On Mon, 19 Jan 2026 02:36:18 +0800
kernel test robot [off-list ref] wrote:

Hi Eric,

...

vim +/__arch_xprod_64 +138 include/asm-generic/div64.h

461a5e51060c93 Nicolas Pitre 2015-10-30  125  
f682b27c57aec2 Nicolas Pitre 2015-10-30  126  #ifndef __arch_xprod_64
f682b27c57aec2 Nicolas Pitre 2015-10-30  127  /*
f682b27c57aec2 Nicolas Pitre 2015-10-30  128   * Default C implementation for __arch_xprod_64()
f682b27c57aec2 Nicolas Pitre 2015-10-30  129   *
f682b27c57aec2 Nicolas Pitre 2015-10-30  130   * Prototype: uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
f682b27c57aec2 Nicolas Pitre 2015-10-30  131   * Semantic:  retval = ((bias ? m : 0) + m * n) >> 64
f682b27c57aec2 Nicolas Pitre 2015-10-30  132   *
f682b27c57aec2 Nicolas Pitre 2015-10-30  133   * The product is a 128-bit value, scaled down to 64 bits.
00a31dd3acea0f Nicolas Pitre 2024-10-03  134   * Hoping for compile-time optimization of  conditional code.
f682b27c57aec2 Nicolas Pitre 2015-10-30  135   * Architectures may provide their own optimized assembly implementation.
f682b27c57aec2 Nicolas Pitre 2015-10-30  136   */
5f712d70e20a46 Eric Dumazet  2026-01-18  137  static inline_for_performance
d533cb2d2af400 Nicolas Pitre 2024-10-03 @138  uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
f682b27c57aec2 Nicolas Pitre 2015-10-30  139  {
f682b27c57aec2 Nicolas Pitre 2015-10-30  140  	uint32_t m_lo = m;
f682b27c57aec2 Nicolas Pitre 2015-10-30  141  	uint32_t m_hi = m >> 32;
f682b27c57aec2 Nicolas Pitre 2015-10-30  142  	uint32_t n_lo = n;
f682b27c57aec2 Nicolas Pitre 2015-10-30  143  	uint32_t n_hi = n >> 32;
00a31dd3acea0f Nicolas Pitre 2024-10-03  144  	uint64_t x, y;
f682b27c57aec2 Nicolas Pitre 2015-10-30  145  
00a31dd3acea0f Nicolas Pitre 2024-10-03  146  	/* Determine if overflow handling can be dispensed with. */
00a31dd3acea0f Nicolas Pitre 2024-10-03  147  	bool no_ovf = __builtin_constant_p(m) &&
00a31dd3acea0f Nicolas Pitre 2024-10-03  148  		      ((m >> 32) + (m & 0xffffffff) < 0x100000000);

Can that ever have got compiled?
Won't the compiler complain about 0x100000000 being out of range?
Lots of alternatives...

If u128 exists this should probably just be:
	return ((u128)m * n + (bias ? m : 0)) >> 64;

Which is probably the only alternative an architecture might provide (none do AFAICT).

	David