Re: [PATCH v2 1/3] lib: extract 32bit mul/div helpers from nios2 into GENERIC_LIB

[Vineet Gupta <vgupta@kernel.org>]

On 11/3/26 05:30, Simon Schuster wrote:
> From: Dinh Nguyen <dinguyen@kernel.org>
>
> As suggested in 426c73b7-b4dc-4b68-9aa5-6145a13280bc@app.fastmail.com,
> the software emulation of div/mul/mod on 32-bit platforms from nios2
> might be of use to other smaller architectures as well when trying to
> drop their libgcc dependency (e.g., arc, openrisc).
> Therefore, extract and add them to lib/, guarded by respective
> CONFIG_GENERIC_LIB_* options.
>
> Symbols covered:
> s32 __mulsi3(s32 a, s32 b);
> s32 __divsi3(s32 a, s32 b);
> s32 __udivsi3(s32 a, s32 b);
> s32 __modsi3(s32 a, s32 b);
> s32 __umodsi3(s32 a, s32 b);
>
> Suggested-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
> Co-developed-by: Simon Schuster <schuster.simon@siemens-energy.com>
> Signed-off-by: Simon Schuster <schuster.simon@siemens-energy.com>
> ---
>   include/linux/libgcc.h | 10 ++++++
>   lib/Kconfig            | 15 ++++++++
>   lib/Makefile           | 10 ++++++
>   lib/divmod.c           | 94 ++++++++++++++++++++++++++++++++++++++++++++++++++
>   lib/mulsi3.c           | 24 +++++++++++++
>   5 files changed, 153 insertions(+)
>
> diff --git a/include/linux/libgcc.h b/include/linux/libgcc.h
> index 0d68f9d6a6a7e..f5db35a9bc146 100644
> --- a/include/linux/libgcc.h
> +++ b/include/linux/libgcc.h
> @@ -7,6 +7,7 @@
>   #define __LIB_LIBGCC_H
>   
>   #include <asm/byteorder.h>
> +#include <linux/types.h>
>   
>   typedef int word_type __attribute__ ((mode (__word__)));
>   
> @@ -34,6 +35,15 @@ long long notrace __lshrdi3(long long u, word_type b);
>   long long notrace __muldi3(long long u, long long v);
>   word_type notrace __ucmpdi2(unsigned long long a, unsigned long long b);
>   
> +/*
> + * Software 32-bit multiply for cores built without the hardware multiplier.
> + */
> +s32 notrace __mulsi3(s32 a, s32 b);
> +s32 notrace __divsi3(s32 a, s32 b);
> +s32 notrace __udivsi3(s32 a, s32 b);
> +s32 notrace __modsi3(s32 a, s32 b);
> +s32 notrace __umodsi3(s32 a, s32 b);

Shouldn't the 'u' prefixed functions have unsigned int args, result.
It seems libgcc/?divmod.c doesn't but ARC does have nint_t for those 
which is unsigned.
Nios it seems was removed from gcc?

