From: George Hu <integral@archlinux.org>
To: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>
Cc: Andrii Nakryiko <andrii@kernel.org>,
Eduard Zingerman <eddyz87@gmail.com>,
Kumar Kartikeya Dwivedi <memxor@gmail.com>,
Martin KaFai Lau <martin.lau@linux.dev>,
Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
Jiri Olsa <jolsa@kernel.org>,
Emil Tsalapatis <emil@etsalapatis.com>,
Ihor Solodrai <ihor.solodrai@linux.dev>,
Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H . Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, bpf@vger.kernel.org,
linux-kernel@vger.kernel.org, George Hu <integral@archlinux.org>
Subject: [PATCH] bpf, x86: use 32-bit zero idiom for shorter encoding
Date: Fri, 25 Sep 2026 00:01:54 +0800 [thread overview]
Message-ID: <20260924160154.12807-1-integral@archlinux.org> (raw)
Emit "xor eax, eax" instead of "xor rax, rax".
In 64-bit mode, zeroing EAX implicitly clears RAX, and the 32-bit
form encodes one byte smaller while keeping identical semantics.
Signed-off-by: George Hu <integral@archlinux.org>
---
arch/x86/net/bpf_jit_comp.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 9fbef7504e51..2e3c8e9861a4 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -579,8 +579,11 @@ static void emit_prologue(u8 **pprog, u8 *ip, u32 stack_depth, bool ebpf_from_cb
if (tail_call_reachable && !is_subprog)
/* When it's the entry of the whole tailcall context,
* zeroing rax means initialising tail_call_cnt.
+ *
+ * xor eax, eax
+ * equivalent to 'xor rax, rax', but one byte less
*/
- EMIT3(0x48, 0x31, 0xC0); /* xor rax, rax */
+ EMIT2(0x31, 0xC0);
else
/* Keep the same instruction layout. */
emit_nops(&prog, 3); /* nop3 */
--
2.55.0
next reply other threads:[~2026-09-24 16:02 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-24 16:01 George Hu [this message]
2026-09-24 16:23 ` Alexei Starovoitov
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