Update comments and simplify miri allocator handling a bit

Author: bjorn3

Diff for: compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -1,6 +1,6 @@
 use std::collections::hash_map::Entry::*;
 
-use rustc_ast::expand::allocator::{ALLOCATOR_METHODS, NO_ALLOC_SHIM_IS_UNSTABLE};
+use rustc_ast::expand::allocator::NO_ALLOC_SHIM_IS_UNSTABLE;
 use rustc_data_structures::unord::UnordMap;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LOCAL_CRATE, LocalDefId};
@@ -207,10 +207,8 @@ fn exported_symbols_provider_local(
 
     // Mark allocator shim symbols as exported only if they were generated.
     if needs_allocator_shim(tcx) {
-        for symbol_name in ALLOCATOR_METHODS
-            .iter()
-            .map(|method| format!("__rust_{}", method.name))
-            .chain(["__rust_alloc_error_handler".to_string(), OomStrategy::SYMBOL.to_string()])
+        for symbol_name in
+            ["__rust_alloc_error_handler".to_string(), OomStrategy::SYMBOL.to_string()]
         {
             let exported_symbol = ExportedSymbol::NoDefId(SymbolName::new(tcx, &symbol_name));
 

Diff for: compiler/rustc_monomorphize/src/partitioning.rs

@@ -889,18 +889,9 @@ fn mono_item_visibility<'tcx>(
         //
         // FIXME update comment
         // * Second is "std internal symbols". Currently this is primarily used
-        //   for allocator symbols. Allocators are a little weird in their
-        //   implementation, but the idea is that the compiler, at the last
-        //   minute, defines an allocator with an injected object file. The
-        //   `alloc` crate references these symbols (`__rust_alloc`) and the
-        //   definition doesn't get hooked up until a linked crate artifact is
-        //   generated.
-        //
-        //   The symbols synthesized by the compiler (`__rust_alloc`) are thin
-        //   veneers around the actual implementation, some other symbol which
-        //   implements the same ABI. These symbols (things like `__rg_alloc`,
-        //   `__rdl_alloc`, `__rde_alloc`, etc), are all tagged with "std
-        //   internal symbols".
+        //   for allocator symbols and the unwinder runtime to allow cyclic
+        //   dependencies between the defining and using crate and to allow
+        //   replacing them.
         //
         //   The std-internal symbols here **should not show up in a dll as an
         //   exported interface**, so they return `false` from

Diff for: library/std/src/alloc.rs

@@ -385,8 +385,7 @@ pub mod __default_lib_allocator {
     // These are used as a fallback for implementing the `__rust_alloc`, etc symbols
     // (see `src/liballoc/alloc.rs`) when there is no `#[global_allocator]` attribute.
 
-    // for symbol names src/librustc_ast/expand/allocator.rs
-    // for signatures src/librustc_allocator/lib.rs
+    // for symbol names and signatures see compiler/rustc_ast/src/expand/allocator.rs
 
     // linkage directives are provided as part of the current compiler allocator
     // ABI

Diff for: src/tools/miri/src/shims/alloc.rs

@@ -51,16 +51,6 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         Align::from_bytes(prev_power_of_two(size)).unwrap()
     }
 
-    /// Emulates calling the internal __rust_* allocator functions
-    fn emulate_allocator(&mut self) -> InterpResult<'tcx, EmulateItemResult> {
-        // When `#[global_allocator]` is used, `__rust_*` is defined by the macro expansion
-        // of this attribute. As such we have to call an exported Rust function,
-        // and not execute any Miri shim. Somewhat unintuitively doing so is done
-        // by returning `NotSupported`, which triggers the `lookup_exported_symbol`
-        // fallback case in `emulate_foreign_item`.
-        interp_ok(EmulateItemResult::NotSupported)
-    }
-
     fn malloc(&mut self, size: u64, zero_init: bool) -> InterpResult<'tcx, Pointer> {
         let this = self.eval_context_mut();
         let align = this.malloc_align(size);

Diff for: src/tools/miri/src/shims/foreign_items.rs

@@ -527,80 +527,34 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
             }
 
