[PATCH 0/2] x86: Use "er" asm constriant for add/sub

[PATCH 0/2] x86: Use "er" asm constriant for add/sub

[David Laight]

The x86 instruction set only supports 32bit signed immediate values
for add/sub.
Replace the "ir" constraint with "er" to avoid build errors.

Found by a patch that added used mul_u64_add_u64_div_u64() to
do a rounding divide by 2^32 (perhaps not the best way to do this).

A quick grep only found one other affected file.

David Laight (2):
  lib: mul_u64_add_u64_div_u64: Fix addition of large constants
  x86/local: local_add/local_sub: Support large immediate values

 arch/x86/include/asm/div64.h | 2 +-
 arch/x86/include/asm/local.h | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

-- 
2.39.5

[PATCH 1/2] lib: mul_u64_add_u64_div_u64: Fix addition of large constants

[David Laight]

Adding constants over 2^31 fails to compile because the ADD instruction
only supports 32bit signed immediates.

Replace the "irm" constraint with "erm" so that the compiler loads
large constants into a register.

Found by a patch to drivers/iio/frequency/ad9910.c

Fixes: 6480241f31f5 ("lib: add mul_u64_add_u64_div_u64() and mul_u64_u64_div_u64_roundup()")
Signed-off-by: David Laight <david.laight.linux@gmail.com>
---
 arch/x86/include/asm/div64.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
index 30fd06ede751..8a2d343f977e 100644
--- a/arch/x86/include/asm/div64.h
+++ b/arch/x86/include/asm/div64.h
@@ -111,7 +111,7 @@ static inline u64 mul_u64_add_u64_div_u64(u64 rax, u64 mul, u64 add, u64 div)
 
 	if (!statically_true(!add))
 		asm ("addq %[add], %[lo]; adcq $0, %[hi]" :
-			[lo] "+r" (rax), [hi] "+r" (rdx) : [add] "irm" (add));
+			[lo] "+r" (rax), [hi] "+r" (rdx) : [add] "erm" (add));
 
 	asm ("divq %[div]" : "+a" (rax), "+d" (rdx) : [div] "rm" (div));
 
-- 
2.39.5

Re: [PATCH 1/2] lib: mul_u64_add_u64_div_u64: Fix addition of large constants

[H. Peter Anvin]

On 2026-08-03 02:47, David Laight wrote:
> Adding constants over 2^31 fails to compile because the ADD instruction
> only supports 32bit signed immediates.
> 
> Replace the "irm" constraint with "erm" so that the compiler loads
> large constants into a register.
> 
> Found by a patch to drivers/iio/frequency/ad9910.c
> 
> Fixes: 6480241f31f5 ("lib: add mul_u64_add_u64_div_u64() and mul_u64_u64_div_u64_roundup()")
> Signed-off-by: David Laight <david.laight.linux@gmail.com>

So it is.

Reviewed-by: H. Peter Anvin <hpa@zytor.com>

> ---
>  arch/x86/include/asm/div64.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
> index 30fd06ede751..8a2d343f977e 100644
> --- a/arch/x86/include/asm/div64.h
> +++ b/arch/x86/include/asm/div64.h
> @@ -111,7 +111,7 @@ static inline u64 mul_u64_add_u64_div_u64(u64 rax, u64 mul, u64 add, u64 div)
>  
>  	if (!statically_true(!add))
>  		asm ("addq %[add], %[lo]; adcq $0, %[hi]" :
> -			[lo] "+r" (rax), [hi] "+r" (rdx) : [add] "irm" (add));
> +			[lo] "+r" (rax), [hi] "+r" (rdx) : [add] "erm" (add));
>  
>  	asm ("divq %[div]" : "+a" (rax), "+d" (rdx) : [div] "rm" (div));
>  

[PATCH 2/2] x86/local: local_add/local_sub: Support large immediate values

[David Laight]

