## 從簡單的 C 語言函式來看現代 Compiler 使用 SIMD 的威力

```double data[LEN];

void compute()
{
const double A = 1.1, B = 2.2, C = 3.3;

int i;
for(i=0; i<LEN; i++) {
data[i] = A*i*i + B*i + C;
}
}
```

```void compute()
{
const double A = 1.1, B = 2.2, C = 3.3;
const double A2 = A+A;
double Z = A+B;
double Y = C;

int i;
for(i=0; i<LEN; i++) {
data[i] = Y;
Y += Z;
Z += A2;
}
}
```

```int i;
for (i = 0; i < LEN; i += 4) {
data[i+0] = A*(i+0)*(i+0) + B*(i+0) + C;
data[i+1] = A*(i+1)*(i+1) + B*(i+1) + C;
data[i+2] = A*(i+2)*(i+2) + B*(i+2) + C;
data[i+3] = A*(i+3)*(i+3) + B*(i+3) + C;
}
```

```\$ 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.
```

```13a0:       66 0f 6f c2             movdqa xmm0,xmm2
13a4:       48 83 c0 20             add    rax,0x20
13a8:       66 0f fe d6             paddd  xmm2,xmm6
13ac:       f3 0f e6 f8             cvtdq2pd xmm7,xmm0
13b0:       66 0f 28 cf             movapd xmm1,xmm7
13b4:       66 0f 70 c0 ee          pshufd xmm0,xmm0,0xee
13b9:       66 0f 59 cd             mulpd  xmm1,xmm5
13bd:       f3 0f e6 c0             cvtdq2pd xmm0,xmm0
13c1:       66 0f 59 cf             mulpd  xmm1,xmm7
13c5:       66 0f 59 fc             mulpd  xmm7,xmm4
13c9:       66 0f 58 cf             addpd  xmm1,xmm7
13cd:       66 0f 58 cb             addpd  xmm1,xmm3
13d1:       0f 29 48 e0             movaps XMMWORD PTR [rax-0x20],xmm1
13d5:       66 0f 28 c8             movapd xmm1,xmm0
13d9:       66 0f 59 cd             mulpd  xmm1,xmm5
13dd:       66 0f 59 c8             mulpd  xmm1,xmm0
13e1:       66 0f 59 c4             mulpd  xmm0,xmm4
13e5:       66 0f 58 c1             addpd  xmm0,xmm1
13e9:       66 0f 58 c3             addpd  xmm0,xmm3
13ed:       0f 29 40 f0             movaps XMMWORD PTR [rax-0x10],xmm0
13f1:       48 39 c2                cmp    rdx,rax
13f4:       75 aa                   jne    13a0 <compute+0x40>
```

```1340:       f2 0f 11 08             movsd  QWORD PTR [rax],xmm1
1344:       48 83 c0 08             add    rax,0x8
1348:       f2 0f 58 c8             addsd  xmm1,xmm0
134c:       f2 0f 58 c2             addsd  xmm0,xmm2
1350:       48 39 d0                cmp    rax,rdx
1353:       75 eb                   jne    1340 <compute+0x30>
```

```\$ 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.
```

```11b1:       66 0f ef c9             pxor   xmm1,xmm1
11b5:       f2 0f 2a c8             cvtsi2sd xmm1,eax
11b9:       66 0f 28 c1             movapd xmm0,xmm1
11bd:       f2 0f 59 c4             mulsd  xmm0,xmm4
11c1:       f2 0f 59 c1             mulsd  xmm0,xmm1
11c5:       f2 0f 59 cb             mulsd  xmm1,xmm3
11c9:       f2 0f 58 c1             addsd  xmm0,xmm1
11cd:       f2 0f 58 c2             addsd  xmm0,xmm2
11d1:       f2 0f 11 04 c2          movsd  QWORD PTR [rdx+rax*8],xmm0
11d6:       48 83 c0 01             add    rax,0x1
11da:       48 3d 40 42 0f 00       cmp    rax,0xf4240
11e0:       75 cf                   jne    11b1 <compute+0x28>
```

```11b3:       f2 0f 11 08             movsd  QWORD PTR [rax],xmm1
11b7:       f2 0f 58 c8             addsd  xmm1,xmm0
11bb:       f2 0f 58 c2             addsd  xmm0,xmm2
11bf:       48 83 c0 08             add    rax,0x8
11c3:       48 39 d0                cmp    rax,rdx
11c6:       75 eb                   jne    11b3 <compute+0x2a>
```

```\$ 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.
```

```11c1:       f2 0f 2a 45 e4          cvtsi2sd xmm0,DWORD PTR [rbp-0x1c]
11c6:       66 0f 28 c8             movapd xmm1,xmm0
11ca:       f2 0f 59 4d e8          mulsd  xmm1,QWORD PTR [rbp-0x18]
11cf:       f2 0f 2a 45 e4          cvtsi2sd xmm0,DWORD PTR [rbp-0x1c]
11d4:       f2 0f 59 c8             mulsd  xmm1,xmm0
11d8:       f2 0f 2a 45 e4          cvtsi2sd xmm0,DWORD PTR [rbp-0x1c]
11dd:       f2 0f 59 45 f0          mulsd  xmm0,QWORD PTR [rbp-0x10]
11e2:       f2 0f 58 c1             addsd  xmm0,xmm1
11e6:       f2 0f 58 45 f8          addsd  xmm0,QWORD PTR [rbp-0x8]
11eb:       8b 45 e4                mov    eax,DWORD PTR [rbp-0x1c]
11ee:       48 98                   cdqe
11f0:       48 8d 14 c5 00 00 00    lea    rdx,[rax*8+0x0]
11f7:       00
11f8:       48 8d 05 41 2e 00 00    lea    rax,[rip+0x2e41]
11ff:       f2 0f 11 04 02          movsd  QWORD PTR [rdx+rax*1],xmm0
1204:       83 45 e4 01             add    DWORD PTR [rbp-0x1c],0x1
1208:       81 7d e4 3f 42 0f 00    cmp    DWORD PTR [rbp-0x1c],0xf423f
120f:       7e b0                   jle    11c1 <compute+0x38>
```

```11e8:       8b 45 cc                mov    eax,DWORD PTR [rbp-0x34]
11eb:       48 98                   cdqe
11ed:       48 8d 14 c5 00 00 00    lea    rdx,[rax*8+0x0]
11f4:       00
11f5:       48 8d 05 44 2e 00 00    lea    rax,[rip+0x2e44]
11fc:       f2 0f 10 45 d8          movsd  xmm0,QWORD PTR [rbp-0x28]
1201:       f2 0f 11 04 02          movsd  QWORD PTR [rdx+rax*1],xmm0
1206:       f2 0f 10 45 d8          movsd  xmm0,QWORD PTR [rbp-0x28]
120b:       f2 0f 58 45 d0          addsd  xmm0,QWORD PTR [rbp-0x30]
1210:       f2 0f 11 45 d8          movsd  QWORD PTR [rbp-0x28],xmm0
1215:       f2 0f 10 45 d0          movsd  xmm0,QWORD PTR [rbp-0x30]
121a:       f2 0f 58 45 f8          addsd  xmm0,QWORD PTR [rbp-0x8]
121f:       f2 0f 11 45 d0          movsd  QWORD PTR [rbp-0x30],xmm0
1224:       83 45 cc 01             add    DWORD PTR [rbp-0x34],0x1
1228:       81 7d cc 3f 42 0f 00    cmp    DWORD PTR [rbp-0x34],0xf423f
122f:       7e b7                   jle    11e8 <compute+0x5f>
```

