bpf, x86: detect/optimize loading 0 immediates

When sometimes structs or variables need to be initialized/'memset' to 0 in
an eBPF C program, the x86 BPF JIT converts this to use immediates. We can
however save a couple of bytes (f.e. even up to 7 bytes on a single emmission
of BPF_LD | BPF_IMM | BPF_DW) in the image by detecting such case and use xor
on the dst register instead.

Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>

arch/x86/net/bpf_jit_comp.c

@@ -459,6 +459,18 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
  }
 
  case BPF_ALU | BPF_MOV | BPF_K:
+ /* optimization: if imm32 is zero, use 'xor <dst>,<dst>'
+ * to save 3 bytes.
+ */
+ if (imm32 == 0) {
+ if (is_ereg(dst_reg))
+ EMIT1(add_2mod(0x40, dst_reg, dst_reg));
+ b2 = 0x31; /* xor */
+ b3 = 0xC0;
+ EMIT2(b2, add_2reg(b3, dst_reg, dst_reg));
+ break;
+ }
+
  /* mov %eax, imm32 */
  if (is_ereg(dst_reg))
  EMIT1(add_1mod(0x40, dst_reg));
@@ -473,6 +485,20 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
  return -EINVAL;
  }
 
+ /* optimization: if imm64 is zero, use 'xor <dst>,<dst>'
+ * to save 7 bytes.
+ */
+ if (insn[0].imm == 0 && insn[1].imm == 0) {
+ b1 = add_2mod(0x48, dst_reg, dst_reg);
+ b2 = 0x31; /* xor */
+ b3 = 0xC0;
+ EMIT3(b1, b2, add_2reg(b3, dst_reg, dst_reg));
+
+ insn++;
+ i++;
+ break;
+ }
+
  /* movabsq %rax, imm64 */
  EMIT2(add_1mod(0x48, dst_reg), add_1reg(0xB8, dst_reg));
  EMIT(insn[0].imm, 4);