On Fri, Nov 10, 2023 at 04:48:02PM +0900, Ryusuke Konishi wrote:
> On Fri, Nov 10, 2023 at 6:06 AM Matthew Wilcox (Oracle) wrote:
> > +++ b/fs/buffer.c
> > @@ -199,7 +199,7 @@ __find_get_block_slow(struct block_device *bdev, sector_t block)
> > int all_mapped = 1;
> > static DEFINE_RATELIMIT_STATE(last_warned, HZ, 1);
> >
> > - index = block >> (PAGE_SHIFT - bd_inode->i_blkbits);
> > + index = ((loff_t)block << bd_inode->i_blkbits) / PAGE_SIZE;
>
> Multiple 64-bit divisions are used in this patch, but why not use two
> stage shifts as shown below if there is no left shift overflow and the
> sign bit will not be set ?
>
> index = ((loff_t)block << bd_inode->i_blkbits) >> PAGE_SHIFT;
Here's what the compiler turns that into:
3223: 49 8b 86 80 00 00 00 mov 0x80(%r14),%rax
322a: 4c 89 ee mov %r13,%rsi
322d: ba 01 00 00 00 mov $0x1,%edx
3232: 0f b6 88 c2 00 00 00 movzbl 0xc2(%rax),%ecx
^ this is where we load i_blkbits into ecx
3239: 48 89 45 d0 mov %rax,-0x30(%rbp)
323d: 4c 8b 60 30 mov 0x30(%rax),%r12
3241: 48 d3 e6 shl %cl,%rsi
^ shift left by %cl (the bottom byte of ecx)
3244: 31 c9 xor %ecx,%ecx
3246: 48 c1 ee 0c shr $0xc,%rsi
^ shift right by 12
324a: 4c 89 e7 mov %r12,%rdi
324d: e8 00 00 00 00 call 3252 <__find_get_block+0x82>
324e: R_X86_64_PLT32 __filemap_get_folio-0x4
