這是我建立的一個小測驗。這里我們有兩個場景：
- 場景 1：輸入和輸出為的兩個函式 (
scenario1a和) 以及從 Neon 資料型別 ( ) 加載/存盤。scenario1buint16_t*uint16x8x4_t - 場景 2：與場景 1 相同的功能（在本例中為
scenario2a和scenario2b），但輸入和輸出為uint16x8x4_t*，并且加載和存盤在主函式中完成。
（在 c 代碼下面，我包含了使用 -O3 編譯后生成的反匯編）。
#include <stdio.h>
#include <stdlib.h>
#include <arm_neon.h>
void scenario1a(uint16_t* resultArray, const uint16_t* X);
void scenario1b(uint16_t* resultArray, const uint16_t* X);
void scenario2a(uint16x8x4_t* result, const uint16x8x4_t* temp);
void scenario2b(uint16x8x4_t* result, const uint16x8x4_t* temp);
void scenario1a(uint16_t* resultArray, const uint16_t* X) {
uint16x8x4_t temp, result;
temp = vld1q_u16_x4(X);
result.val[0] = vextq_u16(temp.val[0], vmulq_n_u16(temp.val[1], -1), 2);
result.val[1] = vextq_u16(temp.val[1], vmulq_n_u16(temp.val[2], -1), 2);
result.val[2] = vextq_u16(temp.val[2], vmulq_n_u16(temp.val[3], -1), 2);
result.val[3] = vextq_u16(temp.val[3], vmulq_n_u16(temp.val[0], -1), 2);
vst1q_u16_x4(resultArray, result);
}
void scenario1b(uint16_t* resultArray, const uint16_t* X) {
uint16x8x4_t temp, result;
temp = vld1q_u16_x4(X);
result.val[0] = vaddq_u16(temp.val[0], temp.val[1]);
result.val[1] = vmulq_n_u16(temp.val[1], -2);
result.val[2] = vaddq_u16(temp.val[2], temp.val[3]);
result.val[3] = vmulq_n_u16(temp.val[3], -2);
vst1q_u16_x4(resultArray, result);
}
void scenario2a(uint16x8x4_t* result, const uint16x8x4_t* temp) {
result->val[0] = vextq_u16(temp->val[0], vmulq_n_u16(temp->val[1], -1), 2);
result->val[1] = vextq_u16(temp->val[1], vmulq_n_u16(temp->val[2], -1), 2);
result->val[2] = vextq_u16(temp->val[2], vmulq_n_u16(temp->val[3], -1), 2);
result->val[3] = vextq_u16(temp->val[3], vmulq_n_u16(temp->val[0], -1), 2);
}
void scenario2b(uint16x8x4_t* result, const uint16x8x4_t* temp) {
result->val[0] = vaddq_u16(temp->val[0], temp->val[1]);
result->val[1] = vmulq_n_u16(temp->val[1], -2);
result->val[2] = vaddq_u16(temp->val[2], temp->val[3]);
result->val[3] = vmulq_n_u16(temp->val[3], -2);
}
int main(void) {
uint16_t input[32] = {15,3,1,85,44,156,32,97,3,54,97,17,0,55,9,17,163,23,74,85,96,14,25,36,95,84,76,51,42,63,58,74};
// Scenario 01: Input and output are uint16_t*
uint16_t result01a[32];
uint16_t result01_final[32];
scenario1a(result01a, input);
scenario1b(result01_final, result01a);
// Scenario 02: Input and output are uint16x8x4_t
uint16_t result02_final[32];
uint16x8x4_t temp, result02a, result02b;
temp = vld1q_u16_x4(input);
scenario2a(&result02a, &temp);
scenario2b(&result02b, &result02a);
vst1q_u16_x4(result02_final, result02b);
return 0;
}
拆卸：
test: file format elf64-littleaarch64
Disassembly of section .init:
0000000000000658 <_init>:
658: a9bf7bfd stp x29, x30, [sp, #-16]!
65c: 910003fd mov x29, sp
660: 94000065 bl 7f4 <call_weak_fn>
664: a8c17bfd ldp x29, x30, [sp], #16
668: d65f03c0 ret
Disassembly of section .plt:
0000000000000670 <.plt>:
670: a9bf7bf0 stp x16, x30, [sp, #-16]!
