This patch adds an optimization that uses the asr immediate instruction
for BPF_ALU BPF_ARSH BPF_K, rather than loading the immediate to
a temporary register. This is similar to existing code for handling
BPF_ALU BPF_{LSH,RSH} BPF_K. This optimization saves two instructions
and is more consistent with LSH and RSH.
Example of the code generated for BPF_ALU32_IMM(BPF_ARSH, BPF_REG_0, 5)
before the optimization:
2c: mov r8, #5
30: mov r9, #0
34: asr r0, r0, r8
and after optimization:
2c: asr r0, r0, #5
Tested on QEMU using lib/test_bpf and test_verifier.
Co-developed-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200501020210.32294-3-luke.r.nels@gmail.com
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