Re: [PATCH v2 next 2/4] lib: mul_u64_u64_div_u64() Use BUG_ON() for divide by zero

[David Laight <david.laight.linux@gmail.com>]

On Mon, 19 May 2025 08:10:50 +0200
Uwe Kleine-König <u.kleine-koenig@baylibre.com> wrote:

> On Sun, May 18, 2025 at 02:38:46PM +0100, David Laight wrote:
> > Do an explicit BUG_ON(!divisor) instead of hoping the 'undefined
> > behaviour' the compiler generated for a compile-time 1/0 is in any
> > way useful.
> > 
> > It may be better to define the function to return ~(u64)0 for
> > divide by zero.
> > 
> > Signed-off-by: David Laight <david.laight.linux@gmail.com>
> > ---
> > 
> > A new change for v2 of the patchset.
> > Whereas gcc inserts (IIRC) 'ud2' clang is likely to let the code
> > continue and generate 'random' results for any 'undefined bahaviour'.
> > 
> >  lib/math/div64.c | 10 +++-------
> >  1 file changed, 3 insertions(+), 7 deletions(-)
> > 
> > diff --git a/lib/math/div64.c b/lib/math/div64.c
> > index a5c966a36836..c426fa0660bc 100644
> > --- a/lib/math/div64.c
> > +++ b/lib/math/div64.c
> > @@ -186,6 +186,9 @@ EXPORT_SYMBOL(iter_div_u64_rem);
> >  #ifndef mul_u64_u64_div_u64
> >  u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
> >  {
> > +	/* Trigger exception if divisor is zero */
> > +	BUG_ON(!d);
> > +  
> 
> I'm unsure if I should like the BUG_ON better than return 1/0. My gut
> feeling is that mul_u64_u64_div_u64() should behave in the same way as
> e.g. div64_u64 (which is just `return dividend / divisor;` for 64bit
> archs and thus triggers the same exception as `return 1/0;`.

You need to execute a run-time 1/0 not a compile-time one.
clang is likely to decide it is 'undefined behaviour' and just not
generate any code at all - including removing the 'if (!d)' condition.

For x86 gcc does (sometimes at least) generate 'if (!d) asm("ud2")'
but BUG_ON() adds a table entry for the fault site.

> If BUG_ON should be it, I'd prefer
> 
> 	BUG_ON(unlikely(d == 0));
> 
> which keeps the unlikely() that is already in the check removed below
> and is more explicit that checking for !d.

IIRC there is an 'unlikely' inside BUG_ON() - so the call site doesn't
need one.

	David

> >  	if (ilog2(a) + ilog2(b) <= 62)
> >  		return div64_u64(a * b, d);
> >  
> > @@ -212,13 +215,6 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
> >  
> >  #endif
> >  
> > -	/* make sure d is not zero, trigger exception otherwise */
> > -#pragma GCC diagnostic push
> > -#pragma GCC diagnostic ignored "-Wdiv-by-zero"
> > -	if (unlikely(d == 0))
> > -		return 1/0;
> > -#pragma GCC diagnostic pop
> > -
> >  	int shift = __builtin_ctzll(d);
> >  
> >  	/* try reducing the fraction in case the dividend becomes <= 64 bits */  
> 
> Best regards
> Uwe