vendor/rustc-ap-rustc_target/src/spec/thumb_base.rs

   1 // These `thumbv*` targets cover the ARM Cortex-M family of processors which are widely used in
   2 // microcontrollers. Namely, all these processors:
   3 //
   4 // - Cortex-M0
   5 // - Cortex-M0+
   6 // - Cortex-M1
   7 // - Cortex-M3
   8 // - Cortex-M4(F)
   9 // - Cortex-M7(F)
  10 // - Cortex-M23
  11 // - Cortex-M33
  12 //
  13 // We have opted for these instead of one target per processor (e.g., `cortex-m0`, `cortex-m3`,
  14 // etc) because the differences between some processors like the cortex-m0 and cortex-m1 are almost
  15 // non-existent from the POV of codegen so it doesn't make sense to have separate targets for them.
  16 // And if differences exist between two processors under the same target, rustc flags can be used to
  17 // optimize for one processor or the other.
  18 //
  19 // Also, we have not chosen a single target (`arm-none-eabi`) like GCC does because this makes
  20 // difficult to integrate Rust code and C code. Targeting the Cortex-M4 requires different gcc flags
  21 // than the ones you would use for the Cortex-M0 and with a single target it'd be impossible to
  22 // differentiate one processor from the other.
  23 //
  24 // About arm vs thumb in the name. The Cortex-M devices only support the Thumb instruction set,
  25 // which is more compact (higher code density), and not the ARM instruction set. That's why LLVM
  26 // triples use thumb instead of arm. We follow suit because having thumb in the name let us
  27 // differentiate these targets from our other `arm(v7)-*-*-gnueabi(hf)` targets in the context of
  28 // build scripts / gcc flags.
  29
  30 use crate::spec::{LinkerFlavor, LldFlavor, PanicStrategy, RelocModel, TargetOptions};
  31
  32 pub fn opts() -> TargetOptions {
  33     // See rust-lang/rfcs#1645 for a discussion about these defaults
  34     TargetOptions {
  35         linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
  36         executables: true,
  37         // In most cases, LLD is good enough
  38         linker: Some("rust-lld".to_string()),
  39         // Because these devices have very little resources having an unwinder is too onerous so we
  40         // default to "abort" because the "unwind" strategy is very rare.
  41         panic_strategy: PanicStrategy::Abort,
  42         // Similarly, one almost always never wants to use relocatable code because of the extra
  43         // costs it involves.
  44         relocation_model: RelocModel::Static,
  45         unsupported_abis: super::arm_base::unsupported_abis(),
  46         // When this section is added a volatile load to its start address is also generated. This
  47         // volatile load is a footgun as it can end up loading an invalid memory address, depending
  48         // on how the user set up their linker scripts. This section adds pretty printer for stuff
  49         // like std::Vec, which is not that used in no-std context, so it's best to left it out
  50         // until we figure a way to add the pretty printers without requiring a volatile load cf.
  51         // rust-lang/rust#44993.
  52         emit_debug_gdb_scripts: false,
  53         // LLVM is eager to trash the link register when calling `noreturn` functions, which
  54         // breaks debugging. Preserve LR by default to prevent that from happening.
  55         eliminate_frame_pointer: false,
  56         ..Default::default()
  57     }
  58 }