Re: [PATCH v3 0/2] x86/math64: handle #DE in mul_u64_u64_div_u64()

[David Laight]

On Fri, 15 Aug 2025 18:40:09 +0200
Oleg Nesterov <oleg@xxxxxxxxxx> wrote:

One of my 'idea patches' is to make mul_u64_u64_div_u64() a wrapper for
another function that takes in extra 'int *overflowed' parameter that is
set zero/non-zero for success/overflow.
The 'overflowed' parameter can either be a compile-time NULL or a
valid pointer.

So the x86-x64 asm implementation would use different code - you need
the 'jump around fail label' to write the ~0 return value to *overflowed.
The extra pointer check in the C version normal path may not be worth
worrying about (but the '*overflow = 0' could easily be inlined).

The typical use would be:
	quotient = mul_u64_u64_div_u64_overflow(..., &overflowed);
	if (quotient == ~0ull && overflowed)
		...
That will generate better code than returning 'overflowed' and the
quotient by reference.

Although I wonder how often ~0ull is a valid result?

	David