Files
rust/tests/codegen
Matthias Krüger 6c9e922685 Rollup merge of #131323 - jfrimmel:avr-inline-asm-clobber-abi, r=Amanieu
Support `clobber_abi` in AVR inline assembly

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.

[ABI documentation]: https://gcc.gnu.org/wiki/avr-gcc#Call-Used_Registers

r? ``@Amanieu``

``@rustbot`` label +O-AVR
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The files here use the LLVM FileCheck framework, documented at https://llvm.org/docs/CommandGuide/FileCheck.html.

One extension worth noting is the use of revisions as custom prefixes for FileCheck. If your codegen test has different behavior based on the chosen target or different compiler flags that you want to exercise, you can use a revisions annotation, like so:

// revisions: aaa bbb
// [bbb] compile-flags: --flags-for-bbb

After specifying those variations, you can write different expected, or explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:, like so:

// CHECK: expected code
// aaa-SAME: emitted-only-for-aaa
// aaa-NOT:                        emitted-only-for-bbb
// bbb-NOT:  emitted-only-for-aaa
// bbb-SAME:                       emitted-only-for-bbb