674: 90000090 adrp x16, 10000 <__FRAME_END__ 0xf3d8>
678: f947c611 ldr x17, [x16, #3976]
67c: 913e2210 add x16, x16, #0xf88
680: d61f0220 br x17
684: d503201f nop
688: d503201f nop
68c: d503201f nop
0000000000000690 <__cxa_finalize@plt>:
690: 90000090 adrp x16, 10000 <__FRAME_END__ 0xf3d8>
694: f947ca11 ldr x17, [x16, #3984]
698: 913e4210 add x16, x16, #0xf90
69c: d61f0220 br x17
00000000000006a0 <__libc_start_main@plt>:
6a0: 90000090 adrp x16, 10000 <__FRAME_END__ 0xf3d8>
6a4: f947ce11 ldr x17, [x16, #3992]
6a8: 913e6210 add x16, x16, #0xf98
6ac: d61f0220 br x17
00000000000006b0 <__stack_chk_fail@plt>:
6b0: 90000090 adrp x16, 10000 <__FRAME_END__ 0xf3d8>
6b4: f947d211 ldr x17, [x16, #4000]
6b8: 913e8210 add x16, x16, #0xfa0
6bc: d61f0220 br x17
00000000000006c0 <__gmon_start__@plt>:
6c0: 90000090 adrp x16, 10000 <__FRAME_END__ 0xf3d8>
6c4: f947d611 ldr x17, [x16, #4008]
6c8: 913ea210 add x16, x16, #0xfa8
6cc: d61f0220 br x17
00000000000006d0 <abort@plt>:
6d0: 90000090 adrp x16, 10000 <__FRAME_END__ 0xf3d8>
6d4: f947da11 ldr x17, [x16, #4016]
6d8: 913ec210 add x16, x16, #0xfb0
6dc: d61f0220 br x17
Disassembly of section .text:
00000000000006e0 <main>:
6e0: 90000085 adrp x5, 10000 <__FRAME_END__ 0xf3d8>
6e4: a9a67bfd stp x29, x30, [sp, #-416]!
6e8: 910003fd mov x29, sp
6ec: 90000002 adrp x2, 0 <_init-0x658>
6f0: 91292042 add x2, x2, #0xa48
6f4: 910263e3 add x3, sp, #0x98
6f8: 910363e0 add x0, sp, #0xd8
6fc: 6f008434 mvni v20.8h, #0x1
700: f947f0a5 ldr x5, [x5, #4064]
704: aa0303e1 mov x1, x3
708: 910143e4 add x4, sp, #0x50
70c: a940344c ldp x12, x13, [x2]
710: a9412c4a ldp x10, x11, [x2, #16]
714: f94000a6 ldr x6, [x5]
718: f900cfe6 str x6, [sp, #408]
71c: d2800006 mov x6, #0x0 // #0
720: a9422448 ldp x8, x9, [x2, #32]
724: a9431c46 ldp x6, x7, [x2, #48]
728: 910463e2 add x2, sp, #0x118
72c: a909b7ec stp x12, x13, [sp, #152]
730: a90aafea stp x10, x11, [sp, #168]
734: a90ba7e8 stp x8, x9, [sp, #184]
738: a90c9fe6 stp x6, x7, [sp, #200]
73c: 94000069 bl 8e0 <scenario1a>
740: 4c402400 ld1 {v0.8h-v3.8h}, [x0]
744: 910043e1 add x1, sp, #0x10
748: aa0403e0 mov x0, x4
74c: 4c402470 ld1 {v16.8h-v19.8h}, [x3]
750: 4e619e85 mul v5.8h, v20.8h, v1.8h
754: 4e608424 add v4.8h, v1.8h, v0.8h
758: 4e628466 add v6.8h, v3.8h, v2.8h
75c: 4e639e87 mul v7.8h, v20.8h, v3.8h
760: 4c002030 st1 {v16.16b-v19.16b}, [x1]
764: 4c002444 st1 {v4.8h-v7.8h}, [x2]
768: 94000072 bl 930 <scenario2a>
76c: ad409885 ldp q5, q6, [x4, #16]
770: 90000081 adrp x1, 10000 <__FRAME_END__ 0xf3d8>
774: 910563e2 add x2, sp, #0x158
