx.get_fn_addr(
-                                        ty::Instance::resolve_for_fn_ptr(
-                                            bx.tcx(),
-                                            ty::ParamEnv::reveal_all(),
-                                            def_id,
-                                            substs,
-                                        )
-                                        .unwrap(),
-                                    ),
+                                let instance = ty::Instance::resolve_for_fn_ptr(
+                                    bx.tcx(),
+                                    ty::ParamEnv::reveal_all(),
+                                    def_id,
+                                    substs,
                                 )
+                                .unwrap()
+                                .polymorphize(bx.cx().tcx());
+                                OperandValue::Immediate(bx.get_fn_addr(instance))
                             }
                             _ => bug!("{} cannot be reified to a fn ptr", operand.layout.ty),
                         }
@@ -213,7 +211,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                     def_id,
                                     substs,
                                     ty::ClosureKind::FnOnce,
-                                );
+                                )
+                                .polymorphize(bx.cx().tcx());
                                 OperandValue::Immediate(bx.cx().get_fn_addr(instance))
                             }
                             _ => bug!("{} cannot be cast to a fn ptr", operand.layout.ty),

src/librustc_middle/query/mod.rs

@@ -1309,6 +1309,13 @@ rustc_queries! {
         query codegen_unit(_: Symbol) -> &'tcx CodegenUnit<'tcx> {
             desc { "codegen_unit" }
         }
+        query unused_generic_params(key: DefId) -> u64 {
+            cache_on_disk_if { key.is_local() }
+            desc {
+                |tcx| "determining which generic parameters are unused by `{}`",
+                    tcx.def_path_str(key)
+            }
+        }
         query backend_optimization_level(_: CrateNum) -> OptLevel {
             desc { "optimization level used by backend" }
         }

src/librustc_middle/ty/instance.rs

@@ -1,5 +1,6 @@
 use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use crate::ty::print::{FmtPrinter, Printer};
+use crate::ty::subst::InternalSubsts;
 use crate::ty::{self, SubstsRef, Ty, TyCtxt, TypeFoldable};
 use rustc_errors::ErrorReported;
 use rustc_hir::def::Namespace;
@@ -106,32 +107,9 @@ pub enum InstanceDef<'tcx> {
 }
 
 impl<'tcx> Instance<'tcx> {
-    /// Returns the `Ty` corresponding to this `Instance`,
-    /// with generic substitutions applied and lifetimes erased.
-    ///
-    /// This method can only be called when the 'substs' for this Instance
-    /// are fully monomorphic (no `ty::Param`'s are present).
-    /// This is usually the case (e.g. during codegen).
-    /// However, during constant evaluation, we may want
-    /// to try to resolve a `Instance` using generic parameters
-    /// (e.g. when we are attempting to to do const-propagation).
-    /// In this case, `Instance.ty_env` should be used to provide
-    /// the `ParamEnv` for our generic context.
-    pub fn monomorphic_ty(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
-        let ty = tcx.type_of(self.def.def_id());
-        // There shouldn't be any params - if there are, then
-        // Instance.ty_env should have been used to provide the proper
-        // ParamEnv
-        if self.substs.has_param_types_or_consts() {
-            bug!("Instance.ty called for type {:?} with params in substs: {:?}", ty, self.substs);
-        }
-        tcx.subst_and_normalize_erasing_regions(self.substs, ty::ParamEnv::reveal_all(), &ty)
-    }
-
-    /// Like `Instance.ty`, but allows a `ParamEnv` to be specified for use during
-    /// normalization. This method is only really useful during constant evaluation,
-    /// where we are dealing with potentially generic types.
-    pub fn ty_env(&self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Ty<'tcx> {
+    /// Returns the `Ty` corresponding to this `Instance`, with generic substitutions applied and
+    /// lifetimes erased, allowing a `ParamEnv` to be specified for use during normalization.
+    pub fn ty(&self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Ty<'tcx> {
         let ty = tcx.type_of(self.def.def_id());
         tcx.subst_and_normalize_erasing_regions(self.substs, param_env, &ty)
     }
@@ -486,6 +464,42 @@ impl<'tcx> Instance<'tcx> {
             | InstanceDef::VtableShim(..) => Some(self.substs),
         }
     }
+
+    /// Returns a new `Instance` where generic parameters in `instance.substs` are replaced by
+    /// identify parameters if they are determined to be unused in `instance.def`.
+    pub fn polymorphize(self, tcx: TyCtxt<'tcx>) -> Self {
+        debug!("polymorphize: running polymorphization analysis");
+        if !tcx.sess.opts.debugging_opts.polymorphize {
+            return self;
+        }
+
+        if let InstanceDef::Item(def) = self.def {
+            let results = tcx.unused_generic_params(def.did);
+
+            if results == 0 {
+                // Exit early if every parameter was used.
+                return self;
+            }
+
+            debug!("polymorphize: results={:064b}", results);
+            let polymorphized_substs =
+                InternalSubsts::for_item(tcx, def.did, |param, _| match param.kind {
+                // If parameter is a const or type parameter..
+                ty::GenericParamDefKind::Const | ty::GenericParamDefKind::Type { .. } if
+                    // ..and is within range and unused..
+                    param.index < 64 && ((results >> param.index) & 1) == 1 =>
+                        // ..then use the identity for this parameter.
+                        tcx.mk_param_from_def(param),
+                // Otherwise, use the parameter as before.
+                _ => self.substs[param.index as usize],
+            });
+
+            debug!("polymorphize: self={:?} polymorphized_substs={:?}", self, polymorphized_substs);
+            Self { def: self.def, substs: polymorphized_substs }
+        } else {
+            self
+        }
+    }
 }
 
 fn needs_fn_once_adapter_shim(