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bpf, x64: save one byte per shl/shr/sar when imm is 1 

When we shift by one, we can use a different encoding where imm
is not explicitly needed, which saves 1 byte per such op.

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 01:07:58 +01:00 committed by Alexei Starovoitov
parent ee07862f7b
commit 88e69a1fcc
1 changed files with 5 additions and 1 deletions
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6
arch/x86/net/bpf_jit_comp.c
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@ -640,7 +640,11 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
case BPF_RSH: b3 = 0xE8; break;
case BPF_ARSH: b3 = 0xF8; break;
}
EMIT3(0xC1, add_1reg(b3, dst_reg), imm32);
if (imm32 == 1)
EMIT2(0xD1, add_1reg(b3, dst_reg));
else
EMIT3(0xC1, add_1reg(b3, dst_reg), imm32);
break;
case BPF_ALU | BPF_LSH | BPF_X: