From: Daniel Borkmann <daniel@iogearbox.net>
To: ast@kernel.org
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org,
Daniel Borkmann <daniel@iogearbox.net>
Subject: [PATCH bpf-next 1/2] bpf, x86: Small optimization in comparing against imm0
Date: Thu, 3 Oct 2019 01:45:11 +0200 [thread overview]
Message-ID: <20191002234512.25902-1-daniel@iogearbox.net> (raw)
Replace 'cmp reg, 0' with 'test reg, reg' for comparisons against
zero. Saves 1 byte of instruction encoding per occurrence. The flag
results of test 'reg, reg' are identical to 'cmp reg, 0' in all
cases except for AF which we don't use/care about. In terms of
macro-fusibility in combination with a subsequent conditional jump
instruction, both have the same properties for the jumps used in
the JIT translation. For example, same JITed Cilium program can
shrink a bit from e.g. 12,455 to 12,317 bytes as tests with 0 are
used quite frequently.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
arch/x86/net/bpf_jit_comp.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 991549a1c5f3..3ad2ba1ad855 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -909,6 +909,16 @@ xadd: if (is_imm8(insn->off))
case BPF_JMP32 | BPF_JSLT | BPF_K:
case BPF_JMP32 | BPF_JSGE | BPF_K:
case BPF_JMP32 | BPF_JSLE | BPF_K:
+ /* test dst_reg, dst_reg to save one extra byte */
+ if (imm32 == 0) {
+ if (BPF_CLASS(insn->code) == BPF_JMP)
+ EMIT1(add_2mod(0x48, dst_reg, dst_reg));
+ else if (is_ereg(dst_reg))
+ EMIT1(add_2mod(0x40, dst_reg, dst_reg));
+ EMIT2(0x85, add_2reg(0xC0, dst_reg, dst_reg));
+ goto emit_cond_jmp;
+ }
+
/* cmp dst_reg, imm8/32 */
if (BPF_CLASS(insn->code) == BPF_JMP)
EMIT1(add_1mod(0x48, dst_reg));
--
2.17.1
next reply other threads:[~2019-10-02 23:45 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-02 23:45 Daniel Borkmann [this message]
2019-10-02 23:45 ` [PATCH bpf-next 2/2] bpf: Add loop test case with 32 bit reg comparison against 0 Daniel Borkmann
2019-10-03 20:56 ` Song Liu
2019-10-03 20:52 ` [PATCH bpf-next 1/2] bpf, x86: Small optimization in comparing against imm0 Song Liu
2019-10-03 21:08 ` John Fastabend
2019-10-04 19:36 ` Alexei Starovoitov
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