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Summary: Changelog: [Internal] Reviewed By: motiz88 Differential Revision: D21995098 fbshipit-source-id: b387bd45b44fe93743d34d2a3435c165d15f054f
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Oct 16, 2020
Summary:
changelog: [internal]
Prevents 2 type converions:
1. int <-> size_t
2. int <-> int32_t
# Why is using size_t better when working with indexes.
## 1. Type conversion isn't for free.
Take this example
```
size_t calculate(int number) {
return number + 1;
}
```
It generates following assembly (generated with armv8-a clang 10.0.0):
```
calculate(int): // calculate(int)
sub sp, sp, #16 // =16
str w0, [sp, #12]
ldr w8, [sp, #12]
add w9, w8, #1 // =1
mov w8, w9
sxtw x0, w8
add sp, sp, #16 // =16
ret
```
That's 9 instructions.
If we get rid of type conversion:
```
size_t calculate(size_t number) {
return number + 1;
}
```
Assembly (generated with armv8-a clang 10.0.0):
```
calculate(unsigned long): // calculate(unsigned long)
sub sp, sp, #16 // =16
str x0, [sp, #8]
ldr x8, [sp, #8]
add x0, x8, #1 // =1
add sp, sp, #16 // =16
ret
```
Compiler now produces only 7 instructions.
## Semantics
When using int for indexing, the type doesn't say much. By using `size_t`, just by looking at the type, it gives the reader more information about where it is coming from.
Reviewed By: JoshuaGross
Differential Revision: D24332248
fbshipit-source-id: 87ef982829ec14906ed9e002ea2e875fda4a0cd8
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