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Date: Thu, 18 Jan 2024 14:38:46 -0800
From: Charlie Jenkins <charlie@rivosinc.com>
To: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Conor Dooley <conor@kernel.org>,
	Paul Walmsley <paul.walmsley@sifive.com>, aou@eecs.berkeley.edu,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: lib: Support csum on GCC <11
Message-ID: <ZamodtvgqscAqW8N@ghost>
References: <20240118-museum-ample-11d649f95de8@spud>
 <mhng-6127df88-9768-4f9d-8d78-a1aeb4213451@palmer-ri-x1c9>
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In-Reply-To: <mhng-6127df88-9768-4f9d-8d78-a1aeb4213451@palmer-ri-x1c9>

On Thu, Jan 18, 2024 at 02:20:42PM -0800, Palmer Dabbelt wrote:
> On Thu, 18 Jan 2024 14:05:44 PST (-0800), Conor Dooley wrote:
> > On Thu, Jan 18, 2024 at 01:53:59PM -0800, Charlie Jenkins wrote:
> > > The OutputOperands field for asm goto statements is only supported
> > > starting from GCC 11. Split the asm goto to remove the use of this
> > > feature.
> > > 
> > > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > > Fixes: a04c192eabfb ("riscv: Add checksum library")
> > > ---
> > > The OutputOperands field for asm goto statements is only supported
> > > starting from GCC 11. Split the asm goto to remove the use of this
> > > feature.
> > 
> > Maybe this is a super naive question, but is it possible to just not
> > use the custom csum code for gcc older than 11?
> 
> Charlie and I were talking, these old GCC versions also don't support ZBB.
> So I think we can get away with something like

I sent an updated version that uses CC_HAS_ASM_GOTO_TIED_OUTPUT since
there may exist a compiler that supports ZBB but doesn't support output
in asm goto.