## 用 objdump 學到的一些東西...

```tcc -c true.c
objdump -d true.o```

```0000000000000000 <main>:
0:   f3 0f 1e fa             endbr64
4:   55                      push   %rbp
5:   48 89 e5                mov    %rsp,%rbp
8:   b8 00 00 00 00          mov    \$0x0,%eax
d:   5d                      pop    %rbp
e:   c3                      retq```

## LLVM 的更換授權進展

Hacker News Daily 上看到「LLVM relicensing update & call for help」這篇，在講 LLVM 計畫從 UIUC licenseMIT license 授權轉成 Apache License 2.0 的進展，在 Hacker News 上的討論「LLVM relicensing update and call for help (llvm.org)」也可以翻一下。

The run time libraries were dual licensed under the UIUC and MIT license; the rest of the code only under the UIUC license. Therefore, we could not easily move code to run time libraries from other parts. The reason run time libraries were dual licensed was to enable linking to run time library binaries without requiring attribution to LLVM.

As an exception, if, as a result of your compiling your source code, portions of this Software are embedded into an Object form of such source code, you may redistribute such embedded portions in such Object form without complying with the conditions of Sections 4(a), 4(b) and 4(d) of the License.

In addition, if you combine or link compiled forms of this Software with software that is licensed under the GPLv2 ("Combined Software") and if a court of competent jurisdiction determines that the patent provision (Section 3), the indemnity provision (Section 9) or other Section of the License conflicts with the conditions of the GPLv2, you may retroactively and prospectively choose to deem waived or otherwise exclude such Section(s) of the License, but only in their entirety and only with respect to the Combined Software.

