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bpf, x64: save several bytes by using mov over movabsq when possible 

While analyzing some of the more complex BPF programs from Cilium,
I found that LLVM generally prefers to emit LD_IMM64 instead of MOV32
BPF instructions for loading unsigned 32-bit immediates into a
register. Given we cannot change the current/stable LLVM versions
that are already out there, lets optimize this case such that the
JIT prefers to emit 'mov %eax, imm32' over 'movabsq %rax, imm64'
whenever suitable in order to reduce the image size by 4-5 bytes per
such load in the typical case, reducing image size on some of the
bigger programs by up to 4%. emit_mov_imm32() and emit_mov_imm64()
have been added as helpers.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Daniel Borkmann 2018-02-24 01:07:59 +01:00 committed by Alexei Starovoitov
parent 88e69a1fcc
commit 6fe8b9c1f4
1 changed files with 74 additions and 51 deletions
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125
arch/x86/net/bpf_jit_comp.c
View File

@ -60,7 +60,12 @@ static bool is_imm8(int value)
static bool is_simm32(s64 value)
{
return value == (s64) (s32) value;
return value == (s64)(s32)value;
}
static bool is_uimm32(u64 value)
{
return value == (u64)(u32)value;
}
/* mov dst, src */
@ -355,6 +360,68 @@ static void emit_load_skb_data_hlen(u8 **pprog)
*pprog = prog;
}
static void emit_mov_imm32(u8 **pprog, bool sign_propagate,
u32 dst_reg, const u32 imm32)
{
u8 *prog = *pprog;
u8 b1, b2, b3;
int cnt = 0;
/* optimization: if imm32 is positive, use 'mov %eax, imm32'
* (which zero-extends imm32) to save 2 bytes.
*/
if (sign_propagate && (s32)imm32 < 0) {
/* 'mov %rax, imm32' sign extends imm32 */
b1 = add_1mod(0x48, dst_reg);
b2 = 0xC7;
b3 = 0xC0;
EMIT3_off32(b1, b2, add_1reg(b3, dst_reg), imm32);
goto done;
}
/* optimization: if imm32 is zero, use 'xor %eax, %eax'
* 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));
goto done;
}
/* mov %eax, imm32 */
if (is_ereg(dst_reg))
EMIT1(add_1mod(0x40, dst_reg));
EMIT1_off32(add_1reg(0xB8, dst_reg), imm32);
done:
*pprog = prog;
}
static void emit_mov_imm64(u8 **pprog, u32 dst_reg,
const u32 imm32_hi, const u32 imm32_lo)
{
u8 *prog = *pprog;
int cnt = 0;
if (is_uimm32(((u64)imm32_hi << 32) | (u32)imm32_lo)) {
/* For emitting plain u32, where sign bit must not be
* propagated LLVM tends to load imm64 over mov32
* directly, so save couple of bytes by just doing
* 'mov %eax, imm32' instead.
*/
emit_mov_imm32(&prog, false, dst_reg, imm32_lo);
} else {
/* movabsq %rax, imm64 */
EMIT2(add_1mod(0x48, dst_reg), add_1reg(0xB8, dst_reg));
EMIT(imm32_lo, 4);
EMIT(imm32_hi, 4);
}
*pprog = prog;
}
static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
int oldproglen, struct jit_context *ctx)
{
@ -377,7 +444,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
const s32 imm32 = insn->imm;
u32 dst_reg = insn->dst_reg;
u32 src_reg = insn->src_reg;
u8 b1 = 0, b2 = 0, b3 = 0;
u8 b2 = 0, b3 = 0;
s64 jmp_offset;
u8 jmp_cond;
bool reload_skb_data;
@ -485,58 +552,13 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
break;
case BPF_ALU64 | BPF_MOV | BPF_K:
/* optimization: if imm32 is positive,
* use 'mov eax, imm32' (which zero-extends imm32)
* to save 2 bytes
*/
if (imm32 < 0) {
/* 'mov rax, imm32' sign extends imm32 */
b1 = add_1mod(0x48, dst_reg);
b2 = 0xC7;
b3 = 0xC0;
EMIT3_off32(b1, b2, add_1reg(b3, dst_reg), imm32);
break;
}
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));
EMIT1_off32(add_1reg(0xB8, dst_reg), imm32);
emit_mov_imm32(&prog, BPF_CLASS(insn->code) == BPF_ALU64,
dst_reg, imm32);
break;
case BPF_LD | BPF_IMM | BPF_DW:
/* 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);
EMIT(insn[1].imm, 4);
emit_mov_imm64(&prog, dst_reg, insn[1].imm, insn[0].imm);
insn++;
i++;
break;
@ -604,7 +626,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
EMIT_mov(BPF_REG_0, src_reg);
else
/* mov rax, imm32 */
EMIT3_off32(0x48, 0xC7, 0xC0, imm32);
emit_mov_imm32(&prog, true,
BPF_REG_0, imm32);
if (BPF_CLASS(insn->code) == BPF_ALU64)
EMIT1(add_1mod(0x48, AUX_REG));