讓我們嘗試使用JMH復(fù)制。
@Benchmark
@Measurement(timeUnit = TimeUnit.NANOSECONDS)
@BenchmarkMode(Mode.AverageTime)
public int first() throws IOException {
return i % 2;
}
@Benchmark
@Measurement(timeUnit = TimeUnit.NANOSECONDS)
@BenchmarkMode(Mode.AverageTime)
public int second() throws IOException {
return i & 0x1;
}
好的，它是可復(fù)制的。的first速度略慢于second。現(xiàn)在讓我們找出原因。使用以下命令運(yùn)行它-prof perfnorm：
Benchmark Mode Cnt Score Error Units
MyBenchmark.first avgt 50 2.674 ± 0.028 ns/op
MyBenchmark.first:CPI avgt 10 0.301 ± 0.002 #/op
MyBenchmark.first:L1-dcache-load-misses avgt 10 0.001 ± 0.001 #/op
MyBenchmark.first:L1-dcache-loads avgt 10 11.011 ± 0.146 #/op
MyBenchmark.first:L1-dcache-stores avgt 10 3.011 ± 0.034 #/op
MyBenchmark.first:L1-icache-load-misses avgt 10 ≈ 10?3 #/op
MyBenchmark.first:LLC-load-misses avgt 10 ≈ 10?? #/op
MyBenchmark.first:LLC-loads avgt 10 ≈ 10?? #/op
MyBenchmark.first:LLC-store-misses avgt 10 ≈ 10?? #/op
MyBenchmark.first:LLC-stores avgt 10 ≈ 10?? #/op
MyBenchmark.first:branch-misses avgt 10 ≈ 10?? #/op
MyBenchmark.first:branches avgt 10 4.006 ± 0.054 #/op
MyBenchmark.first:cycles avgt 10 9.322 ± 0.113 #/op
MyBenchmark.first:dTLB-load-misses avgt 10 ≈ 10?? #/op
MyBenchmark.first:dTLB-loads avgt 10 10.939 ± 0.175 #/op
MyBenchmark.first:dTLB-store-misses avgt 10 ≈ 10?? #/op
MyBenchmark.first:dTLB-stores avgt 10 2.991 ± 0.045 #/op
MyBenchmark.first:iTLB-load-misses avgt 10 ≈ 10?? #/op
MyBenchmark.first:iTLB-loads avgt 10 ≈ 10?? #/op
MyBenchmark.first:instructions avgt 10 30.991 ± 0.427 #/op
MyBenchmark.second avgt 50 2.263 ± 0.015 ns/op
MyBenchmark.second:CPI avgt 10 0.320 ± 0.001 #/op
MyBenchmark.second:L1-dcache-load-misses avgt 10 0.001 ± 0.001 #/op
MyBenchmark.second:L1-dcache-loads avgt 10 11.045 ± 0.152 #/op
MyBenchmark.second:L1-dcache-stores avgt 10 3.014 ± 0.032 #/op
MyBenchmark.second:L1-icache-load-misses avgt 10 ≈ 10?3 #/op
MyBenchmark.second:LLC-load-misses avgt 10 ≈ 10?? #/op
MyBenchmark.second:LLC-loads avgt 10 ≈ 10?? #/op
MyBenchmark.second:LLC-store-misses avgt 10 ≈ 10?? #/op
MyBenchmark.second:LLC-stores avgt 10 ≈ 10?? #/op
MyBenchmark.second:branch-misses avgt 10 ≈ 10?? #/op
MyBenchmark.second:branches avgt 10 4.014 ± 0.045 #/op
MyBenchmark.second:cycles avgt 10 8.024 ± 0.098 #/op
MyBenchmark.second:dTLB-load-misses avgt 10 ≈ 10?? #/op
MyBenchmark.second:dTLB-loads avgt 10 10.989 ± 0.161 #/op
MyBenchmark.second:dTLB-store-misses avgt 10 ≈ 10?? #/op
MyBenchmark.second:dTLB-stores avgt 10 3.004 ± 0.042 #/op
MyBenchmark.second:iTLB-load-misses avgt 10 ≈ 10?? #/op
MyBenchmark.second:iTLB-loads avgt 10 ≈ 10?? #/op
MyBenchmark.second:instructions avgt 10 25.076 ± 0.296 #/op
注意周期和說(shuō)明上的差異?，F(xiàn)在，這很明顯。該first很關(guān)心的跡象，但second不（只按位與）。為了確保這是原因，請(qǐng)看一下程序集片段：
第一的：
0x00007f91111f8355: mov 0xc(%r10),%r11d ;*getfield i
0x00007f91111f8359: mov %r11d,%edx
0x00007f91111f835c: and $0x1,%edx
0x00007f91111f835f: mov %edx,%r10d
0x00007f6bd120a6e2: neg %r10d
0x00007f6bd120a6e5: test %r11d,%r11d
0x00007f6bd120a6e8: cmovl %r10d,%edx
第二：
0x00007ff36cbda580: mov $0x1,%edx
0x00007ff36cbda585: mov 0x40(%rsp),%r10
0x00007ff36cbda58a: and 0xc(%r10),%edx
