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

Conor Dooley
2024-04-12T10:42:03Z

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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?

Thanks,
Conor.

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