Re: [PATCH 3/3] target/arm: Make Thumb T1 hint space UNDEF before v6T2

On 8/21/26 06:25, Peter Maydell wrote:
> The "hint space" is a region of the encoding space which is defined
> to NOP if not specified as an architected instruction, so that future
> instructions can be added there which fall back to NOPs on older
> CPUs.  In the A32 encoding, this hint space was carved out of the MSR
> (imm) insn by using the fact that a field_mask (bits [19:15]) of
> 0b0000 meant that an MSR (imm) would set no parts of the CPSR from
> the immediate, so it was always NOP on existing CPUs.
> 
> For the T1 encoding, the hint space is in a range that used to UNDEF
> in Armv5, and so the hint insns and the NOP region must all UNDEF
> before v6T2.  Rather than putting this check in the trans functions
> for each hint insn and for the NOP space (which is a lot of places,
> and awkward since those trans functions are often shared with the A64
> and A32 encodings), put in a decode line that covers the whole space
> which we check before any of the hints and which will explicitly
> UNDEF if necessary.
> 
> Cc:[email protected]
> Resolves:https://gitlab.com/qemu-project/qemu/-/work_items/4208
> Signed-off-by: Peter Maydell<[email protected]>
> ---
>   target/arm/tcg/t16.decode  |  3 +++
>   target/arm/tcg/translate.c | 18 ++++++++++++++++++
>   2 files changed, 21 insertions(+)

Reviewed-by: Richard Henderson <[email protected]>

r~