bpf, x64: save several bytes when mul dest is r0/r3 anyway (d806a0cf) · Commits · 戴 / test

Instead of unconditionally performing push/pop on rax/rdx in case of multiplication, we can save a few bytes in case of dest register being either BPF r0 (rax) or r3 (rdx) since the result is written in there anyway. Signed-off-by:

Daniel Borkmann <daniel@iogearbox.net> Signed-off-by:

Alexei Starovoitov <ast@kernel.org>

arch/x86/net/bpf_jit_comp.c

+11 −10

Original line number	Diff line number	Diff line
		@@ -615,7 +615,9 @@ static int do_jit(struct bpf_prog bpf_prog, int addrs, u8 *image,
		case BPF_ALU \| BPF_MUL \| BPF_X:
		case BPF_ALU64 \| BPF_MUL \| BPF_K:
		case BPF_ALU64 \| BPF_MUL \| BPF_X:
		if (dst_reg != BPF_REG_0)
		EMIT1(0x50); /* push rax */
		if (dst_reg != BPF_REG_3)
		EMIT1(0x52); /* push rdx */

		/* mov r11, dst_reg */
		@@ -636,14 +638,13 @@ static int do_jit(struct bpf_prog bpf_prog, int addrs, u8 *image,
		/* mul(q) r11 */
		EMIT2(0xF7, add_1reg(0xE0, AUX_REG));

		/* mov r11, rax */
		EMIT_mov(AUX_REG, BPF_REG_0);

		if (dst_reg != BPF_REG_3)
		EMIT1(0x5A); /* pop rdx */
		if (dst_reg != BPF_REG_0) {
		/* mov dst_reg, rax */
		EMIT_mov(dst_reg, BPF_REG_0);
		EMIT1(0x58); /* pop rax */

		/* mov dst_reg, r11 */
		EMIT_mov(dst_reg, AUX_REG);
		}
		break;

		/* shifts */

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