rustc_codegen_llvm: add support for writing summary bitcode

Typical uses of ThinLTO don't have any use for this as a standalone
file, but distributed ThinLTO uses this to make the linker phase more
efficient. With clang you'd do something like `clang -flto=thin
-fthin-link-bitcode=foo.indexing.o -c foo.c` and then get both foo.o
(full of bitcode) and foo.indexing.o (just the summary or index part of
the bitcode). That's then usable by a two-stage linking process that's
more friendly to distributed build systems like bazel, which is why I'm
working on this area.

I talked some to @teresajohnson about naming in this area, as things
seem to be a little confused between various blog posts and build
systems. "bitcode index" and "bitcode summary" tend to be a little too
ambiguous, and she tends to use "thin link bitcode" and "minimized
bitcode" (which matches the descriptions in LLVM). Since the clang
option is thin-link-bitcode, I went with that to try and not add a new
spelling in the world.

Per @dtolnay, you can work around the lack of this by using `lld
--thinlto-index-only` to do the indexing on regular .o files of
bitcode, but that is a bit wasteful on actions when we already have all
the information in rustc and could just write out the matching minimized
bitcode. I didn't test that at all in our infrastructure, because by the
time I learned that I already had this patch largely written.

compiler/rustc_codegen_cranelift/src/driver/aot.rs

@@ -200,7 +200,7 @@ fn produce_final_output_artifacts(
     // to get rid of it.
     for output_type in crate_output.outputs.keys() {
         match *output_type {
-            OutputType::Bitcode => {
+            OutputType::Bitcode | OutputType::ThinLinkBitcode => {
                 // Cranelift doesn't have bitcode
                 // user_wants_bitcode = true;
                 // // Copy to .bc, but always keep the .0.bc. There is a later

compiler/rustc_codegen_gcc/src/lib.rs

@@ -335,6 +335,10 @@ impl ThinBufferMethods for ThinBuffer {
     fn data(&self) -> &[u8] {
         unimplemented!();
     }
+
+    fn thin_link_data(&self) -> &[u8] {
+        unimplemented!();
+    }
 }
 
 pub struct GccContext {

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -687,6 +687,14 @@ impl ThinBufferMethods for ThinBuffer {
             slice::from_raw_parts(ptr, len)
         }
     }
+
+    fn thin_link_data(&self) -> &[u8] {
+        unsafe {
+            let ptr = llvm::LLVMRustThinLTOBufferThinLinkDataPtr(self.0) as *const _;
+            let len = llvm::LLVMRustThinLTOBufferThinLinkDataLen(self.0);
+            slice::from_raw_parts(ptr, len)
+        }
+    }
 }
 
 impl Drop for ThinBuffer {

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -708,6 +708,8 @@ pub(crate) unsafe fn codegen(
         //   asm from LLVM and use `gcc` to create the object file.
 
         let bc_out = cgcx.output_filenames.temp_path(OutputType::Bitcode, module_name);
+        let bc_index_out =
+            cgcx.output_filenames.temp_path(OutputType::ThinLinkBitcode, module_name);
         let obj_out = cgcx.output_filenames.temp_path(OutputType::Object, module_name);
 
         if config.bitcode_needed() {
@@ -716,6 +718,7 @@ pub(crate) unsafe fn codegen(
                 .generic_activity_with_arg("LLVM_module_codegen_make_bitcode", &*module.name);
             let thin = ThinBuffer::new(llmod, config.emit_thin_lto);
             let data = thin.data();
+            let index_data = thin.thin_link_data();
 
             if let Some(bitcode_filename) = bc_out.file_name() {
                 cgcx.prof.artifact_size(
@@ -725,14 +728,33 @@ pub(crate) unsafe fn codegen(
                 );
             }
 
+            if let Some(thin_link_bitcode_filename) = bc_index_out.file_name() {
+                cgcx.prof.artifact_size(
+                    "llvm_bitcode_summary",
+                    thin_link_bitcode_filename.to_string_lossy(),
+                    index_data.len() as u64,
+                );
+
+                let _timer = cgcx.prof.generic_activity_with_arg(
+                    "LLVM_module_codegen_emit_bitcode_index",
+                    &*module.name,
+                );
+                if let Err(err) = fs::write(&bc_index_out, index_data) {
+                    dcx.emit_err(WriteBytecode { path: &bc_index_out, err });
+                }
+            }
+
             if config.emit_bc || config.emit_obj == EmitObj::Bitcode {
-                let _timer = cgcx
+                {
-                    .prof
+                    let _timer = cgcx.prof.generic_activity_with_arg(
-                    .generic_activity_with_arg("LLVM_module_codegen_emit_bitcode", &*module.name);
+                        "LLVM_module_codegen_emit_bitcode",
+                        &*module.name,
+                    );
                     if let Err(err) = fs::write(&bc_out, data) {
                         dcx.emit_err(WriteBytecode { path: &bc_out, err });
                     }
                 }
+            }
 
             if config.emit_obj == EmitObj::ObjectCode(BitcodeSection::Full) {
                 let _timer = cgcx