778: 3dc00c84 ldr q4, [x4, #48]
77c: 3dc017e7 ldr q7, [sp, #80]
780: f947f021 ldr x1, [x1, #4064]
784: 4e749c83 mul v3.8h, v4.8h, v20.8h
788: 4e668482 add v2.8h, v4.8h, v6.8h
78c: 4e749ca1 mul v1.8h, v5.8h, v20.8h
790: 4e6784a0 add v0.8h, v5.8h, v7.8h
794: 4c002440 st1 {v0.8h-v3.8h}, [x2]
798: f940cfe0 ldr x0, [sp, #408]
79c: f9400022 ldr x2, [x1]
7a0: eb020000 subs x0, x0, x2
7a4: d2800002 mov x2, #0x0 // #0
7a8: 54000081 b.ne 7b8 <main 0xd8> // b.any
7ac: 52800000 mov w0, #0x0 // #0
7b0: a8da7bfd ldp x29, x30, [sp], #416
7b4: d65f03c0 ret
7b8: 97ffffbe bl 6b0 <__stack_chk_fail@plt>
00000000000007bc <_start>:
7bc: d280001d mov x29, #0x0 // #0
7c0: d280001e mov x30, #0x0 // #0
7c4: aa0003e5 mov x5, x0
7c8: f94003e1 ldr x1, [sp]
7cc: 910023e2 add x2, sp, #0x8
7d0: 910003e6 mov x6, sp
7d4: 90000080 adrp x0, 10000 <__FRAME_END__ 0xf3d8>
7d8: f947f800 ldr x0, [x0, #4080]
7dc: 90000083 adrp x3, 10000 <__FRAME_END__ 0xf3d8>
7e0: f947f463 ldr x3, [x3, #4072]
7e4: 90000084 adrp x4, 10000 <__FRAME_END__ 0xf3d8>
7e8: f947e084 ldr x4, [x4, #4032]
7ec: 97ffffad bl 6a0 <__libc_start_main@plt>
7f0: 97ffffb8 bl 6d0 <abort@plt>
00000000000007f4 <call_weak_fn>:
7f4: 90000080 adrp x0, 10000 <__FRAME_END__ 0xf3d8>
7f8: f947ec00 ldr x0, [x0, #4056]
7fc: b4000040 cbz x0, 804 <call_weak_fn 0x10>
800: 17ffffb0 b 6c0 <__gmon_start__@plt>
804: d65f03c0 ret
808: d503201f nop
80c: d503201f nop
0000000000000810 <deregister_tm_clones>:
810: b0000080 adrp x0, 11000 <__data_start>
814: 91004000 add x0, x0, #0x10
818: b0000081 adrp x1, 11000 <__data_start>
81c: 91004021 add x1, x1, #0x10
820: eb00003f cmp x1, x0
824: 540000c0 b.eq 83c <deregister_tm_clones 0x2c> // b.none
828: 90000081 adrp x1, 10000 <__FRAME_END__ 0xf3d8>
82c: f947e421 ldr x1, [x1, #4040]
830: b4000061 cbz x1, 83c <deregister_tm_clones 0x2c>
834: aa0103f0 mov x16, x1
838: d61f0200 br x16
83c: d65f03c0 ret
0000000000000840 <register_tm_clones>:
840: b0000080 adrp x0, 11000 <__data_start>
844: 91004000 add x0, x0, #0x10
848: b0000081 adrp x1, 11000 <__data_start>
84c: 91004021 add x1, x1, #0x10
850: cb000021 sub x1, x1, x0
854: d37ffc22 lsr x2, x1, #63
858: 8b810c41 add x1, x2, x1, asr #3
85c: 9341fc21 asr x1, x1, #1
860: b40000c1 cbz x1, 878 <register_tm_clones 0x38>
864: 90000082 adrp x2, 10000 <__FRAME_END__ 0xf3d8>
868: f947fc42 ldr x2, [x2, #4088]
86c: b4000062 cbz x2, 878 <register_tm_clones 0x38>
870: aa0203f0 mov x16, x2
874: d61f0200 br x16
878: d65f03c0 ret
87c: d503201f nop
0000000000000880 <__do_global_dtors_aux>:
880: a9be7bfd stp x29, x30, [sp, #-32]!
