Be aware, however, that in some circumstances, some versions of ldd may attempt to obtain the dependency information by directly executing the program. Thus, you should never employ ldd on an untrusted executable, since this may result in the execution of arbitrary code.
macOS and other Darwin-based systems, which use Mach-O rather than ELF, and have an 4.x/SVR4-inspired dynamic linking mechanism (not surprising, given that the person who did a lot of the work on the 4.x system left Sun to go to NeXT), but don't have an "ldd" program. Instead, there's "otool -L", which produces output such as [...]
首先是在 -O3 的情況下 (也就是作者使用的參數),可以看到類似的結果:(我桌機的 CPU 是定速,沒有跑動態調整)
$ repeat 10 ./a
[-] Took: 248830 ns.
[-] Took: 249150 ns.
[-] Took: 248760 ns.
[-] Took: 248730 ns.
[-] Took: 248770 ns.
[-] Took: 248861 ns.
[-] Took: 248760 ns.
[-] Took: 253050 ns.
[-] Took: 248640 ns.
[-] Took: 249211 ns.
$ repeat 10 ./b
[-] Took: 686660 ns.
[-] Took: 696090 ns.
[-] Took: 696310 ns.
[-] Took: 694431 ns.
[-] Took: 691971 ns.
[-] Took: 697690 ns.
[-] Took: 693241 ns.
[-] Took: 692900 ns.
[-] Took: 654751 ns.
[-] Took: 679101 ns.
從版本 A 的 objdump -d -S -M intel a 可以看到作者 screenshot 內也有看的 unroll 與 SSE2 指令集:
$ repeat 10 ./a
[-] Took: 571140 ns.
[-] Took: 570280 ns.
[-] Took: 571271 ns.
[-] Took: 573971 ns.
[-] Took: 571981 ns.
[-] Took: 569650 ns.
[-] Took: 566361 ns.
[-] Took: 571600 ns.
[-] Took: 571330 ns.
[-] Took: 571030 ns.
$ repeat 10 ./b
[-] Took: 697521 ns.
[-] Took: 696961 ns.
[-] Took: 696201 ns.
[-] Took: 694921 ns.
[-] Took: 696930 ns.
[-] Took: 695001 ns.
[-] Took: 701661 ns.
[-] Took: 698100 ns.
[-] Took: 702430 ns.
[-] Took: 702641 ns.
從 objdump 可以看到版本 A 的變化,退化成一次只處理一個,但把所有的數字都用 xmmN 存放計算:
$ repeat 10 ./a
[-] Took: 1097091 ns.
[-] Took: 1092941 ns.
[-] Took: 1092501 ns.
[-] Took: 1091991 ns.
[-] Took: 1092441 ns.
[-] Took: 1093970 ns.
[-] Took: 1091341 ns.
[-] Took: 1093931 ns.
[-] Took: 1094111 ns.
[-] Took: 1092231 ns.
$ repeat 10 ./b
[-] Took: 2703282 ns.
[-] Took: 2705933 ns.
[-] Took: 2703582 ns.
[-] Took: 2702622 ns.
[-] Took: 2703043 ns.
[-] Took: 2702262 ns.
[-] Took: 2703352 ns.
[-] Took: 2703532 ns.
[-] Took: 2703112 ns.
[-] Took: 2702533 ns.