[PATCH bpf-next 3/6] bpf, x64: save several bytes when mul dest is r0/r3 anyway

[Daniel Borkmann]

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 <dan...@iogearbox.net>
---
 arch/x86/net/bpf_jit_comp.c | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index f3e5cd8..9895ca3 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -615,8 +615,10 @@ 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:
-                       EMIT1(0x50); /* push rax */
-                       EMIT1(0x52); /* push rdx */
+                       if (dst_reg != BPF_REG_0)
+                               EMIT1(0x50); /* push rax */
+                       if (dst_reg != BPF_REG_3)
+                               EMIT1(0x52); /* push rdx */
 
                        /* mov r11, dst_reg */
                        EMIT_mov(AUX_REG, 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);
-
-                       EMIT1(0x5A); /* pop rdx */
-                       EMIT1(0x58); /* pop rax */
-
-                       /* mov dst_reg, r11 */
-                       EMIT_mov(dst_reg, AUX_REG);
+                       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 */
+                       }
                        break;
 
                        /* shifts */
-- 
2.9.5