884: 910003fd mov x29, sp
888: f9000bf3 str x19, [sp, #16]
88c: b0000093 adrp x19, 11000 <__data_start>
890: 39404260 ldrb w0, [x19, #16]
894: 35000140 cbnz w0, 8bc <__do_global_dtors_aux 0x3c>
898: 90000080 adrp x0, 10000 <__FRAME_END__ 0xf3d8>
89c: f947e800 ldr x0, [x0, #4048]
8a0: b4000080 cbz x0, 8b0 <__do_global_dtors_aux 0x30>
8a4: b0000080 adrp x0, 11000 <__data_start>
8a8: f9400400 ldr x0, [x0, #8]
8ac: 97ffff79 bl 690 <__cxa_finalize@plt>
8b0: 97ffffd8 bl 810 <deregister_tm_clones>
8b4: 52800020 mov w0, #0x1 // #1
8b8: 39004260 strb w0, [x19, #16]
8bc: f9400bf3 ldr x19, [sp, #16]
8c0: a8c27bfd ldp x29, x30, [sp], #32
8c4: d65f03c0 ret
8c8: d503201f nop
8cc: d503201f nop
00000000000008d0 <frame_dummy>:
8d0: 17ffffdc b 840 <register_tm_clones>
8d4: d503201f nop
8d8: d503201f nop
8dc: d503201f nop
00000000000008e0 <scenario1a>:
8e0: 4c402420 ld1 {v0.8h-v3.8h}, [x1]
8e4: 6e60b833 neg v19.8h, v1.8h
8e8: 6e60b852 neg v18.8h, v2.8h
8ec: 6e60b871 neg v17.8h, v3.8h
8f0: 6e60b810 neg v16.8h, v0.8h
8f4: 6e132004 ext v4.16b, v0.16b, v19.16b, #4
8f8: 6e122025 ext v5.16b, v1.16b, v18.16b, #4
8fc: 6e112046 ext v6.16b, v2.16b, v17.16b, #4
900: 6e102067 ext v7.16b, v3.16b, v16.16b, #4
904: 4c002404 st1 {v4.8h-v7.8h}, [x0]
908: d65f03c0 ret
90c: d503201f nop
0000000000000910 <scenario1b>:
910: 4c402420 ld1 {v0.8h-v3.8h}, [x1]
914: 6f008430 mvni v16.8h, #0x1
918: 4e619e05 mul v5.8h, v16.8h, v1.8h
91c: 4e608424 add v4.8h, v1.8h, v0.8h
920: 4e628466 add v6.8h, v3.8h, v2.8h
924: 4e639e07 mul v7.8h, v16.8h, v3.8h
928: 4c002404 st1 {v4.8h-v7.8h}, [x0]
92c: d65f03c0 ret
0000000000000930 <scenario2a>:
930: ad400025 ldp q5, q0, [x1]
934: ad408423 ldp q3, q1, [x1, #16]
938: 3dc00c24 ldr q4, [x1, #48]
93c: 6e60b800 neg v0.8h, v0.8h
940: 4ea11c22 mov v2.16b, v1.16b
944: 6e60b821 neg v1.8h, v1.8h
948: 6e0020a5 ext v5.16b, v5.16b, v0.16b, #4
94c: 4ea41c80 mov v0.16b, v4.16b
950: 6e60b884 neg v4.8h, v4.8h
954: 6e012063 ext v3.16b, v3.16b, v1.16b, #4
958: 3d800005 str q5, [x0]
95c: 3dc00021 ldr q1, [x1]
960: 6e042042 ext v2.16b, v2.16b, v4.16b, #4
964: ad008803 stp q3, q2, [x0, #16]
968: 6e60b821 neg v1.8h, v1.8h
96c: 6e012000 ext v0.16b, v0.16b, v1.16b, #4
970: 3d800c00 str q0, [x0, #48]
974: d65f03c0 ret
978: d503201f nop
97c: d503201f nop
0000000000000980 <scenario2b>:
980: ad401022 ldp q2, q4, [x1]
984: 6f008420 mvni v0.8h, #0x1
988: ad410c21 ldp q1, q3, [x1, #32]
98c: 4e609c85 mul v5.8h, v4.8h, v0.8h
990: 4e648442 add v2.8h, v2.8h, v4.8h
994: 4e609c60 mul v0.8h, v3.8h, v0.8h
998: 4e638421 add v1.8h, v1.8h, v3.8h
99c: ad001402 stp q2, q5, [x0]
9a0: ad010001 stp q1, q0, [x0, #32]
9a4: d65f03c0 ret
00000000000009a8 <__libc_csu_init>:
9a8: a9bc7bfd stp x29, x30, [sp, #-64]!
