Re: [PATCH 0/2] target/arm: Deprecate iwMMXt emulation and associated CPUs

[Richard Henderson <richard.henderson@linaro.org>]

On 1/27/25 03:27, Peter Maydell wrote:
> This patchset marks all the CPUs that support iwMMXt as deprecated,
> because I don't believe anybody is using them, and we have no way to
> test the thousands of lines of code we have that's only there for
> iwMMXt emulation.  (See for instance the recent thread where a patch
> was submitted to fix an issue detected by a static analyzer: we
> didn't take it, largely because we had no way to know if the
> behaviour change the patch would produce was correct or not.)
> 
> The pxa2xx CPUs are now only useful with user-mode emulation, because
> we dropped all the machine types that used them in 9.2.  (Technically
> you could alse use "-cpu pxa270" with a board model like versatilepb
> which doesn't sanity-check the CPU type, but that has never been a
> supported config.)
> 
> To use them (or iwMMXt emulation) with QEMU user-mode you would need
> to explicitly select them with the -cpu option or the QEMU_CPU
> environment variable, because the '-cpu max' default CPU does not
> include iwMMXt emulation.  A google search finds no examples of
> anybody doing this in the last decade.
> 
> I asked some of the Linaro GCC folks if they were using QEMU to test
> their iwMMXt codegen, or knew anybody doing that upstream, and the
> answer was "no". In fact, GCC is in the process of dropping support
> for iwMMXt entirely.
>                      
> We have one test case in check-tcg which purports to be checking
> iwMMXt. In fact it is doing no such thing: it runs the test without
> selecting an iwMMXt CPU, which means the iwMMXt insns are interpreted
> as FPA11 insns by the linux-user emulate_arm_fpa11() code. So the test
> prints garbage and then succeeds anyway. Modern distro toolchains
> can't generate a binary that will run with -cpu pxa270 (because their
> crt startup code uses Thumb insns); rather than putting in a lot of
> effort trying to salvage the test case to really test a feature we've
> deprecated, I opted to just remove the test.
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~