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Update cpp.md

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Signed-off-by: Dennis Eichhorn <spl1nes.com@googlemail.com>
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@ -41,19 +41,15 @@ When writing code keep the following topics in mind:
Branched code
```c++
for (int i = 0; i < N; i++)
if (a[i] < 50) {
s += a[i];
}
if (a < 50) {
b += a;
}
```
Branchless code
```c++
for (int i = 0; i < N; i++)
s += (a[i] < 50) * a[i];
}
b += (a < 50) * a;
```
### Instruction table latency
@ -72,6 +68,18 @@ for (int i = 0; i < N; i++)
https://www.agner.org/optimize/instruction_tables.pdf
### Cache sizes
| CPU Category | Stat |
|--------------|---------|
| L1 Cache | 32 - 48 KB |
| L2 Cache | 2 - 4 MB |
| L3 Cache | 8 - 36 MB |
| L4 Cache | 0 - 128 MB |
| Clock speed | 3.5 - 6.2 Ghz |
| Cache Line | 64 B |
| Page Size | 4 KB |
### Cache line sharing between CPU cores
When working with multi-threading you may choose to use atomic variables and atomic operations to reduce the locking in your application. You may think that a variable value `a[0]` used by thread 1 on core 1 and a variable value `a[1]` used by thread 2 on core 2 will have no performance impact. However, this is wrong. Core 1 and core 2 both have different L1 and L2 caches BUT the CPU doesn't just load individual variables, it loads entire cache lines (e.g. 64 bytes). This means that if you define `int a[2]`, it has a high chance of being on the same cache line and therfore thread 1 and thread 2 both have to wait on each other when doing atomic writes.