Add a target option "merge-functions" taking values in ("disabled",

"trampolines", or "aliases (the default)) to allow targets to opt out of
the MergeFunctions LLVM pass. Also add a corresponding -Z option with
the same name and values.

This works around: https://github.com/rust-lang/rust/issues/57356

Motivation:

Basically, the problem is that the MergeFunctions pass, which rustc
currently enables by default at -O2 and -O3, and `extern "ptx-kernel"`
functions (specific to the NVPTX target) are currently not compatible
with each other. If the MergeFunctions pass is allowed to run, rustc can
generate invalid PTX assembly (i.e. a PTX file that is not accepted by
the native PTX assembler ptxas). Therefore we would like a way to opt
out of the MergeFunctions pass, which is what our target option does.

Related work:

The current behavior of rustc is to enable MergeFunctions at -O2 and -O3,
and also to enable the use of function aliases within MergeFunctions.
MergeFunctions both with and without function aliases is incompatible with
the NVPTX target.

clang's "solution" is to have a "-fmerge-functions" flag that opts in to
the MergeFunctions pass, but it is not enabled by default.

1 files changed, 9 insertions, 1 deletions

diff --git a/src/librustc_codegen_llvm/llvm_util.rs b/src/librustc_codegen_llvm/llvm_util.rs
index ad9ebbcde8a..dc70ebcf943 100644
--- a/src/librustc_codegen_llvm/llvm_util.rs
+++ b/src/librustc_codegen_llvm/llvm_util.rs
@@ -3,6 +3,7 @@ use back::write::create_target_machine;
 use llvm;
 use rustc::session::Session;
 use rustc::session::config::PrintRequest;
+use rustc_target::spec::MergeFunctions;
 use libc::c_int;
 use std::ffi::CString;
 use syntax::feature_gate::UnstableFeatures;
@@ -61,7 +62,14 @@ unsafe fn configure_llvm(sess: &Session) {
             add("-disable-preinline");
         }
         if llvm::LLVMRustIsRustLLVM() {
-            add("-mergefunc-use-aliases");
+            match sess.opts.debugging_opts.merge_functions
+                  .unwrap_or(sess.target.target.options.merge_functions) {
+                MergeFunctions::Disabled |
+                MergeFunctions::Trampolines => {}
+                MergeFunctions::Aliases => {
+                    add("-mergefunc-use-aliases");
+                }
+            }
         }
 
         // HACK(eddyb) LLVM inserts `llvm.assume` calls to preserve align attributes