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Date: Fri, 12 Jan 2024 17:24:18 -0800
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Subject: Re: [PATCH bpf-next] bpf: Minor improvements for bpf_cmp.
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To: Alexei Starovoitov <alexei.starovoitov@gmail.com>, bpf@vger.kernel.org
Cc: daniel@iogearbox.net, andrii@kernel.org, martin.lau@kernel.org,
 kernel-team@fb.com
References: <20240112220134.71209-1-alexei.starovoitov@gmail.com>
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From: Yonghong Song <yonghong.song@linux.dev>
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On 1/12/24 2:01 PM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
>
> Few minor improvements for bpf_cmp() macro:
> . reduce number of args in __bpf_cmp()
> . rename NOFLIP to UNLIKELY
> . add a comment about 64-bit truncation in "i" constraint
> . use "ri" constraint for sizeof(rhs) <= 4
> . improve error message for bpf_cmp_likely()
>
> Before:
> progs/iters_task_vma.c:31:7: error: variable 'ret' is uninitialized when used here [-Werror,-Wuninitialized]
>     31 |                 if (bpf_cmp_likely(seen, <==, 1000))
>        |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ../bpf/bpf_experimental.h:325:3: note: expanded from macro 'bpf_cmp_likely'
>    325 |                 ret;
>        |                 ^~~
> progs/iters_task_vma.c:31:7: note: variable 'ret' is declared here
> ../bpf/bpf_experimental.h:310:3: note: expanded from macro 'bpf_cmp_likely'
>    310 |                 bool ret;
>        |                 ^
>
> After:
> progs/iters_task_vma.c:31:7: error: invalid operand for instruction
>     31 |                 if (bpf_cmp_likely(seen, <==, 1000))
>        |                     ^
> ../bpf/bpf_experimental.h:324:17: note: expanded from macro 'bpf_cmp_likely'
>    324 |                         asm volatile("r0 " #OP " invalid compare");
>        |                                      ^
> <inline asm>:1:5: note: instantiated into assembly here
>      1 |         r0 <== invalid compare
>        |            ^
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Acked-by: Yonghong Song <yonghong.song@linux.dev>