Add support for storing code model to LLVM module IR

This patch avoids undefined behavior by linking different object files.
Also this would it could be propagated properly to LTO.

See https://reviews.llvm.org/D52322 and https://reviews.llvm.org/D52323.

Author: kubo39

src/librustc_codegen_llvm/context.rs

@@ -22,7 +22,7 @@ use rustc_session::Session;
 use rustc_span::source_map::{Span, DUMMY_SP};
 use rustc_span::symbol::Symbol;
 use rustc_target::abi::{HasDataLayout, LayoutOf, PointeeInfo, Size, TargetDataLayout, VariantIdx};
-use rustc_target::spec::{HasTargetSpec, RelocModel, Target, TlsModel};
+use rustc_target::spec::{CodeModel, HasTargetSpec, RelocModel, Target, TlsModel};
 
 use std::cell::{Cell, RefCell};
 use std::ffi::CStr;
@@ -99,6 +99,16 @@ fn to_llvm_tls_model(tls_model: TlsModel) -> llvm::ThreadLocalMode {
     }
 }
 
+fn to_llvm_code_model(code_model: CodeModel) -> llvm::CodeModel {
+    match code_model {
+        CodeModel::Tiny => llvm::CodeModel::Tiny,
+        CodeModel::Small => llvm::CodeModel::Small,
+        CodeModel::Kernel => llvm::CodeModel::Kernel,
+        CodeModel::Medium => llvm::CodeModel::Medium,
+        CodeModel::Large => llvm::CodeModel::Large,
+    }
+}
+
 fn strip_function_ptr_alignment(data_layout: String) -> String {
     // FIXME: Make this more general.
     data_layout.replace("-Fi8-", "-")
@@ -181,6 +191,10 @@ pub unsafe fn create_module(
         }
     }
 
+    if let Some(code_model) = sess.code_model() {
+        llvm::LLVMRustSetModuleCodeModel(llmod, to_llvm_code_model(code_model));
+    }
+
     // If skipping the PLT is enabled, we need to add some module metadata
     // to ensure intrinsic calls don't use it.
     if !sess.needs_plt() {

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -2129,6 +2129,7 @@ extern "C" {
     pub fn LLVMRustUnsetComdat(V: &Value);
     pub fn LLVMRustSetModulePICLevel(M: &Module);
     pub fn LLVMRustSetModulePIELevel(M: &Module);
+    pub fn LLVMRustSetModuleCodeModel(M: &Module, Model: CodeModel);
     pub fn LLVMRustModuleBufferCreate(M: &Module) -> &'static mut ModuleBuffer;
     pub fn LLVMRustModuleBufferPtr(p: &ModuleBuffer) -> *const u8;
     pub fn LLVMRustModuleBufferLen(p: &ModuleBuffer) -> usize;

src/rustllvm/PassWrapper.cpp

@@ -1163,6 +1163,20 @@ extern "C" void LLVMRustSetModulePIELevel(LLVMModuleRef M) {
   unwrap(M)->setPIELevel(PIELevel::Level::Large);
 }
 
+//  Linking object files with different code models is undefined behavior
+//  because the compiler would have to generate additional code (to span
+//  longer jumps) if a larger code model is used with a smaller one.
+//  Therefore we will treat attempts to mix code models as an error.
+//
+// See https://reviews.llvm.org/D52322 and https://reviews.llvm.org/D52323.
+extern "C" void LLVMRustSetModuleCodeModel(LLVMModuleRef M,
+                                           LLVMRustCodeModel Model) {
+  auto CM = fromRust(Model);
+  if (!CM.hasValue())
+    return;
+  unwrap(M)->setCodeModel(*CM);
+}
+
 // Here you'll find an implementation of ThinLTO as used by the Rust compiler
 // right now. This ThinLTO support is only enabled on "recent ish" versions of
 // LLVM, and otherwise it's just blanket rejected from other compilers.

src/test/codegen/codemodels.rs

@@ -0,0 +1,21 @@
+// revisions: NOMODEL MODEL-TINY MODEL-SMALL MODEL-KERNEL MODEL-MEDIUM MODEL-LARGE
+//[NOMODEL] compile-flags:
+//[MODEL-TINY] compile-flags: --target=riscv32i-unknown-none-elf -C code-model=tiny
+//[MODEL-SMALL] compile-flags: -C code-model=small
+//[MODEL-KERNEL] compile-flags: --target=x86_64-unknown-linux-gnu -C code-model=kernel
+//[MODEL-MEDIUM] compile-flags: --target=x86_64-unknown-linux-gnu -C code-model=medium
+//[MODEL-LARGE] compile-flags: -C code-model=large
+
+#![crate_type = "lib"]
+
+// MODEL-TINY: !llvm.module.flags = !{{{.*}}}
+// MODEL-TINY: !(([0-9]+)) = !(i32 1, !"Code-Model", i32 0)
+// MODEL-SMALL: !llvm.module.flags = !{{{.*}}}
+// MODEL-SMALL: !{{[0-9]+}} = !{i32 1, !"Code Model", i32 1}
+// MODEL-KERNEL: !llvm.module.flags = !{{{.*}}}
+// MODEL-KERNEL: !{{[0-9]+}} = !{i32 1, !"Code Model", i32 2}
+// MODEL-MEDIUM: !llvm.module.flags = !{{{.*}}}
+// MODEL-MEDIUM: !{{[0-9]+}} = !{i32 1, !"Code Model", i32 3}
+// MODEL-LARGE: !llvm.module.flags = !{{{.*}}}
+// MODEL-LARGE: !{{[0-9]+}} = !{i32 1, !"Code Model", i32 4}
+// NOMODEL-NOT: Code Model