Auto merge of #56358 - nikic:mergefunc-aliases, r=rkruppe

Enable -mergefunc-use-aliases

If the Rust LLVM fork is used, enable the -mergefunc-use-aliases
flag, which will create aliases for merged functions, rather than
inserting a call from one to the other.

A number of codegen tests needed to be adjusted, because functions
that previously fell below the thunk limit are now being merged.
Merging is prevented in various ways now.

I expect that this is going to break something, somewhere, because
it isn't able to deal with aliases properly, but we won't find out
until we try :)

This fixes #52651.

r? @rkruppe

3 files changed, 9 insertions, 4 deletions

diff --git a/src/librustc_codegen_llvm/back/write.rs b/src/librustc_codegen_llvm/back/write.rs
index 4eb920324bd..2ddbd0c299a 100644
--- a/src/librustc_codegen_llvm/back/write.rs
+++ b/src/librustc_codegen_llvm/back/write.rs
@@ -371,15 +371,16 @@ pub(crate) unsafe fn optimize(cgcx: &CodegenContext<LlvmCodegenBackend>,
                     .unwrap_or(llvm::CodeGenOptLevel::None);
                 let prepare_for_thin_lto = cgcx.lto == Lto::Thin || cgcx.lto == Lto::ThinLocal ||
                     (cgcx.lto != Lto::Fat && cgcx.opts.debugging_opts.cross_lang_lto.enabled());
+                with_llvm_pmb(llmod, &config, opt_level, prepare_for_thin_lto, &mut |b| {
+                    llvm::LLVMPassManagerBuilderPopulateFunctionPassManager(b, fpm);
+                    llvm::LLVMPassManagerBuilderPopulateModulePassManager(b, mpm);
+                });
+
                 have_name_anon_globals_pass = have_name_anon_globals_pass || prepare_for_thin_lto;
                 if using_thin_buffers && !prepare_for_thin_lto {
                     assert!(addpass("name-anon-globals"));
                     have_name_anon_globals_pass = true;
                 }
-                with_llvm_pmb(llmod, &config, opt_level, prepare_for_thin_lto, &mut |b| {
-                    llvm::LLVMPassManagerBuilderPopulateFunctionPassManager(b, fpm);
-                    llvm::LLVMPassManagerBuilderPopulateModulePassManager(b, mpm);
-                })
             }
 
             for pass in &config.passes {
diff --git a/src/librustc_codegen_llvm/llvm/ffi.rs b/src/librustc_codegen_llvm/llvm/ffi.rs
index f1a966d7654..9eefeca70cf 100644
--- a/src/librustc_codegen_llvm/llvm/ffi.rs
+++ b/src/librustc_codegen_llvm/llvm/ffi.rs
@@ -1358,6 +1358,7 @@ extern "C" {
     pub fn LLVMRustDebugMetadataVersion() -> u32;
     pub fn LLVMRustVersionMajor() -> u32;
     pub fn LLVMRustVersionMinor() -> u32;
+    pub fn LLVMRustIsRustLLVM() -> bool;
 
     pub fn LLVMRustAddModuleFlag(M: &Module, name: *const c_char, value: u32);
 
diff --git a/src/librustc_codegen_llvm/llvm_util.rs b/src/librustc_codegen_llvm/llvm_util.rs
index 267d7e0d54b..fdb6373bea1 100644
--- a/src/librustc_codegen_llvm/llvm_util.rs
+++ b/src/librustc_codegen_llvm/llvm_util.rs
@@ -70,6 +70,9 @@ unsafe fn configure_llvm(sess: &Session) {
         if sess.opts.debugging_opts.disable_instrumentation_preinliner {
             add("-disable-preinline");
         }
+        if llvm::LLVMRustIsRustLLVM() {
+            add("-mergefunc-use-aliases");
+        }
 
         for arg in &sess.opts.cg.llvm_args {
             add(&(*arg));