Add avr_target_feature by taiki-e · Pull Request #146900 · rust-lang/rust (original) (raw)

This adds the following unstable target features (tracking issue: #146889):

• The following two are particularly important for properly supporting inline assembly:
  • tinyencoding: AVR has devices that reduce the number of registers, similar to RISC-V's RV32E. This feature is necessary to support inline assembly in such devices. (see also Support AVRTiny devices in AVR inline assembly #146901)
  • lowbytefirst: AVR's memory access is per 8-bit, and when writing 16-bit ports, the bytes must be written in a specific order. This order depends on devices, making this feature necessary to write proper inline assembly for such use cases. (see also llvm/llvm-project@2a52876)
• The followings help recognizing whether specific instructions are available:
  • addsubiw
  • break
  • eijmpcall
  • elpm
  • elpmx
  • ijmpcall
  • jmpcall
  • lpm
  • lpmx
  • movw
  • mul
  • rmw
  • spm
  • spmx
  • sram
    Of these, all except addsubiw, break, ijmpcall, lpm, rmw, spm, spmx, and sram have corresponding conditional codes in avr-libc. LLVM also has des feature, but I excluded it from this PR because DES is insecure.

LLVM also has smallstack, wrappingrjmp, and memmappedregs features, but I skipped them because they didn't seem to belong to either of the above categories, but I might have missed something.

(The feature names are match with definitions in LLVM.)

cc @Patryk27 @Rahix
r? workingjubilee

@rustbot label +O-AVR +A-target-feature