Support clobber_abi in AVR inline assembly by jfrimmel · Pull Request #131323 · rust-lang/rust (original) (raw)

This PR implements the clobber_abi part necessary to eventually stabilize the inline assembly for AVR. This is tracked in #93335.
This is heavily inspired by the sibling-PR #131310 for the MSP430. I've explained my reasoning in the first commit message in detail, which is reproduced below for easier reviewing:

This follows the ABI documentation of AVR-GCC:

The [...] call-clobbered general purpose registers (GPRs) are registers that might be destroyed (clobbered) by a function call.

• R18–R27, R30, R31
  These GPRs are call clobbered. An ordinary function may use them without restoring the contents. [...]
• R0, T-Flag
  The temporary register and the T-flag in SREG are also call-clobbered, but this knowledge is not exposed explicitly to the compiler (R0 is a fixed register).

Therefore this commit lists the aforementioned registers r18–r27, r30 and r31 as clobbered registers. Since the r0 register (listed above as well) is not available in inline assembly at all (potentially because the AVR-GCC considers it a fixed register causing the register to never be used in register allocation and LLVM adopting this), there is no need to list it in the clobber list (the r0-variant is not even available). A comment was added to ensure, that the r0 gets added to the clobber-list once the register gets usable in inline ASM.
Since the SREG is normally considered clobbered anyways (unless the user supplies the preserve_flags-option), there is no need to explicitly list a bit in this register (which is not possible to list anyways).

Note, that this commit completely ignores the case of interrupts (that are described in the ABI-specification), since every register touched in an ISR need to be saved anyways.

r? @Amanieu

@rustbot label +O-AVR