Auto merge of #83044 - kubo39:set-llvm-code-model, r=nikic

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.

This patch is based on https://github.com/rust-lang/rust/pull/74002

3 files changed, 10 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_llvm/src/back/write.rs b/compiler/rustc_codegen_llvm/src/back/write.rs
index c224da7885b..4b7bcf05501 100644
--- a/compiler/rustc_codegen_llvm/src/back/write.rs
+++ b/compiler/rustc_codegen_llvm/src/back/write.rs
@@ -140,7 +140,7 @@ fn to_llvm_relocation_model(relocation_model: RelocModel) -> llvm::RelocModel {
     }
 }
 
-fn to_llvm_code_model(code_model: Option<CodeModel>) -> llvm::CodeModel {
+pub(crate) fn to_llvm_code_model(code_model: Option<CodeModel>) -> llvm::CodeModel {
     match code_model {
         Some(CodeModel::Tiny) => llvm::CodeModel::Tiny,
         Some(CodeModel::Small) => llvm::CodeModel::Small,
diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index 9f6a2ae3ca1..21473f3b114 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -1,4 +1,5 @@
 use crate::attributes;
+use crate::back::write::to_llvm_code_model;
 use crate::callee::get_fn;
 use crate::coverageinfo;
 use crate::debuginfo;
@@ -181,6 +182,13 @@ pub unsafe fn create_module(
         }
     }
 
+    // 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.
+    //
+    // See https://reviews.llvm.org/D52322 and https://reviews.llvm.org/D52323.
+    llvm::LLVMRustSetModuleCodeModel(llmod, to_llvm_code_model(sess.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() {
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index e6d60044c84..82cd1be3b3b 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -2326,6 +2326,7 @@ 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;