Guillaume Gomez 470c4f94e8
Rollup merge of #133452 - taiki-e:hexagon-asm-pred, r=Amanieu
Support predicate registers (clobber-only) in Hexagon inline assembly

The result of the Hexagon instructions such as comparison, store conditional, etc. is stored in predicate registers (`p[0-3]`), but currently there is no way to mark it as clobbered in `asm!`.

This is also needed for `clobber_abi` (although implementing `clobber_abi` will require the addition of support for [several more register classes](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/Hexagon/HexagonRegisterInfo.cpp#L71-L90). see also https://github.com/rust-lang/rust/issues/93335#issuecomment-2395210055).

Refs:
- [Section 6 "Conditional Execution" in Qualcomm Hexagon V73 Programmer’s Reference Manual](https://docs.qualcomm.com/bundle/publicresource/80-N2040-53_REV_AB_Qualcomm_Hexagon_V73_Programmers_Reference_Manual.pdf#page=90)
- [Register definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/Hexagon/HexagonRegisterInfo.td#L155)

cc `@androm3da` (target maintainer of hexagon-unknown-{[none-elf](https://doc.rust-lang.org/nightly/rustc/platform-support/hexagon-unknown-none-elf.html#target-maintainers),[linux-musl](https://doc.rust-lang.org/nightly/rustc/platform-support/hexagon-unknown-linux-musl.html#target-maintainers)})

r? `@Amanieu`

`@rustbot` label +A-inline-assembly
(Currently there is no O-hexagon label...)
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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