Ensure the allocator shim never participates in LTO

Making it participate in LTO would be incorrect if you compile a crate
as both a dylib (which needs it) and rlib (which must not include it) in
the same rustc invocation. With linker plugin LTO, the allocator shim
will still participate in LTO as it is safe to do so in that case.

Author: bjorn3

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -809,19 +809,12 @@ pub(crate) fn compute_per_cgu_lto_type(
     sess_lto: &Lto,
     opts: &config::Options,
     sess_crate_types: &[CrateType],
-    module_kind: ModuleKind,
 ) -> ComputedLtoType {
     // If the linker does LTO, we don't have to do it. Note that we
     // keep doing full LTO, if it is requested, as not to break the
     // assumption that the output will be a single module.
     let linker_does_lto = opts.cg.linker_plugin_lto.enabled();
 
-    // When we're automatically doing ThinLTO for multi-codegen-unit
-    // builds we don't actually want to LTO the allocator modules if
-    // it shows up. This is due to various linker shenanigans that
-    // we'll encounter later.
-    let is_allocator = module_kind == ModuleKind::Allocator;
-
     // We ignore a request for full crate graph LTO if the crate type
     // is only an rlib, as there is no full crate graph to process,
     // that'll happen later.
@@ -833,7 +826,7 @@ pub(crate) fn compute_per_cgu_lto_type(
     let is_rlib = matches!(sess_crate_types, [CrateType::Rlib]);
 
     match sess_lto {
-        Lto::ThinLocal if !linker_does_lto && !is_allocator => ComputedLtoType::Thin,
+        Lto::ThinLocal if !linker_does_lto => ComputedLtoType::Thin,
         Lto::Thin if !linker_does_lto && !is_rlib => ComputedLtoType::Thin,
         Lto::Fat if !is_rlib => ComputedLtoType::Fat,
         _ => ComputedLtoType::No,
@@ -855,7 +848,16 @@ fn execute_optimize_work_item<B: ExtraBackendMethods>(
     // back to the coordinator thread for further LTO processing (which
     // has to wait for all the initial modules to be optimized).
 
-    let lto_type = compute_per_cgu_lto_type(&cgcx.lto, &cgcx.opts, &cgcx.crate_types, module.kind);
+    // When we're automatically doing ThinLTO for multi-codegen-unit
+    // builds we don't actually want to LTO the allocator modules if
+    // it shows up. This is due to various linker shenanigans that
+    // we'll encounter later.
+    if module.kind == ModuleKind::Allocator {
+        let module = B::codegen(cgcx, module, module_config);
+        return WorkItemResult::Finished(module);
+    }
+
+    let lto_type = compute_per_cgu_lto_type(&cgcx.lto, &cgcx.opts, &cgcx.crate_types);
 
     // If we're doing some form of incremental LTO then we need to be sure to
     // save our module to disk first.

compiler/rustc_codegen_ssa/src/base.rs

@@ -46,9 +46,7 @@ use crate::meth::load_vtable;
 use crate::mir::operand::OperandValue;
 use crate::mir::place::PlaceRef;
 use crate::traits::*;
-use crate::{
-    CachedModuleCodegen, CodegenLintLevels, CrateInfo, ModuleCodegen, ModuleKind, errors, meth, mir,
-};
+use crate::{CachedModuleCodegen, CodegenLintLevels, CrateInfo, ModuleCodegen, errors, meth, mir};
 
 pub(crate) fn bin_op_to_icmp_predicate(op: BinOp, signed: bool) -> IntPredicate {
     match (op, signed) {
@@ -1135,12 +1133,7 @@ pub fn determine_cgu_reuse<'tcx>(tcx: TyCtxt<'tcx>, cgu: &CodegenUnit<'tcx>) ->
         // We can re-use either the pre- or the post-thinlto state. If no LTO is
         // being performed then we can use post-LTO artifacts, otherwise we must
         // reuse pre-LTO artifacts
-        match compute_per_cgu_lto_type(
-            &tcx.sess.lto(),
-            &tcx.sess.opts,
-            tcx.crate_types(),
-            ModuleKind::Regular,
-        ) {
+        match compute_per_cgu_lto_type(&tcx.sess.lto(), &tcx.sess.opts, tcx.crate_types()) {
             ComputedLtoType::No => CguReuse::PostLto,
             _ => CguReuse::PreLto,
         }