Replace the "ir" constraint with "er" so that constants over 2^31
get loaded into a register.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
---
 arch/x86/include/asm/local.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
index 4957018fef3e..68af7b74450a 100644
--- a/arch/x86/include/asm/local.h
+++ b/arch/x86/include/asm/local.h
@@ -32,14 +32,14 @@ static inline void local_add(long i, local_t *l)
 {
 	asm volatile(_ASM_ADD "%1,%0"
 		     : "+m" (l->a.counter)
-		     : "ir" (i));
+		     : "er" (i));
 }
 
 static inline void local_sub(long i, local_t *l)
 {
 	asm volatile(_ASM_SUB "%1,%0"
 		     : "+m" (l->a.counter)
-		     : "ir" (i));
+		     : "er" (i));
 }
 
 /**
-- 
2.39.5

Re: [PATCH 2/2] x86/local: local_add/local_sub: Support large immediate values

[Peter Zijlstra]

On Mon, Aug 03, 2026 at 10:47:02AM +0100, David Laight wrote:
> Replace the "ir" constraint with "er" so that constants over 2^31
> get loaded into a register.
> 
> Signed-off-by: David Laight <david.laight.linux@gmail.com>
> ---
>  arch/x86/include/asm/local.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
> index 4957018fef3e..68af7b74450a 100644
> --- a/arch/x86/include/asm/local.h
> +++ b/arch/x86/include/asm/local.h
> @@ -32,14 +32,14 @@ static inline void local_add(long i, local_t *l)
>  {
>  	asm volatile(_ASM_ADD "%1,%0"
>  		     : "+m" (l->a.counter)
> -		     : "ir" (i));
> +		     : "er" (i));
>  }
>  
>  static inline void local_sub(long i, local_t *l)
>  {
>  	asm volatile(_ASM_SUB "%1,%0"
>  		     : "+m" (l->a.counter)
> -		     : "ir" (i));
> +		     : "er" (i));
>  }

And the other two sites already have "er", how inconsistent :/

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Re: [PATCH 2/2] x86/local: local_add/local_sub: Support large immediate values

[H. Peter Anvin]

On 2026-08-03 02:47, David Laight wrote:
> Replace the "ir" constraint with "er" so that constants over 2^31
> get loaded into a register.
> 
> Signed-off-by: David Laight <david.laight.linux@gmail.com>

Yep, that's right.

Reviewed-by: H. Peter Anvin <hpa@zytor.com>

> ---
>  arch/x86/include/asm/local.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
> index 4957018fef3e..68af7b74450a 100644
> --- a/arch/x86/include/asm/local.h
> +++ b/arch/x86/include/asm/local.h
> @@ -32,14 +32,14 @@ static inline void local_add(long i, local_t *l)
>  {
>  	asm volatile(_ASM_ADD "%1,%0"
>  		     : "+m" (l->a.counter)
> -		     : "ir" (i));
> +		     : "er" (i));
>  }
>  
>  static inline void local_sub(long i, local_t *l)
>  {
>  	asm volatile(_ASM_SUB "%1,%0"
>  		     : "+m" (l->a.counter)
> -		     : "ir" (i));
> +		     : "er" (i));
>  }
>  
>  /**

Re: [PATCH 0/2] x86: Use "er" asm constriant for add/sub

[Peter Zijlstra]

On Mon, Aug 03, 2026 at 10:47:00AM +0100, David Laight wrote:
> The x86 instruction set only supports 32bit signed immediate values
> for add/sub.
> Replace the "ir" constraint with "er" to avoid build errors.
> 
> Found by a patch that added used mul_u64_add_u64_div_u64() to
> do a rounding divide by 2^32 (perhaps not the best way to do this).
> 
> A quick grep only found one other affected file.

I suppose the percpu stuff works because of __pcpu_cast_4 truncating the
value to u32 and then ignoring the whole signed/unsigned business. And
__pcpu_reg_imm_8 has "re".

Re: [PATCH 0/2] x86: Use "er" asm constriant for add/sub

[Nicolas Pitre]

On Mon, 3 Aug 2026, David Laight wrote:

> The x86 instruction set only supports 32bit signed immediate values
> for add/sub.
> Replace the "ir" constraint with "er" to avoid build errors.
> 
> Found by a patch that added used mul_u64_add_u64_div_u64() to
> do a rounding divide by 2^32 (perhaps not the best way to do this).
> 
> A quick grep only found one other affected file.