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2354,6 +2354,8 @@ extern "C" {
     pub fn LLVMRustThinLTOBufferFree(M: &'static mut ThinLTOBuffer);
     pub fn LLVMRustThinLTOBufferPtr(M: &ThinLTOBuffer) -> *const c_char;
     pub fn LLVMRustThinLTOBufferLen(M: &ThinLTOBuffer) -> size_t;
+    pub fn LLVMRustThinLTOBufferThinLinkDataPtr(M: &ThinLTOBuffer) -> *const c_char;
+    pub fn LLVMRustThinLTOBufferThinLinkDataLen(M: &ThinLTOBuffer) -> size_t;
     pub fn LLVMRustCreateThinLTOData(
         Modules: *const ThinLTOModule,
         NumModules: c_uint,

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -107,6 +107,7 @@ pub struct ModuleConfig {
     pub emit_asm: bool,
     pub emit_obj: EmitObj,
     pub emit_thin_lto: bool,
+    pub emit_thin_lto_index: bool,
     pub bc_cmdline: String,
 
     // Miscellaneous flags. These are mostly copied from command-line
@@ -231,6 +232,10 @@ impl ModuleConfig {
             ),
             emit_obj,
             emit_thin_lto: sess.opts.unstable_opts.emit_thin_lto,
+            emit_thin_lto_index: if_regular!(
+                sess.opts.output_types.contains_key(&OutputType::ThinLinkBitcode),
+                false
+            ),
             bc_cmdline: sess.target.bitcode_llvm_cmdline.to_string(),
 
             verify_llvm_ir: sess.verify_llvm_ir(),
@@ -282,6 +287,7 @@ impl ModuleConfig {
 
     pub fn bitcode_needed(&self) -> bool {
         self.emit_bc
+            || self.emit_thin_lto_index
             || self.emit_obj == EmitObj::Bitcode
             || self.emit_obj == EmitObj::ObjectCode(BitcodeSection::Full)
     }
@@ -629,6 +635,9 @@ fn produce_final_output_artifacts(
                 // them for making an rlib.
                 copy_if_one_unit(OutputType::Bitcode, true);
             }
+            OutputType::ThinLinkBitcode => {
+                copy_if_one_unit(OutputType::ThinLinkBitcode, false);
+            }
             OutputType::LlvmAssembly => {
                 copy_if_one_unit(OutputType::LlvmAssembly, false);
             }

compiler/rustc_codegen_ssa/src/traits/write.rs

@@ -62,6 +62,7 @@ pub trait WriteBackendMethods: 'static + Sized + Clone {
 
 pub trait ThinBufferMethods: Send + Sync {
     fn data(&self) -> &[u8];
+    fn thin_link_data(&self) -> &[u8];
 }
 
 pub trait ModuleBufferMethods: Send + Sync {

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -1488,6 +1488,7 @@ LLVMRustPrepareThinLTOImport(const LLVMRustThinLTOData *Data, LLVMModuleRef M,
 // a ThinLTO summary attached.
 struct LLVMRustThinLTOBuffer {
   std::string data;
+  std::string thin_link_data;
 };
 
 extern "C" LLVMRustThinLTOBuffer*
@@ -1495,6 +1496,7 @@ LLVMRustThinLTOBufferCreate(LLVMModuleRef M, bool is_thin) {
   auto Ret = std::make_unique<LLVMRustThinLTOBuffer>();
   {
     auto OS = raw_string_ostream(Ret->data);
+    auto ThinLinkOS = raw_string_ostream(Ret->thin_link_data);
     {
       if (is_thin) {
         PassBuilder PB;
@@ -1508,7 +1510,7 @@ LLVMRustThinLTOBufferCreate(LLVMModuleRef M, bool is_thin) {
         PB.registerLoopAnalyses(LAM);
         PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
         ModulePassManager MPM;
-        MPM.addPass(ThinLTOBitcodeWriterPass(OS, nullptr));
+        MPM.addPass(ThinLTOBitcodeWriterPass(OS, &ThinLinkOS));
         MPM.run(*unwrap(M), MAM);
       } else {
         WriteBitcodeToFile(*unwrap(M), OS);
@@ -1533,6 +1535,16 @@ LLVMRustThinLTOBufferLen(const LLVMRustThinLTOBuffer *Buffer) {
   return Buffer->data.length();
 }
 
