Re: [PATCH liburing v1 2/2] setup: Mark the exported functions as __cold

[Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>]

On Sun, Jul 3, 2022 at 6:59 PM Ammar Faizi wrote:
>
> From: Ammar Faizi <ammarfaizi2@gnuweeb.org>
>
> These functions are called at initialization, which are slow-paths.
> Mark them as __cold so that the compiler will optimize for code size.
>
> Here is the result compiling with Ubuntu clang
> 15.0.0-++20220601012204+ec2711b35411-1~exp1~20220601012300.510
>
> Without this patch:
>
>   $ wc -c src/liburing.so.2.3
>   71288 src/liburing.so.2.3
>
> With this patch:
>
>   $ wc -c src/liburing.so.2.3
>   69448 src/liburing.so.2.3
>
> Take one slow-path function example, using __cold avoids aggresive
> inlining.
>
> Without this patch:
>
>   00000000000024f0 <io_uring_queue_init>:
>     24f0: pushq  %r14
>     24f2: pushq  %rbx
>     24f3: subq   $0x78,%rsp
>     24f7: movq   %rsi,%r14
>     24fa: xorps  %xmm0,%xmm0
>     24fd: movaps %xmm0,(%rsp)
>     2501: movaps %xmm0,0x60(%rsp)
>     2506: movaps %xmm0,0x50(%rsp)
>     250b: movaps %xmm0,0x40(%rsp)
>     2510: movaps %xmm0,0x30(%rsp)
>     2515: movaps %xmm0,0x20(%rsp)
>     251a: movaps %xmm0,0x10(%rsp)
>     251f: movq   $0x0,0x70(%rsp)
>     2528: movl   %edx,0x8(%rsp)
>     252c: movq   %rsp,%rsi
>     252f: movl   $0x1a9,%eax
>     2534: syscall
>     2536: movq   %rax,%rbx
>     2539: testl  %ebx,%ebx
>     253b: js     256a <io_uring_queue_init+0x7a>
>     253d: movq   %rsp,%rsi
>     2540: movl   %ebx,%edi
>     2542: movq   %r14,%rdx
>     2545: callq  2080 <io_uring_queue_mmap@plt>
>     254a: testl  %eax,%eax
>     254c: je     255d <io_uring_queue_init+0x6d>
>     254e: movl   %eax,%edx
>     2550: movl   $0x3,%eax
>     2555: movl   %ebx,%edi
>     2557: syscall
>     2559: movl   %edx,%ebx
>     255b: jmp    256a <io_uring_queue_init+0x7a>
>     255d: movl   0x14(%rsp),%eax
>     2561: movl   %eax,0xc8(%r14)
>     2568: xorl   %ebx,%ebx
>     256a: movl   %ebx,%eax
>     256c: addq   $0x78,%rsp
>     2570: popq   %rbx
>     2571: popq   %r14
>     2573: retq
>
> With this patch:
>
>   000000000000240c <io_uring_queue_init>:
>     240c: subq   $0x78,%rsp
>     2410: xorps  %xmm0,%xmm0
>     2413: movq   %rsp,%rax
>     2416: movaps %xmm0,(%rax)
>     2419: movaps %xmm0,0x60(%rax)
>     241d: movaps %xmm0,0x50(%rax)
>     2421: movaps %xmm0,0x40(%rax)
>     2425: movaps %xmm0,0x30(%rax)
>     2429: movaps %xmm0,0x20(%rax)
>     242d: movaps %xmm0,0x10(%rax)
>     2431: movq   $0x0,0x70(%rax)
>     2439: movl   %edx,0x8(%rax)
>     243c: movq   %rax,%rdx
>     243f: callq  2090 <io_uring_queue_init_params@plt>
>     2444: addq   $0x78,%rsp
>     2448: retq
>
> Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>

Reviewed-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>

tq

-- Viro