From b5361d0d41871874926bd489b8dc014a016c3f52 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Thu, 21 Dec 2017 07:03:16 -0800
Subject: [PATCH] rustc: Set release mode cgus to 16 by default

This commit is the next attempt to enable multiple codegen units by default in
release mode, getting some of those sweet, sweet parallelism wins by running
codegen in parallel. Performance should not be lost due to ThinLTO being on by
default as well.

Closes #45320
---
 src/librustc/session/mod.rs    |  5 +----
 src/librustc_llvm/ffi.rs       |  1 +
 src/librustc_trans/back/lto.rs | 15 +++++++++++++++
 src/rustllvm/PassWrapper.cpp   | 15 +++++++++++++++
 4 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index 60a218500ca78..748a734d1d7ac 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -785,10 +785,7 @@ impl Session {
         // As a result 16 was chosen here! Mostly because it was a power of 2
         // and most benchmarks agreed it was roughly a local optimum. Not very
         // scientific.
-        match self.opts.optimize {
-            config::OptLevel::No => 16,
-            _ => 1, // FIXME(#46346) this should be 16
-        }
+        16
     }
 
     /// Returns whether ThinLTO is enabled for this compilation
diff --git a/src/librustc_llvm/ffi.rs b/src/librustc_llvm/ffi.rs
index cb385923067c4..7d65349446516 100644
--- a/src/librustc_llvm/ffi.rs
+++ b/src/librustc_llvm/ffi.rs
@@ -1732,4 +1732,5 @@ extern "C" {
                                            CU1: *mut *mut c_void,
                                            CU2: *mut *mut c_void);
     pub fn LLVMRustThinLTOPatchDICompileUnit(M: ModuleRef, CU: *mut c_void);
+    pub fn LLVMRustThinLTORemoveAvailableExternally(M: ModuleRef);
 }
diff --git a/src/librustc_trans/back/lto.rs b/src/librustc_trans/back/lto.rs
index ba8c26bc819be..298716840926a 100644
--- a/src/librustc_trans/back/lto.rs
+++ b/src/librustc_trans/back/lto.rs
@@ -726,6 +726,21 @@ impl ThinModule {
         run_pass_manager(cgcx, tm, llmod, config, true);
         cgcx.save_temp_bitcode(&mtrans, "thin-lto-after-pm");
         timeline.record("thin-done");
+
+        // FIXME: this is a hack around a bug in LLVM right now. Discovered in
+        // #46910 it was found out that on 32-bit MSVC LLVM will hit a codegen
+        // error if there's an available_externally function in the LLVM module.
+        // Typically we don't actually use these functions but ThinLTO makes
+        // heavy use of them when inlining across modules.
+        //
+        // Tracked upstream at https://bugs.llvm.org/show_bug.cgi?id=35736 this
+        // function call (and its definition on the C++ side of things)
+        // shouldn't be necessary eventually and we can safetly delete these few
+        // lines.
+        llvm::LLVMRustThinLTORemoveAvailableExternally(llmod);
+        cgcx.save_temp_bitcode(&mtrans, "thin-lto-after-rm-ae");
+        timeline.record("no-ae");
+
         Ok(mtrans)
     }
 }
diff --git a/src/rustllvm/PassWrapper.cpp b/src/rustllvm/PassWrapper.cpp
index 776e4a3e65ad5..4e326c9e199b1 100644
--- a/src/rustllvm/PassWrapper.cpp
+++ b/src/rustllvm/PassWrapper.cpp
@@ -1182,6 +1182,15 @@ LLVMRustThinLTOPatchDICompileUnit(LLVMModuleRef Mod, DICompileUnit *Unit) {
   MD->addOperand(Unit);
 }
 
+extern "C" void
+LLVMRustThinLTORemoveAvailableExternally(LLVMModuleRef Mod) {
+  Module *M = unwrap(Mod);
+  for (Function &F : M->functions()) {
+    if (F.hasAvailableExternallyLinkage())
+      F.deleteBody();
+  }
+}
+
 #else
 
 extern "C" bool
@@ -1272,4 +1281,10 @@ extern "C" void
 LLVMRustThinLTOPatchDICompileUnit(LLVMModuleRef Mod) {
   report_fatal_error("ThinLTO not available");
 }
+
+extern "C" void
+LLVMRustThinLTORemoveAvailableExternally(LLVMModuleRef Mod) {
+  report_fatal_error("ThinLTO not available");
+}
+
 #endif // LLVM_VERSION_GE(4, 0)