From 3da3d15f9f150dafd7d635859795c4c714e7b7ad Mon Sep 17 00:00:00 2001 From: Sam Elliott Date: Sat, 30 May 2020 18:24:19 +0100 Subject: [PATCH] [RISC-V] Do not force frame pointers We have been seeing some very inefficient code that went away when using `-Cforce-frame-pointers=no`. For instance `core::ptr::drop_in_place` at `-Oz` was compiled into a function which consisted entirely of saving registers to the stack, then using the frame pointer to restore the same registers (without any instructions between the prolog and epilog). The RISC-V LLVM backend supports frame pointer elimination, so it makes sense to allow this to happen when using Rust. It's not clear to me that frame pointers have ever been required in the general case. In rust-lang/rust#61675 it was pointed out that this made reassembling stack traces easier, which is true, but there is a code generation option for forcing frame pointers, and I feel the default should not be to require frame pointers, given it demonstrably makes code size worse (around 10% in some embedded applications). The kinds of targets mentioned in rust-lang/rust#61675 are popular, but should not dictate that code generation should be worse for all RISC-V targets, especially as there is a way to use CFI information to reconstruct the stack when the frame pointer is eliminated. It is also a misconception that `fp` is always used for the frame pointer. `fp` is an ABI name for `x8` (aka `s0`), and if no frame pointer is required, `x8` may be used for other callee-saved values. This commit does ensure that the standard library is built with unwind tables, so that users do not need to rebuild the standard library in order to get a backtrace that includes standard library calls (which is the original reason for forcing frame pointers). --- src/bootstrap/compile.rs | 10 ++++++++++ src/librustc_target/spec/riscv32i_unknown_none_elf.rs | 1 - .../spec/riscv32imac_unknown_none_elf.rs | 1 - .../spec/riscv32imc_unknown_none_elf.rs | 1 - src/librustc_target/spec/riscv64gc_unknown_none_elf.rs | 1 - .../spec/riscv64imac_unknown_none_elf.rs | 1 - 6 files changed, 10 insertions(+), 5 deletions(-) diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs index b3999118e3de4..46c43be992ded 100644 --- a/src/bootstrap/compile.rs +++ b/src/bootstrap/compile.rs @@ -244,6 +244,16 @@ pub fn std_cargo(builder: &Builder<'_>, target: Interned, stage: u32, ca if stage >= 1 { cargo.rustflag("-Cembed-bitcode=yes"); } + + // By default, rustc does not include unwind tables unless they are required + // for a particular target. They are not required by RISC-V targets, but + // compiling the standard library with them means that users can get + // backtraces without having to recompile the standard library themselves. + // + // This choice was discussed in https://github.com/rust-lang/rust/pull/69890 + if target.contains("riscv") { + cargo.rustflag("-Cforce-unwind-tables=yes"); + } } #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)] diff --git a/src/librustc_target/spec/riscv32i_unknown_none_elf.rs b/src/librustc_target/spec/riscv32i_unknown_none_elf.rs index aade1e708232e..d7b3e7e15307a 100644 --- a/src/librustc_target/spec/riscv32i_unknown_none_elf.rs +++ b/src/librustc_target/spec/riscv32i_unknown_none_elf.rs @@ -25,7 +25,6 @@ pub fn target() -> TargetResult { relocation_model: RelocModel::Static, emit_debug_gdb_scripts: false, abi_blacklist: super::riscv_base::abi_blacklist(), - eliminate_frame_pointer: false, ..Default::default() }, }) diff --git a/src/librustc_target/spec/riscv32imac_unknown_none_elf.rs b/src/librustc_target/spec/riscv32imac_unknown_none_elf.rs index e2990eeb826f7..b93b6fcf8002a 100644 --- a/src/librustc_target/spec/riscv32imac_unknown_none_elf.rs +++ b/src/librustc_target/spec/riscv32imac_unknown_none_elf.rs @@ -25,7 +25,6 @@ pub fn target() -> TargetResult { relocation_model: RelocModel::Static, emit_debug_gdb_scripts: false, abi_blacklist: super::riscv_base::abi_blacklist(), - eliminate_frame_pointer: false, ..Default::default() }, }) diff --git a/src/librustc_target/spec/riscv32imc_unknown_none_elf.rs b/src/librustc_target/spec/riscv32imc_unknown_none_elf.rs index 55a4d58dfccca..a16e7e31c6619 100644 --- a/src/librustc_target/spec/riscv32imc_unknown_none_elf.rs +++ b/src/librustc_target/spec/riscv32imc_unknown_none_elf.rs @@ -25,7 +25,6 @@ pub fn target() -> TargetResult { relocation_model: RelocModel::Static, emit_debug_gdb_scripts: false, abi_blacklist: super::riscv_base::abi_blacklist(), - eliminate_frame_pointer: false, ..Default::default() }, }) diff --git a/src/librustc_target/spec/riscv64gc_unknown_none_elf.rs b/src/librustc_target/spec/riscv64gc_unknown_none_elf.rs index 7376a14e951f5..e5147a12ed320 100644 --- a/src/librustc_target/spec/riscv64gc_unknown_none_elf.rs +++ b/src/librustc_target/spec/riscv64gc_unknown_none_elf.rs @@ -26,7 +26,6 @@ pub fn target() -> TargetResult { code_model: Some(CodeModel::Medium), emit_debug_gdb_scripts: false, abi_blacklist: super::riscv_base::abi_blacklist(), - eliminate_frame_pointer: false, ..Default::default() }, }) diff --git a/src/librustc_target/spec/riscv64imac_unknown_none_elf.rs b/src/librustc_target/spec/riscv64imac_unknown_none_elf.rs index a3b0eb5334f40..dc056b55b3868 100644 --- a/src/librustc_target/spec/riscv64imac_unknown_none_elf.rs +++ b/src/librustc_target/spec/riscv64imac_unknown_none_elf.rs @@ -26,7 +26,6 @@ pub fn target() -> TargetResult { code_model: Some(CodeModel::Medium), emit_debug_gdb_scripts: false, abi_blacklist: super::riscv_base::abi_blacklist(), - eliminate_frame_pointer: false, ..Default::default() }, })