Re: [PATCH] riscv: lib: Support csum on GCC <11

[Palmer Dabbelt]

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@xxxxxxxxxxxx>
> > 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

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