rust/compiler/rustc_codegen_llvm/src/callee.rs

//! Handles codegen of callees as well as other call-related
//! things. Callees are a superset of normal rust values and sometimes
//! have different representations. In particular, top-level fn items
//! and methods are represented as just a fn ptr and not a full
//! closure.

use crate::attributes;
use crate::common;
use crate::context::CodegenCx;
use crate::llvm;
use crate::value::Value;

use rustc_middle::ty::layout::{FnAbiOf, HasTyCtxt};
use rustc_middle::ty::{self, Instance, TypeVisitableExt};
use tracing::debug;

/// Codegens a reference to a fn/method item, monomorphizing and
/// inlining as it goes.
///
/// # Parameters
///
/// - `cx`: the crate context
/// - `instance`: the instance to be instantiated
pub fn get_fn<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'tcx>) -> &'ll Value {
    let tcx = cx.tcx();

    debug!("get_fn(instance={:?})", instance);

    assert!(!instance.args.has_infer());
    assert!(!instance.args.has_escaping_bound_vars());

    if let Some(&llfn) = cx.instances.borrow().get(&instance) {
        return llfn;
    }

    let sym = tcx.symbol_name(instance).name;
    debug!(
        "get_fn({:?}: {:?}) => {}",
        instance,
        instance.ty(cx.tcx(), ty::ParamEnv::reveal_all()),
        sym
    );

    let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());

    let llfn = if let Some(llfn) = cx.get_declared_value(sym) {
        llfn
    } else {
        let instance_def_id = instance.def_id();
        let llfn = if tcx.sess.target.arch == "x86"
            && let Some(dllimport) = common::get_dllimport(tcx, instance_def_id, sym)
        {
            // Fix for https://github.com/rust-lang/rust/issues/104453
            // On x86 Windows, LLVM uses 'L' as the prefix for any private
            // global symbols, so when we create an undecorated function symbol
            // that begins with an 'L' LLVM misinterprets that as a private
            // global symbol that it created and so fails the compilation at a
            // later stage since such a symbol must have a definition.
            //
            // To avoid this, we set the Storage Class to "DllImport" so that
            // LLVM will prefix the name with `__imp_`. Ideally, we'd like the
            // existing logic below to set the Storage Class, but it has an
            // exemption for MinGW for backwards compatibility.
            let llfn = cx.declare_fn(
                &common::i686_decorated_name(
                    dllimport,
                    common::is_mingw_gnu_toolchain(&tcx.sess.target),
                    true,
                ),
                fn_abi,
                Some(instance),
            );
            unsafe {
                llvm::LLVMSetDLLStorageClass(llfn, llvm::DLLStorageClass::DllImport);
            }
            llfn
        } else {
            cx.declare_fn(sym, fn_abi, Some(instance))
        };
        debug!("get_fn: not casting pointer!");

        attributes::from_fn_attrs(cx, llfn, instance);

        // Apply an appropriate linkage/visibility value to our item that we
        // just declared.
        //
        // This is sort of subtle. Inside our codegen unit we started off
        // compilation by predefining all our own `MonoItem` instances. That
        // is, everything we're codegenning ourselves is already defined. That
        // means that anything we're actually codegenning in this codegen unit
        // will have hit the above branch in `get_declared_value`. As a result,
        // we're guaranteed here that we're declaring a symbol that won't get
        // defined, or in other words we're referencing a value from another
        // codegen unit or even another crate.
        //
        // So because this is a foreign value we blanket apply an external
        // linkage directive because it's coming from a different object file.
        // The visibility here is where it gets tricky. This symbol could be
        // referencing some foreign crate or foreign library (an `extern`
        // block) in which case we want to leave the default visibility. We may
        // also, though, have multiple codegen units. It could be a
        // monomorphization, in which case its expected visibility depends on
        // whether we are sharing generics or not. The important thing here is
        // that the visibility we apply to the declaration is the same one that
        // has been applied to the definition (wherever that definition may be).
        unsafe {
            llvm::LLVMRustSetLinkage(llfn, llvm::Linkage::ExternalLinkage);

            let is_generic =
                instance.args.non_erasable_generics(tcx, instance.def_id()).next().is_some();

            if is_generic {
                // This is a monomorphization. Its expected visibility depends
                // on whether we are in share-generics mode.

                if cx.tcx.sess.opts.share_generics() {
                    // We are in share_generics mode.

                    if let Some(instance_def_id) = instance_def_id.as_local() {
                        // This is a definition from the current crate. If the
                        // definition is unreachable for downstream crates or
                        // the current crate does not re-export generics, the
                        // definition of the instance will have been declared
                        // as `hidden`.
                        if cx.tcx.is_unreachable_local_definition(instance_def_id)
                            || !cx.tcx.local_crate_exports_generics()
                        {
                            llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
                        }
                    } else {
                        // This is a monomorphization of a generic function
                        // defined in an upstream crate.
                        if instance.upstream_monomorphization(tcx).is_some() {
                            // This is instantiated in another crate. It cannot
                            // be `hidden`.
                        } else {
                            // This is a local instantiation of an upstream definition.
                            // If the current crate does not re-export it
                            // (because it is a C library or an executable), it
                            // will have been declared `hidden`.
                            if !cx.tcx.local_crate_exports_generics() {
                                llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
                            }
                        }
                    }
                } else {
                    // When not sharing generics, all instances are in the same
                    // crate and have hidden visibility
                    llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
                }
            } else {
                // This is a non-generic function
                if cx.tcx.is_codegened_item(instance_def_id) {
                    // This is a function that is instantiated in the local crate

                    if instance_def_id.is_local() {
                        // This is function that is defined in the local crate.
                        // If it is not reachable, it is hidden.
                        if !cx.tcx.is_reachable_non_generic(instance_def_id) {
                            llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
                        }
                    } else {
                        // This is a function from an upstream crate that has
                        // been instantiated here. These are always hidden.
                        llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
                    }
                }
            }

            // MinGW: For backward compatibility we rely on the linker to decide whether it
            // should use dllimport for functions.
            if cx.use_dll_storage_attrs
                && let Some(library) = tcx.native_library(instance_def_id)
                && library.kind.is_dllimport()
                && !matches!(tcx.sess.target.env.as_ref(), "gnu" | "uclibc")
            {
                llvm::LLVMSetDLLStorageClass(llfn, llvm::DLLStorageClass::DllImport);
            }

            if cx.should_assume_dso_local(llfn, true) {
                llvm::LLVMRustSetDSOLocal(llfn, true);
            }
        }

        llfn
    };

    cx.instances.borrow_mut().insert(instance, llfn);

    llfn
}