Re: [PATCH v4 next 3/9] lib: mul_u64_u64_div_u64() simplify check for a 64bit product

[Nicolas Pitre]

On Fri, 31 Oct 2025, David Laight wrote:

> On Fri, 31 Oct 2025 13:26:41 -0400 (EDT)
> Nicolas Pitre <npitre@xxxxxxxxxxxx> wrote:
> 
> > On Fri, 31 Oct 2025, David Laight wrote:
> > 
> > > On Wed, 29 Oct 2025 14:11:08 -0400 (EDT)
> > > Nicolas Pitre <npitre@xxxxxxxxxxxx> wrote:
> > >   
> > > > On Wed, 29 Oct 2025, David Laight wrote:
> > > >   
> > > > > If the product is only 64bits div64_u64() can be used for the divide.
> > > > > Replace the pre-multiply check (ilog2(a) + ilog2(b) <= 62) with a
> > > > > simple post-multiply check that the high 64bits are zero.
> > > > > 
> > > > > This has the advantage of being simpler, more accurate and less code.
> > > > > It will always be faster when the product is larger than 64bits.
> > > > > 
> > > > > Most 64bit cpu have a native 64x64=128 bit multiply, this is needed
> > > > > (for the low 64bits) even when div64_u64() is called - so the early
> > > > > check gains nothing and is just extra code.
> > > > > 
> > > > > 32bit cpu will need a compare (etc) to generate the 64bit ilog2()
> > > > > from two 32bit bit scans - so that is non-trivial.
> > > > > (Never mind the mess of x86's 'bsr' and any oddball cpu without
> > > > > fast bit-scan instructions.)
> > > > > Whereas the additional instructions for the 128bit multiply result
> > > > > are pretty much one multiply and two adds (typically the 'adc $0,%reg'
> > > > > can be run in parallel with the instruction that follows).
> > > > > 
> > > > > The only outliers are 64bit systems without 128bit mutiply and
> > > > > simple in order 32bit ones with fast bit scan but needing extra
> > > > > instructions to get the high bits of the multiply result.
> > > > > I doubt it makes much difference to either, the latter is definitely
> > > > > not mainstream.
> > > > > 
> > > > > If anyone is worried about the analysis they can look at the
> > > > > generated code for x86 (especially when cmov isn't used).
> > > > > 
> > > > > Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>    
> > > > 
> > > > Comment below.
> > > > 
> > > >   
> > > > > ---
> > > > > 
> > > > > Split from patch 3 for v2, unchanged since.
> > > > > 
> > > > >  lib/math/div64.c | 6 +++---
> > > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > > > 
> > > > > diff --git a/lib/math/div64.c b/lib/math/div64.c
> > > > > index 1092f41e878e..7158d141b6e9 100644
> > > > > --- a/lib/math/div64.c
> > > > > +++ b/lib/math/div64.c
> > > > > @@ -186,9 +186,6 @@ EXPORT_SYMBOL(iter_div_u64_rem);
> > > > >  #ifndef mul_u64_u64_div_u64
> > > > >  u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
> > > > >  {
> > > > > -	if (ilog2(a) + ilog2(b) <= 62)
> > > > > -		return div64_u64(a * b, d);
> > > > > -
> > > > >  #if defined(__SIZEOF_INT128__)
> > > > >  
> > > > >  	/* native 64x64=128 bits multiplication */
> > > > > @@ -224,6 +221,9 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
> > > > >  		return ~0ULL;
> > > > >  	}
> > > > >  
> > > > > +	if (!n_hi)
> > > > > +		return div64_u64(n_lo, d);    
> > > > 
> > > > I'd move this before the overflow test. If this is to be taken then 
> > > > you'll save one test. same cost otherwise.
> > > >   
> > > 
> > > I wanted the 'divide by zero' result to be consistent.  
> > 
> > It is. div64_u64(x, 0) will produce the same result/behavior.
> 
> Are you sure, for all architectures?

At least all the ones I'm familiar with.

> > 
> > > Additionally the change to stop the x86-64 version panicking on
> > > overflow also makes it return ~0 for divide by zero.
> > > If that is done then this version needs to be consistent and
> > > return ~0 for divide by zero - which div64_u64() won't do.  
> > 
> > Well here I disagree. If that is some x86 peculiarity then x86 should 
> > deal with it and not impose it on everybody. At least having most other 
> > architectures raising SIGFPE when encountering a divide by 0 should 
> > provide enough coverage to have such obviously buggy code fixed.
> 
> The issue here is that crashing the kernel isn't really acceptable.

Encountering a div-by-0 _will_ crash the kernel (or at least kill the 
current task) with most CPUs. They do raise an exception already with 
the other division types. This is no different.

> An extra parameter could be added to return the 'status',
> but that makes the calling interface horrid.

No please.

> So returning ~0 on overflow and divide-by-zero makes it possible
> for the caller to check for errors.

The caller should check for a possible zero divisor _before_ performing 
a division not after. Relying on the div-by_0 CPU behavior is a bug.

> Ok, you lose ~0 as a valid result - but that is very unlikely to
> need to be treated differently to 'overflow'.

I disagree. You need to check for a zero divisor up front and not rely 
on the division to tell you about it.  This is true whether you do
a = b/c; a = div64_u64(b, c); or a = mul_u64_u64_div_u64(a, b, c);.
Most architectures will simply raise an exception if you attempt a div 
by 0, some will return a plain 0. You can't rely on that.

But you need to perform the mul+div before you know there is an 
overflow. Maybe the handling of those cases is the same for the caller 
but this is certainly not universal.


Nicolas