Add LLVM module flags required for the VFE opt

To apply the optimization the `Virtual Function Elim` module flag has to
be set. To apply this optimization post-link the `LTOPostLink` module
flag has to be set.

3 files changed, 23 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_llvm/src/back/lto.rs b/compiler/rustc_codegen_llvm/src/back/lto.rs
index c7497bfd355..38402e04313 100644
--- a/compiler/rustc_codegen_llvm/src/back/lto.rs
+++ b/compiler/rustc_codegen_llvm/src/back/lto.rs
@@ -586,9 +586,21 @@ pub(crate) fn run_pass_manager(
     // LTO-specific optimization passes that LLVM provides.
     //
     // This code is based off the code found in llvm's LTO code generator:
-    //      tools/lto/LTOCodeGenerator.cpp
+    //      llvm/lib/LTO/LTOCodeGenerator.cpp
     debug!("running the pass manager");
     unsafe {
+        if !llvm::LLVMRustHasModuleFlag(
+            module.module_llvm.llmod(),
+            "LTOPostLink".as_ptr().cast(),
+            11,
+        ) {
+            llvm::LLVMRustAddModuleFlag(
+                module.module_llvm.llmod(),
+                llvm::LLVMModFlagBehavior::Error,
+                "LTOPostLink\0".as_ptr().cast(),
+                1,
+            );
+        }
         if llvm_util::should_use_new_llvm_pass_manager(
             &config.new_llvm_pass_manager,
             &cgcx.target_arch,
diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index 5544f0d3f60..27fe60161d5 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -326,6 +326,15 @@ pub unsafe fn create_module<'ll>(
         )
     }
 
+    if sess.opts.debugging_opts.virtual_function_elimination {
+        llvm::LLVMRustAddModuleFlag(
+            llmod,
+            llvm::LLVMModFlagBehavior::Error,
+            "Virtual Function Elim\0".as_ptr().cast(),
+            1,
+        );
+    }
+
     llmod
 }
 
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 1d9a4655db6..4f7b082dac4 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -1936,6 +1936,7 @@ extern "C" {
         name: *const c_char,
         value: u32,
     );
+    pub fn LLVMRustHasModuleFlag(M: &Module, name: *const c_char, len: size_t) -> bool;
 
     pub fn LLVMRustMetadataAsValue<'a>(C: &'a Context, MD: &'a Metadata) -> &'a Value;