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>

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));