linux-kernel - Re: [PATCH 07/19] riscv: Optimize riscv_cpu_isa_extension

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Date: Fri, 12 Apr 2024 11:40:38 +0100
From: Conor Dooley <conor.dooley@...rochip.com>
To: Charlie Jenkins <charlie@...osinc.com>
CC: Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>, Krzysztof
 Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Paul Walmsley
	<paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
	<aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>, Conor Dooley
	<conor+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>, Jernej Skrabec
	<jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, Evan Green
	<evan@...osinc.com>, Clément Léger
	<cleger@...osinc.com>, Jonathan Corbet <corbet@....net>, Shuah Khan
	<shuah@...nel.org>, <linux-riscv@...ts.infradead.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Palmer Dabbelt
	<palmer@...osinc.com>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-sunxi@...ts.linux.dev>, <linux-doc@...r.kernel.org>,
	<linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 07/19] riscv: Optimize
 riscv_cpu_isa_extension_(un)likely()

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@...osinc.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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