Commit 38f51c07 authored Oct 03, 2019 by Daniel Borkmann Committed by Alexei Starovoitov Oct 04, 2019

bpf, x86: Small optimization in comparing against imm0



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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>

parent c5881463

arch/x86/net/bpf_jit_comp.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -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));

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