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

[Charlie Jenkins]

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@xxxxxxxxxxxx>
> > ---
> 
> >  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.
>