             // Rust allocation
-            "__rust_alloc" | "miri_alloc" => {
-                let default = |ecx: &mut MiriInterpCx<'tcx>| {
-                    // Only call `check_shim` when `#[global_allocator]` isn't used. When that
-                    // macro is used, we act like no shim exists, so that the exported function can run.
-                    let [size, align] = ecx.check_shim(abi, ExternAbi::Rust, link_name, args)?;
-                    let size = ecx.read_target_usize(size)?;
-                    let align = ecx.read_target_usize(align)?;
-
-                    ecx.check_rustc_alloc_request(size, align)?;
-
-                    let memory_kind = match link_name.as_str() {
-                        "__rust_alloc" => MiriMemoryKind::Rust,
-                        "miri_alloc" => MiriMemoryKind::Miri,
-                        _ => unreachable!(),
-                    };
+            "miri_alloc" => {
+                let [size, align] = this.check_shim(abi, ExternAbi::Rust, link_name, args)?;
+                let size = this.read_target_usize(size)?;
+                let align = this.read_target_usize(align)?;
 
-                    let ptr = ecx.allocate_ptr(
-                        Size::from_bytes(size),
-                        Align::from_bytes(align).unwrap(),
-                        memory_kind.into(),
-                    )?;
+                this.check_rustc_alloc_request(size, align)?;
 
-                    ecx.write_pointer(ptr, dest)
-                };
+                let ptr = this.allocate_ptr(
+                    Size::from_bytes(size),
+                    Align::from_bytes(align).unwrap(),
+                    MiriMemoryKind::Miri.into(),
+                )?;
 
-                match link_name.as_str() {
-                    "__rust_alloc" => return this.emulate_allocator(),
-                    "miri_alloc" => {
-                        default(this)?;
-                        return interp_ok(EmulateItemResult::NeedsReturn);
-                    }
-                    _ => unreachable!(),
-                }
+                this.write_pointer(ptr, dest);
             }
-            "__rust_alloc_zeroed" => {
-                return this.emulate_allocator();
-            }
-            "__rust_dealloc" | "miri_dealloc" => {
-                let default = |ecx: &mut MiriInterpCx<'tcx>| {
-                    // See the comment for `__rust_alloc` why `check_shim` is only called in the
-                    // default case.
-                    let [ptr, old_size, align] =
-                        ecx.check_shim(abi, ExternAbi::Rust, link_name, args)?;
-                    let ptr = ecx.read_pointer(ptr)?;
-                    let old_size = ecx.read_target_usize(old_size)?;
-                    let align = ecx.read_target_usize(align)?;
-
-                    let memory_kind = match link_name.as_str() {
-                        "__rust_dealloc" => MiriMemoryKind::Rust,
-                        "miri_dealloc" => MiriMemoryKind::Miri,
-                        _ => unreachable!(),
-                    };
-
-                    // No need to check old_size/align; we anyway check that they match the allocation.
-                    ecx.deallocate_ptr(
-                        ptr,
-                        Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
-                        memory_kind.into(),
-                    )
-                };
+            "miri_dealloc" => {
+                let [ptr, old_size, align] =
+                    this.check_shim(abi, ExternAbi::Rust, link_name, args)?;
+                let ptr = this.read_pointer(ptr)?;
+                let old_size = this.read_target_usize(old_size)?;
+                let align = this.read_target_usize(align)?;
 
-                match link_name.as_str() {
-                    "__rust_dealloc" => {
-                        return this.emulate_allocator();
-                    }
-                    "miri_dealloc" => {
-                        default(this)?;
-                        return interp_ok(EmulateItemResult::NeedsReturn);
-                    }
-                    _ => unreachable!(),
-                }
-            }
-            "__rust_realloc" => {
-                return this.emulate_allocator();
+                // No need to check old_size/align; we anyway check that they match the allocation.
+                this.deallocate_ptr(
+                    ptr,
+                    Some((Size::from_bytes(old_size), Align::from_bytes(align).unwrap())),
+                    MiriMemoryKind::Miri.into(),
+                );
             }
 
             // C memory handling functions