Add vsx register support for ppc inline asm, and implement preserves_flag option

[pmur]

This should address the last(?) missing pieces of inline asm for ppc:

• Explicit VSX register support. ISA 2.06 (POWER7) added a 64x128b register overlay extending the fpr's to 128b, and unifies them with the vmx (altivec) registers. Implementations details within gcc/llvm percolate up, and require using the x template modifier. I have updated the inline asm to implicitly include this for vsx arguments which do not specify it. Support for the gcc codegen backend is still a todo.

• Implement the preserves_flags option. All ABI's, and all ISAs store their flags in cr, and the carry bit lives inside xer. The other status registers hold sticky bits or control bits which do not affect branch instructions.

There is some interest in the e500 (powerpcspe) port. Architecturally, it has a very different FP ISA, and includes a simd extension called SPR (which is not IBM's cell SPE). Notably, it does not have altivec/fpr/vsx registers. It also has an SPE accumulator register which its ABI marks as volatile, but I am not sure if the compiler uses it.

[rustbot]

r? @jackh726

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[pmur]

r? @Amanieu
cc @taiki-e

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking tempfile v3.20.0
[RUSTC-TIMING] tempfile test:false 0.113
[RUSTC-TIMING] object test:false 1.952
    Checking rustc_codegen_gcc v0.1.0 (/checkout/compiler/rustc_codegen_gcc)
error[E0004]: non-exhaustive patterns: `rustc_target::asm::InlineAsmRegClass::PowerPC(rustc_target::asm::PowerPCInlineAsmRegClass::vsreg)` not covered
   --> compiler/rustc_codegen_gcc/src/asm.rs:654:11
    |
654 |     match reg_class {
    |           ^^^^^^^^^ pattern `rustc_target::asm::InlineAsmRegClass::PowerPC(rustc_target::asm::PowerPCInlineAsmRegClass::vsreg)` not covered
    |
note: `rustc_target::asm::InlineAsmRegClass` defined here
   --> /checkout/compiler/rustc_target/src/asm/mod.rs:474:1
    |
474 | pub enum InlineAsmRegClass {
---
    = note: the matched value is of type `rustc_target::asm::InlineAsmRegClass`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
740 ~         InlineAsmRegClass::Err => unreachable!(),
741 ~         rustc_target::asm::InlineAsmRegClass::PowerPC(rustc_target::asm::PowerPCInlineAsmRegClass::vsreg) => todo!(),
    |

error[E0004]: non-exhaustive patterns: `rustc_target::asm::InlineAsmRegClass::PowerPC(rustc_target::asm::PowerPCInlineAsmRegClass::vsreg)` not covered
   --> compiler/rustc_codegen_gcc/src/asm.rs:747:11
    |
747 |     match reg {
    |           ^^^ pattern `rustc_target::asm::InlineAsmRegClass::PowerPC(rustc_target::asm::PowerPCInlineAsmRegClass::vsreg)` not covered
    |
note: `rustc_target::asm::InlineAsmRegClass` defined here
   --> /checkout/compiler/rustc_target/src/asm/mod.rs:474:1
    |
474 | pub enum InlineAsmRegClass {
---
    = note: the matched value is of type `rustc_target::asm::InlineAsmRegClass`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
837 ~         InlineAsmRegClass::Err => unreachable!(),
838 ~         rustc_target::asm::InlineAsmRegClass::PowerPC(rustc_target::asm::PowerPCInlineAsmRegClass::vsreg) => todo!(),
    |

For more information about this error, try `rustc --explain E0004`.
[RUSTC-TIMING] rustc_codegen_gcc test:false 1.206
error: could not compile `rustc_codegen_gcc` (lib) due to 2 previous errors