Re: [PATCH 07/19] riscv: Optimize riscv_cpu_isa_extension_(un)likely()

Charlie Jenkins
2024-04-12T17:34:55Z

On Fri, Apr 12, 2024 at 11:40:38AM +0100, Conor Dooley wrote:
> On Thu, Apr 11, 2024 at 09:11:13PM -0700, Charlie Jenkins wrote:
> > When alternatives are disabled, riscv_cpu_isa_extension_(un)likely()
> > checks if the current cpu supports the selected extension if not all
> > cpus support the extension. It is sufficient to only check if the
> > current cpu supports the extension.
> >
> > The alternatives code to handle if all cpus support an extension is
> > factored out into a new function to support this.
> >
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
>
> > static __always_inline bool riscv_cpu_has_extension_unlikely(int cpu, const unsigned long ext)
> > {
> > - if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE) && riscv_has_extension_unlikely(ext))
> > - return true;
> > + compiletime_assert(ext < RISCV_ISA_EXT_MAX,
> > + "ext must be < RISCV_ISA_EXT_MAX");
> >
> > - return __riscv_isa_extension_available(hart_isa[cpu].isa, ext);
> > + if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE) && __riscv_has_extension_unlikely_alternatives(ext))
> > + return true;
> > + else
> > + return __riscv_isa_extension_available(hart_isa[cpu].isa, ext);
> > }
>
> static __always_inline bool riscv_cpu_has_extension_likely(int cpu, const unsigned long ext)
> {
> if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE) && riscv_has_extension_likely(ext))
> return true;
>
> return __riscv_isa_extension_available(hart_isa[cpu].isa, ext);
> }
>
> This is the code as things stand. If alternatives are disabled, the if
> statement becomes if (0 && foo) which will lead to the function call
> getting constant folded away and all you end up with is the call to
> __riscv_isa_extension_available(). Unless I am missing something, I don't
> think this patch has any affect?

Yeah I fumbled this one it appears. I got thrown off by the nested
IS_ENABLED(CONFIG_RISCV_ALTERNATIVE). This patch eliminates the need for
this and maybe can avoid avoid confusion in the future.

- Charlie

>
> Thanks,
> Conor.