Remove register allocator bug workaround

[dylanmckay]

Bugzilla Link	38224
Version	trunk
OS	All

Extended Description

The register allocator does not handle certain functions that use the 'LDDWRdPtrQ' instructions which have high register pressure.

The error message is:

LLVM ERROR: ran out of registers during register allocation

In order to work around this, we restricted the allowable registers of LDDWRdPtrQ to disallow Z.

First noticed in avr-rust/rust-legacy-fork#37

[llvmbot]

I think this is the same issues as llvm/llvm-bugzilla-archive#39391 . The proposed patch resolves this as well.

[dylanmckay]

Huh, thanks for fixing the root cause Nirav.

I've raised avr-rust/rust-legacy-fork#126 to track the removal of our AVR-specific workaround (the DREGS_WITHOUT_Z_WORKAROUND register class).

[benshi001]

Now LDDWRdPtrQ can use Z as base address without any error.

int aff(int *arr) {
        return arr[4];
}

is compiled to

aff:                                    ; @aff
; %bb.0:                                ; %entry
        movw    r30, r24
        ldd     r24, Z+8
        ldd     r25, Z+9
        ret