- Charlie

> 
> diff --git a/arch/riscv/lib/csum.c b/arch/riscv/lib/csum.c
> index 06ce8e7250d9..17f883b612c9 100644
> --- a/arch/riscv/lib/csum.c
> +++ b/arch/riscv/lib/csum.c
> @@ -158,10 +158,16 @@ do_csum_with_alignment(const unsigned char *buff, int len)
> 
> 	/*
> 	 * Zbb support saves 6 instructions, so not worth checking without
> -	 * alternatives if supported
> +	 * alternatives if supported.
> +	 *
> +	 * Note that we pull the check for ZBB into the preprocessor proper
> +	 * here, as otherwise GCC will attempt to compile the code inside the
> +	 * "if (IS_ENABLED(ZBB)" block which fails because GCC10 doesn't
> +	 * support ASM goto output operands.  GCC 10 also doesn't support ZBB,
> +	 * so we're safe with that check here.
> 	 */
> -	if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) &&
> -	    IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
> +#if defined(CONFIG_RISCV_ISA_ZBB)
> +	if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
> 		unsigned long fold_temp;
> 
> 		/*
> @@ -213,6 +219,7 @@ do_csum_with_alignment(const unsigned char *buff, int len)
> end:
> 		return csum >> 16;
> 	}
> +#endif /*CONFIG_RISCV_ISA_ZBB*/
> no_zbb:
> #ifndef CONFIG_32BIT
> 	csum += ror64(csum, 32);
> @@ -244,10 +251,11 @@ do_csum_no_alignment(const unsigned char *buff, int len)
> 
> 	/*
> 	 * Zbb support saves 6 instructions, so not worth checking without
> -	 * alternatives if supported
> +	 * alternatives if supported.  See above for the ZBB preprocessor
> +	 * check.
> 	 */
> -	if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) &&
> -	    IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
> +#if defined(CONFIG_RISCV_ISA_ZBB)
> +	if (IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
> 		unsigned long fold_temp;
> 
> 		/*
> @@ -287,6 +295,7 @@ do_csum_no_alignment(const unsigned char *buff, int len)
> #endif /* !CONFIG_32BIT */
> 		return csum >> 16;
> 	}
> +#endif /*CONFIG_RISCV_ISA_ZBB*/
> no_zbb:
> #ifndef CONFIG_32BIT
> 	csum += ror64(csum, 32);
> 
> which is building for me on GCC-10/defconfig.
> 
> > 
> > > ---
> > >  arch/riscv/lib/csum.c | 42 ++++++++++++++++++++++++++++++------------
> > >  1 file changed, 30 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/arch/riscv/lib/csum.c b/arch/riscv/lib/csum.c
> > > index 06ce8e7250d9..23be289f52b6 100644
> > > --- a/arch/riscv/lib/csum.c
> > > +++ b/arch/riscv/lib/csum.c
> > > @@ -177,22 +177,35 @@ do_csum_with_alignment(const unsigned char *buff, int len)
> > >  				  : no_zbb);
> > >  #ifdef CONFIG_32BIT
> > > -		asm_volatile_goto(".option push			\n\
> > > +		/*
> > > +		 * OutputOperands in asm goto is not supported until GCC 11, so
> > > +		 * this asm has to be split to be compatible.
> > > +		 */
> > > +		asm (".option push				\n\
> > >  		.option arch,+zbb				\n\
> > >  			rori	%[fold_temp], %[csum], 16	\n\
> > >  			andi	%[offset], %[offset], 1		\n\
> > >  			add	%[csum], %[fold_temp], %[csum]	\n\
> > > -			beq	%[offset], zero, %l[end]	\n\
> > > -			rev8	%[csum], %[csum]		\n\
> > >  		.option pop"
> > >  			: [csum] "+r" (csum), [fold_temp] "=&r" (fold_temp)
> > > -			: [offset] "r" (offset)
> > > -			:
> > > -			: end);
> > > +			: [offset] "r" (offset));
> > > +
> > > +		if (offset == 0)
> > > +			goto end;
> > > +
> > > +		asm (".option push				\n\
> > > +		.option arch, +zbb				\n\
> > > +			rev8	%[csum], %[csum]		\n\
> > > +		.option pop"
> > > +			: [csum] "+r" (csum));
> > >  		return (unsigned short)csum;
> > >  #else /* !CONFIG_32BIT */
> > > -		asm_volatile_goto(".option push			\n\
> > > +		/*
> > > +		 * OutputOperands in asm goto is not supported until GCC 11, so
> > > +		 * this asm has to be split to be compatible.
> > > +		 */
> > > +		asm (".option push				\n\
> > >  		.option arch,+zbb				\n\
> > >  			rori	%[fold_temp], %[csum], 32	\n\
> > >  			add	%[csum], %[fold_temp], %[csum]	\n\
> > > @@ -200,13 +213,18 @@ do_csum_with_alignment(const unsigned char *buff, int len)
> > >  			roriw	%[fold_temp], %[csum], 16	\n\
> > >  			addw	%[csum], %[fold_temp], %[csum]	\n\
> > >  			andi	%[offset], %[offset], 1		\n\
> > > -			beq	%[offset], zero, %l[end]	\n\
> > > -			rev8	%[csum], %[csum]		\n\
> > >  		.option pop"
> > >  			: [csum] "+r" (csum), [fold_temp] "=&r" (fold_temp)
> > > -			: [offset] "r" (offset)
> > > -			:
> > > -			: end);
> > > +			: [offset] "r" (offset));
> > > +
> > > +		if (offset == 0)
> > > +			goto end;
> > > +
> > > +		asm (".option push				\n\
> > > +		.option arch, +zbb				\n\
> > > +			rev8	%[csum], %[csum]		\n\
> > > +		.option pop"
> > > +			: [csum] "+r" (csum));
> > >  		return (csum << 16) >> 48;
> > >  #endif /* !CONFIG_32BIT */
> > > 
> > > ---
> > > base-commit: 080c4324fa5e81ff3780206a138223abfb57a68e
> > > change-id: 20240118-csum_remove_output_operands_asm_goto-49922c141ce7
> > > -- 
> > > - Charlie
> > > 
> > >