+extern "C" const void*
+LLVMRustThinLTOBufferThinLinkDataPtr(const LLVMRustThinLTOBuffer *Buffer) {
+  return Buffer->thin_link_data.data();
+}
+
+extern "C" size_t
+LLVMRustThinLTOBufferThinLinkDataLen(const LLVMRustThinLTOBuffer *Buffer) {
+  return Buffer->thin_link_data.length();
+}
+
 // This is what we used to parse upstream bitcode for actual ThinLTO
 // processing. We'll call this once per module optimized through ThinLTO, and
 // it'll be called concurrently on many threads.

compiler/rustc_session/src/config.rs

@@ -465,6 +465,7 @@ impl FromStr for SplitDwarfKind {
 #[derive(Encodable, Decodable)]
 pub enum OutputType {
     Bitcode,
+    ThinLinkBitcode,
     Assembly,
     LlvmAssembly,
     Mir,
@@ -492,6 +493,7 @@ impl OutputType {
         match *self {
             OutputType::Exe | OutputType::DepInfo | OutputType::Metadata => true,
             OutputType::Bitcode
+            | OutputType::ThinLinkBitcode
             | OutputType::Assembly
             | OutputType::LlvmAssembly
             | OutputType::Mir
@@ -502,6 +504,7 @@ impl OutputType {
     pub fn shorthand(&self) -> &'static str {
         match *self {
             OutputType::Bitcode => "llvm-bc",
+            OutputType::ThinLinkBitcode => "thin-link-bitcode",
             OutputType::Assembly => "asm",
             OutputType::LlvmAssembly => "llvm-ir",
             OutputType::Mir => "mir",
@@ -518,6 +521,7 @@ impl OutputType {
             "llvm-ir" => OutputType::LlvmAssembly,
             "mir" => OutputType::Mir,
             "llvm-bc" => OutputType::Bitcode,
+            "thin-link-bitcode" => OutputType::ThinLinkBitcode,
             "obj" => OutputType::Object,
             "metadata" => OutputType::Metadata,
             "link" => OutputType::Exe,
@@ -528,8 +532,9 @@ impl OutputType {
 
     fn shorthands_display() -> String {
         format!(
-            "`{}`, `{}`, `{}`, `{}`, `{}`, `{}`, `{}`, `{}`",
+            "`{}`, `{}`, `{}`, `{}`, `{}`, `{}`, `{}`, `{}`, `{}`",
             OutputType::Bitcode.shorthand(),
+            OutputType::ThinLinkBitcode.shorthand(),
             OutputType::Assembly.shorthand(),
             OutputType::LlvmAssembly.shorthand(),
             OutputType::Mir.shorthand(),
@@ -543,6 +548,7 @@ impl OutputType {
     pub fn extension(&self) -> &'static str {
         match *self {
             OutputType::Bitcode => "bc",
+            OutputType::ThinLinkBitcode => "indexing.o",
             OutputType::Assembly => "s",
             OutputType::LlvmAssembly => "ll",
             OutputType::Mir => "mir",
@@ -559,9 +565,11 @@ impl OutputType {
             | OutputType::LlvmAssembly
             | OutputType::Mir
             | OutputType::DepInfo => true,
-            OutputType::Bitcode | OutputType::Object | OutputType::Metadata | OutputType::Exe => {
+            OutputType::Bitcode
-                false
+            | OutputType::ThinLinkBitcode
-            }
+            | OutputType::Object
+            | OutputType::Metadata
+            | OutputType::Exe => false,
         }
     }
 }
@@ -644,6 +652,7 @@ impl OutputTypes {
     pub fn should_codegen(&self) -> bool {
         self.0.keys().any(|k| match *k {
             OutputType::Bitcode
+            | OutputType::ThinLinkBitcode
             | OutputType::Assembly
             | OutputType::LlvmAssembly
             | OutputType::Mir
@@ -657,6 +666,7 @@ impl OutputTypes {
     pub fn should_link(&self) -> bool {
         self.0.keys().any(|k| match *k {
             OutputType::Bitcode
+            | OutputType::ThinLinkBitcode
             | OutputType::Assembly
             | OutputType::LlvmAssembly
             | OutputType::Mir
@@ -1769,6 +1779,12 @@ fn parse_output_types(
                         display = OutputType::shorthands_display(),
                     ))
                 });
+                if output_type == OutputType::ThinLinkBitcode && !unstable_opts.unstable_options {
+                    early_dcx.early_fatal(format!(
+                        "{} requested but -Zunstable-options not specified",
+                        OutputType::ThinLinkBitcode.shorthand()
+                    ));
+                }
                 output_types.insert(output_type, path);
             }
         }