## Fork 自微軟的 Pyjion 專案的 Python 3.10 + JIT 方案

Hacker News 上看到「Pyjion – A Python JIT Compiler (trypyjion.com)」這個專案，也是一個想要透過 JIT 加速 Python 的專案：

Pyjion is a drop-in JIT Compiler for Python 3.10. It can be pip installed into a CPython 3.10 installation on Linux, Mac OS X, or Windows.

Development has moved to https://github.com/tonybaloney/Pyjion

`import pyjion; pyjion.enable()`

zatarc 3 days ago | unvote | prev | next [–]

Pyjion requires: CPython 3.10 and .NET 6

.NET 6 Release: 19 hours ago (https://github.com/dotnet/core/blob/main/release-notes/6.0/6...)

... ok.

## YJIT 帶給 Ruby 大量的效能提昇

Hacker News 首頁上看到的消息，由 Shopify 贊助的 YJIT 被 Ruby 官方接受了：「Merge YJIT: an in-process JIT compiler (github.com/ruby)」。

YJIT currently provides average speedups of 23% over the CRuby interpreter on realistic benchmarks, and near-instant warm-up time.

That being said, according to our benchmarks, we’ve been able to achieve speedups over the CRuby interpreter of 7% on railsbench, 19% on liquid template rendering, and 19% on activerecord.

Currently, only about 50% of instructions in railsbench are executed by YJIT, and the rest run in the interpreter, meaning that there is still a lot we can do to improve upon our current results.

## Windows 上的 Chrome 改用 Clang 編譯

We conducted extensive A/B testing of performance. Performance telemetry numbers are about the same for MSVC-built and clang-built Chrome – some metrics get better, some get worse, but all of them are within 5% of each other.

## 讀書時間：Spectre 的攻擊方式

Spectre 的精華在於 CPU 支援 branch prediction 與 out-of-order execution，也就是 CPU 遇到 branch 時會學習怎麼跑，這個資訊提供給 out-of-order execution 就可以大幅提昇執行速度。可以參考以前在「CPU Branch Prediction 的成本...」提到的效率問題。

```if (false but mispredicts as true)

Suppose register R1 contains a secret value. If the speculatively executed memory read of array1[R1] is a cache hit, then nothing will go on the memory bus and the read from [R2] will initiate quickly. If the read of array1[R1] is a cache miss, then the second read may take longer, resulting in different timing for the victim thread.

```if (false but mispredicts as true)
multiply R1, R2
multiply R3, R4```

Spectre 論文提到的 mitigation (workaround) 是透過 `mfence``lfence` 強制程式碼的順序，但這表示 compiler 要針對所有的 branch 加上這段，對效能影響應該蠻明顯的：

In addition, of the three user-mode serializing instructions listed by Intel, only cpuid can be used in normal code, and it destroys many registers. The mfence and lfence (but not sfence) instructions also appear to work, with the added benefit that they do not destroy register contents. Their behavior with respect to speculative execution is not defined, however, so they may not work in all CPUs or system configurations.

Google 推出的 Retpoline 則是想要避免這個問題。Google 在「Retpoline: a software construct for preventing branch-target-injection」這邊詳細說明了 Retpoline 的原理與方法，採取的方向是控制 speculative execution：

However, we may manipulate its generation to control speculative execution while modifying the visible, on-stack value to direct how the branch is actually retired.

```retpoline_r11_trampoline:
call set_up_target;
capture_spec:
pause;
jmp capture_spec;
set_up_target:
mov %r11, (%rsp);
ret;```

## GCC 的警告參數

`-Wduplicated-cond -Wduplicated-branches -Wlogical-op -Wrestrict -Wnull-dereference -Wold-style-cast -Wuseless-cast -Wjump-misses-init -Wdouble-promotion -Wshadow -Wformat=2`

## Webkit 推出 B3 JIT Compiler (Bare Bones Backend)

Webkit 推出了 B3 加快 optimization 的速度，取代原來 LLVM 的工作：「Introducing the B3 JIT Compiler」。

B3 is not yet complete. We still need to finish porting B3 to ARM64. B3 passes all tests, but we haven’t finished optimizing performance on ARM. Once all platforms that used the FTL switch to B3, we plan to remove LLVM support from the FTL JIT.

## Android NDK 宣佈向 Clang 靠攏...

Hacker News Daily 上看到 Android NDK 宣佈向 Clang 靠過去的消息：「Changelog for NDK Build 2490520」。

Android NDK 做為效能的加速手段而使用到 C 或是 C++，所以會使用對應的 compiler suite：

The NDK is a toolset that allows you to implement parts of your app using native-code languages such as C and C++. Typically, good use cases for the NDK are CPU-intensive applications such as game engines, signal processing, and physics simulation.

PSA: Everyone should be switching to Clang.

GCC in the NDK is now deprecated.