bpf, x86: detect/optimize loading 0 immediates (606c88a8) · Commits · 戴 / test

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 <daniel@iogearbox.net> Acked-by:

Alexei Starovoitov <ast@kernel.org> Signed-off-by:

David S. Miller <davem@davemloft.net>

arch/x86/net/bpf_jit_comp.c

+26 −0

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

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