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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
- `-###` dry-run, outputting exact compiler/linker invocations
- `-print-multi-lib` print available multilib configurations
- `--help=<class>` print description of cmdline options for given class, eg
`warnings`, `optimizers`, `target`, `c`, `c++`
### Target options
```bash
# List all target options with their description.
gcc --help=target
# Configure for current cpu arch and query (-Q) value of options.
gcc -march=native -Q --help=target
```
### Warnings / optimizations
```bash
# List available warnings with short description.
gcc --help=warnings
# List available optimizations with short description.
gcc --help=optimizers
# Prepend --help with `-Q` to print wheter options are enabled or disabled
# instead showing their description.
```
## [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).
The semantics of this hint are as follows, the compiler prioritises `expr ==
cond`. So `__builtin_expect(expr, 0)` means that we expect the `expr` to be `0`
most of the time.
```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
```x86asm
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
```
## ABI (Linux)
- C ABI - [SystemV ABI][sysvabi]
- C++ ABI - [C++ Itanium ABI][cppabi]
[builtins]: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
[sysvabi]: https://www.uclibc.org/docs/psABI-x86_64.pdf
[cppabi]: https://itanium-cxx-abi.github.io/cxx-abi
|