9ac: 910003fd mov x29, sp
9b0: a90153f3 stp x19, x20, [sp, #16]
9b4: 90000094 adrp x20, 10000 <__FRAME_END__ 0xf3d8>
9b8: 9135c294 add x20, x20, #0xd70
9bc: a9025bf5 stp x21, x22, [sp, #32]
9c0: 90000095 adrp x21, 10000 <__FRAME_END__ 0xf3d8>
9c4: 9135a2b5 add x21, x21, #0xd68
9c8: cb150294 sub x20, x20, x21
9cc: 2a0003f6 mov w22, w0
9d0: a90363f7 stp x23, x24, [sp, #48]
9d4: aa0103f7 mov x23, x1
9d8: aa0203f8 mov x24, x2
9dc: 97ffff1f bl 658 <_init>
9e0: eb940fff cmp xzr, x20, asr #3
9e4: 54000160 b.eq a10 <__libc_csu_init 0x68> // b.none
9e8: 9343fe94 asr x20, x20, #3
9ec: d2800013 mov x19, #0x0 // #0
9f0: f8737aa3 ldr x3, [x21, x19, lsl #3]
9f4: aa1803e2 mov x2, x24
9f8: 91000673 add x19, x19, #0x1
9fc: aa1703e1 mov x1, x23
a00: 2a1603e0 mov w0, w22
a04: d63f0060 blr x3
a08: eb13029f cmp x20, x19
a0c: 54ffff21 b.ne 9f0 <__libc_csu_init 0x48> // b.any
a10: a94153f3 ldp x19, x20, [sp, #16]
a14: a9425bf5 ldp x21, x22, [sp, #32]
a18: a94363f7 ldp x23, x24, [sp, #48]
a1c: a8c47bfd ldp x29, x30, [sp], #64
a20: d65f03c0 ret
a24: d503201f nop
0000000000000a28 <__libc_csu_fini>:
a28: d65f03c0 ret
Disassembly of section .fini:
0000000000000a2c <_fini>:
a2c: a9bf7bfd stp x29, x30, [sp, #-16]!
a30: 910003fd mov x29, sp
a34: a8c17bfd ldp x29, x30, [sp], #16
a38: d65f03c0 ret
問題
通常人們從指標加載資料（使用
vld1q_u16_x4），使用 Neon 資料型別進行操作，然后存盤回另一個指標（使用vst1q_u16_x4），并且不要使用我在場景 2 中使用的方法（將 Neon 資料型別作為輸入發送） /輸出）。有一個普遍的原因嗎？我檢查了場景 1a（從 line 開始
8e0）與場景 2a（從 line 開始）的反匯編930。似乎場景 2a 有更多的資料移動。這會在所有情況下發生嗎？那么按照我在問題 1 中提出的要求更快嗎？如果是這樣，那么為什么這在場景 1b 和 2b 中不會發生（分別是行910和980）。在 main 函式中，在 Scenario1a 和 2a 之后都有一些 add/mul（在 和 行中
750,754,758,75c）784,788,78c,790，但我的 main 函式沒有乘法或加法。為什么會這樣？（我只是好奇）
謝謝你的幫助！
uj5u.com熱心網友回復：
絕對沒有理由使用指向霓虹燈資料型別的指標作為引數。記憶體不關心資料型別。編譯器非常保守和官僚主義，他們必須這樣做。這就像向當局提交申請：一個錯誤的復選標記，您的申請將落入錯誤的手中，造成巨大的不必要的麻煩。
簡短：保持簡單。不要試圖以任何方式給編譯器或審閱者留下深刻印象。上次我告訴過你要明確記憶體加載和存盤。您在方案 2 中直接從/到記憶體進行計算。永遠不要這樣做。堅持加載->計算->存盤。區域變數是你最好的朋友。（
__restrict指令可能會有所幫助）
同樣，不要試圖給編譯器或審閱者留下深刻印象。您的方案 2 只是自找麻煩。一場徹頭徹尾的災難。如果您幸運并且沒有立即被解雇，審閱者將立即舉起紅旗，并密切關注您和您的所有代碼。您不應該將被呼叫者與呼叫者放在同一個檔案中。通常，呼叫者會行內短的非靜態被呼叫者，這使得分析更加困難。
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標籤：C部件手臂simd氖