From 4ed846ad4d4e0be96efc4837fa416aabce1882db Mon Sep 17 00:00:00 2001 From: Javier Blazquez Date: Mon, 11 Oct 2021 12:09:32 -0700 Subject: [PATCH] Add -Z no-unique-section-names to reduce ELF header bloat. This change adds a new compiler flag that can help reduce the size of ELF binaries that contain many functions. By default, when enabling function sections (which is the default for most targets), the LLVM backend will generate different section names for each function. For example, a function "func" would generate a section called ".text.func". Normally this is fine because the linker will merge all those sections into a single one in the binary. However, starting with LLVM 12 (llvm/llvm-project@ee5d1a0), the backend will also generate unique section names for exception handling, resulting in thousands of ".gcc_except_table.*" sections ending up in the final binary because some linkers don't currently merge or strip these EH sections. This can bloat the ELF headers and string table significantly in binaries that contain many functions. The new option is analogous to Clang's -fno-unique-section-names, and instructs LLVM to generate the same ".text" and ".gcc_except_table" section for each function, resulting in smaller object files and potentially a smaller final binary. --- compiler/rustc_codegen_llvm/src/back/write.rs | 2 ++ compiler/rustc_codegen_llvm/src/llvm/ffi.rs | 1 + compiler/rustc_interface/src/tests.rs | 1 + compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp | 2 ++ compiler/rustc_session/src/options.rs | 2 ++ .../src/compiler-flags/no-unique-section-names.md | 9 +++++++++ 6 files changed, 17 insertions(+) create mode 100644 src/doc/unstable-book/src/compiler-flags/no-unique-section-names.md diff --git a/compiler/rustc_codegen_llvm/src/back/write.rs b/compiler/rustc_codegen_llvm/src/back/write.rs index ab48c56a62663..d6c122aabdbb5 100644 --- a/compiler/rustc_codegen_llvm/src/back/write.rs +++ b/compiler/rustc_codegen_llvm/src/back/write.rs @@ -161,6 +161,7 @@ pub fn target_machine_factory( let ffunction_sections = sess.opts.debugging_opts.function_sections.unwrap_or(sess.target.function_sections); let fdata_sections = ffunction_sections; + let funique_section_names = !sess.opts.debugging_opts.no_unique_section_names; let code_model = to_llvm_code_model(sess.code_model()); @@ -205,6 +206,7 @@ pub fn target_machine_factory( use_softfp, ffunction_sections, fdata_sections, + funique_section_names, trap_unreachable, singlethread, asm_comments, diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs index 436d906827b5b..3e7048e7b80db 100644 --- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs +++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs @@ -2153,6 +2153,7 @@ extern "C" { UseSoftFP: bool, FunctionSections: bool, DataSections: bool, + UniqueSectionNames: bool, TrapUnreachable: bool, Singlethread: bool, AsmComments: bool, diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs index cfe13b1fd4e1f..021afde8c62ac 100644 --- a/compiler/rustc_interface/src/tests.rs +++ b/compiler/rustc_interface/src/tests.rs @@ -740,6 +740,7 @@ fn test_debugging_options_tracking_hash() { tracked!(new_llvm_pass_manager, Some(true)); tracked!(no_generate_arange_section, true); tracked!(no_link, true); + tracked!(no_unique_section_names, true); tracked!(no_profiler_runtime, true); tracked!(osx_rpath_install_name, true); tracked!(panic_abort_tests, true); diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp index ddb5f7dcebfad..42d2c13b783d5 100644 --- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp +++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp @@ -459,6 +459,7 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine( LLVMRustCodeGenOptLevel RustOptLevel, bool UseSoftFloat, bool FunctionSections, bool DataSections, + bool UniqueSectionNames, bool TrapUnreachable, bool Singlethread, bool AsmComments, @@ -488,6 +489,7 @@ extern "C" LLVMTargetMachineRef LLVMRustCreateTargetMachine( } Options.DataSections = DataSections; Options.FunctionSections = FunctionSections; + Options.UniqueSectionNames = UniqueSectionNames; Options.MCOptions.AsmVerbose = AsmComments; Options.MCOptions.PreserveAsmComments = AsmComments; Options.MCOptions.ABIName = ABIStr; diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs index 8ecb7a031ad81..b76ea98e637da 100644 --- a/compiler/rustc_session/src/options.rs +++ b/compiler/rustc_session/src/options.rs @@ -1188,6 +1188,8 @@ options! { "compile without linking"), no_parallel_llvm: bool = (false, parse_no_flag, [UNTRACKED], "run LLVM in non-parallel mode (while keeping codegen-units and ThinLTO)"), + no_unique_section_names: bool = (false, parse_bool, [TRACKED], + "do not use unique names for text and data sections when -Z function-sections is used"), no_profiler_runtime: bool = (false, parse_no_flag, [TRACKED], "prevent automatic injection of the profiler_builtins crate"), normalize_docs: bool = (false, parse_bool, [TRACKED], diff --git a/src/doc/unstable-book/src/compiler-flags/no-unique-section-names.md b/src/doc/unstable-book/src/compiler-flags/no-unique-section-names.md new file mode 100644 index 0000000000000..5c1c7cda7013e --- /dev/null +++ b/src/doc/unstable-book/src/compiler-flags/no-unique-section-names.md @@ -0,0 +1,9 @@ +# `no-unique-section-names` + +------------------------ + +This flag currently applies only to ELF-based targets using the LLVM codegen backend. It prevents the generation of unique ELF section names for each separate code and data item when `-Z function-sections` is also in use, which is the default for most targets. This option can reduce the size of object files, and depending on the linker, the final ELF binary as well. + +For example, a function `func` will by default generate a code section called `.text.func`. Normally this is fine because the linker will merge all those `.text.*` sections into a single one in the binary. However, starting with [LLVM 12](https://github.com/llvm/llvm-project/commit/ee5d1a04), the backend will also generate unique section names for exception handling, so you would see a section name of `.gcc_except_table.func` in the object file and potentially in the final ELF binary, which could add significant bloat to programs that contain many functions. + +This flag instructs LLVM to use the same `.text` and `.gcc_except_table` section name for each function, and it is analogous to Clang's `-fno-unique-section-names` option.