Would be a good idea adding those cases to the test module.


Nicolas

Re: [PATCH 0/2] x86: Use "er" asm constriant for add/sub

[David Laight]

On Mon, 3 Aug 2026 10:49:40 -0400 (EDT)
Nicolas Pitre <nico@fluxnic.net> wrote:

> On Mon, 3 Aug 2026, David Laight wrote:
> 
> > The x86 instruction set only supports 32bit signed immediate values
> > for add/sub.
> > Replace the "ir" constraint with "er" to avoid build errors.
> > 
> > Found by a patch that added used mul_u64_add_u64_div_u64() to
> > do a rounding divide by 2^32 (perhaps not the best way to do this).
> > 
> > A quick grep only found one other affected file.  
> 
> Would be a good idea adding those cases to the test module.

They fail to compile so it isn't a big deal.

	David

> 
> 
> Nicolas

Re: [PATCH 0/2] x86: Use "er" asm constriant for add/sub

[Jonathan Cameron]

On Mon, 3 Aug 2026 16:39:55 +0100
David Laight <david.laight.linux@gmail.com> wrote:

> On Mon, 3 Aug 2026 10:49:40 -0400 (EDT)
> Nicolas Pitre <nico@fluxnic.net> wrote:
> 
> > On Mon, 3 Aug 2026, David Laight wrote:
> >   
> > > The x86 instruction set only supports 32bit signed immediate values
> > > for add/sub.
> > > Replace the "ir" constraint with "er" to avoid build errors.
> > > 
> > > Found by a patch that added used mul_u64_add_u64_div_u64() to
> > > do a rounding divide by 2^32 (perhaps not the best way to do this).
> > > 
> > > A quick grep only found one other affected file.    
> > 
> > Would be a good idea adding those cases to the test module.  
> 
> They fail to compile so it isn't a big deal.
> 
David, thanks for sorting this (and all for reviewing)

Seems like everyone is happy.  What route is this taking upstream?

I held Rodrigo's driver to avoid the build breakage and would like to
get it queued up early in next cycle.  So not that urgent but nice
to be able to tick it off the list!

Thanks,

Jonathan

> 	David
> 
> > 
> > 
> > Nicolas  
> 

Re: [PATCH 0/2] x86: Use "er" asm constriant for add/sub

[Jonathan Cameron]

On Sun, 23 Aug 2026 23:35:36 +0100
Jonathan Cameron <jonathan.cameron@oss.qualcomm.com> wrote:

> On Mon, 3 Aug 2026 16:39:55 +0100
> David Laight <david.laight.linux@gmail.com> wrote:
> 
> > On Mon, 3 Aug 2026 10:49:40 -0400 (EDT)
> > Nicolas Pitre <nico@fluxnic.net> wrote:
> >   
> > > On Mon, 3 Aug 2026, David Laight wrote:
> > >     
> > > > The x86 instruction set only supports 32bit signed immediate values
> > > > for add/sub.
> > > > Replace the "ir" constraint with "er" to avoid build errors.
> > > > 
> > > > Found by a patch that added used mul_u64_add_u64_div_u64() to
> > > > do a rounding divide by 2^32 (perhaps not the best way to do this).
> > > > 
> > > > A quick grep only found one other affected file.      
> > > 
> > > Would be a good idea adding those cases to the test module.    
> > 
> > They fail to compile so it isn't a big deal.
> >   
> David, thanks for sorting this (and all for reviewing)
> 
> Seems like everyone is happy.  What route is this taking upstream?
> 
> I held Rodrigo's driver to avoid the build breakage and would like to
> get it queued up early in next cycle.  So not that urgent but nice
> to be able to tick it off the list!
> 

Given I have a driver queued up behind this, shall I just take it through
the drivers/iio tree?

I can do an immutable branch just in case anyone else needs it this cycle.

Jonathan


> Thanks,
> 
> Jonathan
> 
> > 	David
> >   
> > > 
> > > 
> > > Nicolas    
> >   
>