>   #ifdef CONFIG_HAVE_ARCH_LIBGCC_H
>   #include <asm/libgcc.h>
>   #endif
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 1e42f5167c68b..16c2f2008b82c 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -611,6 +611,21 @@ config GENERIC_LIB_CMPDI2
>   config GENERIC_LIB_UCMPDI2
>   	bool
>   
> +config GENERIC_LIB_DIVSI3
> +	bool
> +
> +config GENERIC_LIB_MODSI3
> +	bool
> +
> +config GENERIC_LIB_UDIVSI3
> +	bool
> +
> +config GENERIC_LIB_UMODSI3
> +	bool
> +
> +config GENERIC_LIB_MULSI3
> +	bool
> +
>   config GENERIC_LIB_DEVMEM_IS_ALLOWED
>   	bool
>   
> diff --git a/lib/Makefile b/lib/Makefile
> index a2afe954805cb..352a012baecba 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -337,6 +337,16 @@ obj-$(CONFIG_GENERIC_LIB_LSHRDI3) += lshrdi3.o
>   obj-$(CONFIG_GENERIC_LIB_MULDI3) += muldi3.o
>   obj-$(CONFIG_GENERIC_LIB_CMPDI2) += cmpdi2.o
>   obj-$(CONFIG_GENERIC_LIB_UCMPDI2) += ucmpdi2.o
> +obj-$(CONFIG_GENERIC_LIB_MULSI3) += mulsi3.o
> +
> +divmod-needed := $(or \
> +	$(CONFIG_GENERIC_LIB_DIVSI3),\
> +	$(CONFIG_GENERIC_LIB_MODSI3),\
> +	$(CONFIG_GENERIC_LIB_UDIVSI3),\
> +	$(CONFIG_GENERIC_LIB_UMODSI3))
> +obj-$(divmod-needed) += divmod.o
> +
> +
>   obj-$(CONFIG_OBJAGG) += objagg.o
>   
>   # pldmfw library
> diff --git a/lib/divmod.c b/lib/divmod.c
> new file mode 100644
> index 0000000000000..cc1df6f9978ce
> --- /dev/null
> +++ b/lib/divmod.c
> @@ -0,0 +1,94 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright Altera Corporation (C) 2026. All rights reserved.
> + */
> +
> +#include <linux/libgcc.h>
> +#include <linux/export.h>
> +#include <linux/types.h>
> +
> +static u32 udivmodsi4(u32 num, u32 den, int modwanted)
> +{
> +	u32 bit = 1;
> +	u32 res = 0;
> +
> +	while (den < num && bit && !(den & (1U << 31))) {
> +		den <<= 1;
> +		bit <<= 1;
> +	}
> +
> +	while (bit) {
> +		if (num >= den) {
> +			num -= den;
> +			res |= bit;
> +		}
> +		bit >>= 1;
> +		den >>= 1;
> +	}
> +
> +	return modwanted ? num : res;
> +}
> +
> +
> +#ifdef CONFIG_GENERIC_LIB_DIVSI3
> +s32 __divsi3(s32 a, s32 b)
> +{
> +	int neg = 0;
> +	s32 res;
> +
> +	if (a < 0) {
> +		a = -a;
> +		neg = !neg;
> +	}
> +
> +	if (b < 0) {
> +		b = -b;
> +		neg = !neg;
> +	}
> +
> +	res = udivmodsi4(a, b, 0);
> +	if (neg)
> +		res = -res;
> +
> +	return res;
> +}
> +EXPORT_SYMBOL(__divsi3);
> +#endif
> +
> +#ifdef CONFIG_GENERIC_LIB_MODSI3
> +s32 __modsi3(s32 a, s32 b)
> +{
> +	int neg = 0;
> +	s32 res;
> +
> +	if (a < 0) {
> +		a = -a;
> +		neg = 1;
> +	}
> +
> +	if (b < 0)
> +		b = -b;
> +
> +	res = udivmodsi4(a, b, 1);
> +	if (neg)
> +		res = -res;
> +
> +	return res;
> +}
> +EXPORT_SYMBOL(__modsi3);
> +#endif
> +
> +#ifdef CONFIG_GENERIC_LIB_UDIVSI3
> +s32 __udivsi3(s32 a, s32 b)
> +{
> +	return udivmodsi4(a, b, 0);
> +}
> +EXPORT_SYMBOL(__udivsi3);
> +#endif
> +
> +#ifdef CONFIG_GENERIC_LIB_UMODSI3
> +s32 __umodsi3(s32 a, s32 b)
> +{
> +	return udivmodsi4(a, b, 1);
> +}
> +EXPORT_SYMBOL(__umodsi3);
> +#endif
> diff --git a/lib/mulsi3.c b/lib/mulsi3.c
> new file mode 100644
> index 0000000000000..6596a6598de05
> --- /dev/null
> +++ b/lib/mulsi3.c
> @@ -0,0 +1,24 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Copyright Altera Corporation (C) 2026. All rights reserved.
> + */
> +
> +#include <linux/libgcc.h>
> +#include <linux/export.h>
> +#include <linux/types.h>
> +
> +s32 __mulsi3(s32 a, s32 b)
> +{
> +	s32 res = 0;
> +	u32 cnt = a;
> +
> +	while (cnt) {
> +		if (cnt & 1)
> +			res += b;
> +
> +		b <<= 1;
> +		cnt >>= 1;
> +	}
> +
> +	return res;
> +}
> +EXPORT_SYMBOL(__mulsi3);
>