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Miller" , Alexander Potapenko , Andrew Morton , Brendan Jackman , Christoph Lameter , Dennis Zhou , Dmitry Vyukov , dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, iommu@lists.linux.dev, io-uring@vger.kernel.org, Jason Gunthorpe , Jens Axboe , Johannes Weiner , John Hubbard , kasan-dev@googlegroups.com, kvm@vger.kernel.org, "Liam R. Howlett" , linux-arm-kernel@axis.com, linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, linux-ide@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mips@vger.kernel.org, linux-mmc@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, Lorenzo Stoakes , Marco Elver , Marek Szyprowski , Michal Hocko , Mike Rapoport , Muchun Song , netdev@vger.kernel.org, Oscar Salvador , Peter Xu , Robin Murphy , Suren Baghdasaryan , Tejun Heo , virtualization@lists.linux.dev, Vlastimil Babka , wireguard@lists.zx2c4.com, x86@kernel.org, Zi Yan Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, Aug 21, 2025 at 4:29=E2=80=AFPM David Hildenbrand wrote: > > Because doing a 64-bit shift on x86-32 is like three cycles. Doing a > > 64-bit signed division by a simple constant is something like ten > > strange instructions even if the end result is only 32-bit. > > I would have thought that the compiler is smart enough to optimize that? > PAGE_SIZE is a constant. Oh, the compiler optimizes things. But dividing a 64-bit signed value with a constant is still quite complicated. It doesn't generate a 'div' instruction, but it generates something like th= is: movl %ebx, %edx sarl $31, %edx movl %edx, %eax xorl %edx, %edx andl $4095, %eax addl %ecx, %eax adcl %ebx, %edx and that's certainly a lot faster than an actual 64-bit divide would be. An unsigned divide - or a shift - results in just shrdl $12, %ecx, %eax which is still not the fastest instruction (I think shrld gets split into two uops), but it's certainly simpler and easier to read. Linus