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 <dan...@iogearbox.net>
Acked-by: Alexei Starovoitov <a...@kernel.org>
---
arch/x86/net/bpf_jit_comp.c | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index c080e81..4286f36 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/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);
--
1.9.3
--
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