Don't use an allocator shim for #[global_allocator]

This makes it possible to use liballoc/libstd in combination with
`--emit obj` if you use `#[global_allocator]`. Making it work for the
default libstd allocator would require weak functions, which are not
well supported on all systems.

Author: bjorn3

src/allocator.rs

@@ -1,11 +1,12 @@
 #[cfg(feature="master")]
 use gccjit::FnAttribute;
 use gccjit::{FunctionType, GlobalKind, ToRValue};
-use rustc_ast::expand::allocator::{AllocatorKind, AllocatorTy, ALLOCATOR_METHODS};
+use rustc_ast::expand::allocator::{
+    alloc_error_handler_name, AllocatorKind, AllocatorTy, ALLOCATOR_METHODS,
+};
 use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::config::OomStrategy;
-use rustc_span::symbol::sym;
 
 use crate::GccContext;
 
@@ -22,69 +23,71 @@ pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut GccContext, _module_nam
     let i8p = i8.make_pointer();
     let void = context.new_type::<()>();
 
-    for method in ALLOCATOR_METHODS {
-        let mut types = Vec::with_capacity(method.inputs.len());
-        for ty in method.inputs.iter() {
-            match *ty {
-                AllocatorTy::Layout => {
-                    types.push(usize);
-                    types.push(usize);
+    if kind == AllocatorKind::Default {
+        for method in ALLOCATOR_METHODS {
+            let mut types = Vec::with_capacity(method.inputs.len());
+            for ty in method.inputs.iter() {
+                match *ty {
+                    AllocatorTy::Layout => {
+                        types.push(usize);
+                        types.push(usize);
+                    }
+                    AllocatorTy::Ptr => types.push(i8p),
+                    AllocatorTy::Usize => types.push(usize),
+
+                    AllocatorTy::ResultPtr | AllocatorTy::Unit => panic!("invalid allocator arg"),
                 }
-                AllocatorTy::Ptr => types.push(i8p),
-                AllocatorTy::Usize => types.push(usize),
-
-                AllocatorTy::ResultPtr | AllocatorTy::Unit => panic!("invalid allocator arg"),
             }
-        }
-        let output = match method.output {
-            AllocatorTy::ResultPtr => Some(i8p),
-            AllocatorTy::Unit => None,
+            let output = match method.output {
+                AllocatorTy::ResultPtr => Some(i8p),
+                AllocatorTy::Unit => None,
 
-            AllocatorTy::Layout | AllocatorTy::Usize | AllocatorTy::Ptr => {
-                panic!("invalid allocator output")
-            }
-        };
-        let name = format!("__rust_{}", method.name);
+                AllocatorTy::Layout | AllocatorTy::Usize | AllocatorTy::Ptr => {
+                    panic!("invalid allocator output")
+                }
+            };
+            let name = format!("__rust_{}", method.name);
+
+            let args: Vec<_> = types.iter().enumerate()
+                .map(|(index, typ)| context.new_parameter(None, *typ, &format!("param{}", index)))
+                .collect();
+            let func = context.new_function(None, FunctionType::Exported, output.unwrap_or(void), &args, name, false);
 
-        let args: Vec<_> = types.iter().enumerate()
-            .map(|(index, typ)| context.new_parameter(None, *typ, &format!("param{}", index)))
-            .collect();
-        let func = context.new_function(None, FunctionType::Exported, output.unwrap_or(void), &args, name, false);
+            if tcx.sess.target.options.default_hidden_visibility {
+                #[cfg(feature="master")]
+                func.add_attribute(FnAttribute::Visibility(gccjit::Visibility::Hidden));
+            }
+            if tcx.sess.must_emit_unwind_tables() {
+                // TODO(antoyo): emit unwind tables.
+            }
 
-        if tcx.sess.target.options.default_hidden_visibility {
+            let callee = AllocatorKind::Default.fn_name(method.name);
+            let args: Vec<_> = types.iter().enumerate()
+                .map(|(index, typ)| context.new_parameter(None, *typ, &format!("param{}", index)))
+                .collect();
+            let callee = context.new_function(None, FunctionType::Extern, output.unwrap_or(void), &args, callee, false);
             #[cfg(feature="master")]
-            func.add_attribute(FnAttribute::Visibility(gccjit::Visibility::Hidden));
-        }
-        if tcx.sess.must_emit_unwind_tables() {
-            // TODO(antoyo): emit unwind tables.
-        }
+            callee.add_attribute(FnAttribute::Visibility(gccjit::Visibility::Hidden));
+
+            let block = func.new_block("entry");
+
+            let args = args
+                .iter()
+                .enumerate()
+                .map(|(i, _)| func.get_param(i as i32).to_rvalue())
+                .collect::<Vec<_>>();
+            let ret = context.new_call(None, callee, &args);
+            //llvm::LLVMSetTailCall(ret, True);
+            if output.is_some() {
+                block.end_with_return(None, ret);
+            }
+            else {
+                block.end_with_void_return(None);
+            }
 
-        let callee = kind.fn_name(method.name);
-        let args: Vec<_> = types.iter().enumerate()
-            .map(|(index, typ)| context.new_parameter(None, *typ, &format!("param{}", index)))
-            .collect();
-        let callee = context.new_function(None, FunctionType::Extern, output.unwrap_or(void), &args, callee, false);
-        #[cfg(feature="master")]
-        callee.add_attribute(FnAttribute::Visibility(gccjit::Visibility::Hidden));
-
-        let block = func.new_block("entry");
-
-        let args = args
-            .iter()
-            .enumerate()
-            .map(|(i, _)| func.get_param(i as i32).to_rvalue())
-            .collect::<Vec<_>>();
-        let ret = context.new_call(None, callee, &args);
-        //llvm::LLVMSetTailCall(ret, True);
-        if output.is_some() {
-            block.end_with_return(None, ret);
-        }
-        else {
-            block.end_with_void_return(None);
+            // TODO(@Commeownist): Check if we need to emit some extra debugging info in certain circumstances
+            // as described in https://github.com/rust-lang/rust/commit/77a96ed5646f7c3ee8897693decc4626fe380643
         }
-
-        // TODO(@Commeownist): Check if we need to emit some extra debugging info in certain circumstances
-        // as described in https://github.com/rust-lang/rust/commit/77a96ed5646f7c3ee8897693decc4626fe380643
     }
 
     let types = [usize, usize];
@@ -99,7 +102,7 @@ pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut GccContext, _module_nam
         func.add_attribute(FnAttribute::Visibility(gccjit::Visibility::Hidden));
     }
 
-    let callee = alloc_error_handler_kind.fn_name(sym::oom);
+    let callee = alloc_error_handler_name(alloc_error_handler_kind);
     let args: Vec<_> = types.iter().enumerate()
         .map(|(index, typ)| context.new_parameter(None, *typ, &format!("param{}", index)))
         .collect();