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# gcc(1)
## CLI
### Preprocessing
While debugging can be helpful to just pre-process files.
```bash
gcc -E [-dM] ...
```
- `-E` run only preprocessor
- `-dM` list only `#define` statements
## [Builtins][builtins]
### `__builtin_expect(expr, cond)`
Give the compiler a hint which branch is hot, so it can lay out the code
accordingly to reduce number of jump instructions.
See on [compiler explorer](https://godbolt.org/z/MbTHAP).
```bash
echo "
extern void foo();
extern void bar();
void run0(int x) {
if (__builtin_expect(x,0)) { foo(); }
else { bar(); }
}
void run1(int x) {
if (__builtin_expect(x,1)) { foo(); }
else { bar(); }
}
" | gcc -O2 -S -masm=intel -o /dev/stdout -xc -
```
Will generate something similar to the following.
- `run0`: `bar` is on the path without branch
- `run1`: `foo` is on the path without branch
```c
run0:
test edi, edi
jne .L4
xor eax, eax
jmp bar
.L4:
xor eax, eax
jmp foo
run1:
test edi, edi
je .L6
xor eax, eax
jmp foo
.L6:
xor eax, eax
jmp bar
```
[builtins]: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
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