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document combination of flags #756

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document combination of flags
FrancescoV1985 Aug 31, 2025
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updated code according to review's feedback
FrancescoV1985 Sep 25, 2025
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FrancescoV1985 Sep 25, 2025
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FrancescoV1985 Sep 25, 2025
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39 changes: 39 additions & 0 deletions CONTRIBUTING.md
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Expand Up @@ -25,6 +25,45 @@ We encourage new contributors to join our communication channels and introduce t

## Understanding Core Concepts



### Sysroot & compilation flags

#### What *is* the sysroot?
The **sysroot** is the directory that stores the compiled standard
library (`core`, `alloc`, `std`, `test`, …) and compiler built-ins.
Rustup ships these libraries **pre-compiled with LLVM**.

Because **rustc_codegen_gcc** replaces LLVM with the GCC backend, the shipped
LLVM artefacts are **incompatible**.
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They should be compatible. If they are not, this would be an ABI bug.

Therefore the standard library must be **rebuilt with GCC** before ordinary Rust crates can be compiled.

The freshly compiled sysroot ends up in
`build/build_sysroot/...`.

A rebuild of sysroot is needed when

* the backend changes in a way that affects code generation, or
* the user switches toolchains / updates submodules.

Both backend and sysroot can be built using different [profiles](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles).
That is exactly what the `--sysroot`, `--release-sysroot` and `--release` flag supported by the frontend script `y.sh` take care of.
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That is exactly what the `--sysroot`, `--release-sysroot` and `--release` flag supported by the frontend script `y.sh` take care of.
That is exactly what the `--sysroot`, `--release-sysroot` and `--release` flag supported by the build system script `y.sh` take care of.



#### Typical flag combinations

| Command | Backend Profile | Sysroot Profile | Usage Scenario |
|--------------------------------------------|-------------------------------|----------------------------------|------------------------------------------------------------|
| `./y.sh build` |  dev (optimized + debuginfo) |  X |  Optimize backend with debug capabilities |
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According to the Cargo.toml, only the dependencies are built with optimizations.

| `./y.sh build --release` |  release (optimized) |  X |  Optimize backend without rebuilding sysroot |
| `./y.sh build --release --release-sysroot`|  release (optimized) |  X |  Same as --release |
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--release-sysroot should not be used without --sysroot, so I would remove this line and the other doing this from this table and perhaps specify that this outside the table.

| `./y.sh build --release --sysroot` |  release (optimized) |  dev (unoptimized + debuginfo) |  Optimize backend for release without full sysroot rebuild |
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This should build the sysroot since the --sysroot flag is provided.

| `./y.sh build --sysroot` |  dev (optimized + debuginfo) |  dev (unoptimized + debuginfo) |  Full debug capabilities with optimized backend |
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This should build the sysroot since the --sysroot flag is provided.

| `./y.sh build --release-sysroot` |  dev (optimized + debuginfo) |  X |  Build only optimized backend with debug capabilities |
| `./y.sh build --release-sysroot --sysroot`|  dev (optimized + debuginfo) |  release (optimized) |  Build optimized backend and sysroot for debugging and release, respectively |
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This should only build a release sysroot, not a debug sysroot.
And this builds a "debug" (well, dependencies compiled with optimizations + backend not optimized) backend.




### Common Development Tasks

#### Running Specific Tests
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4 changes: 1 addition & 3 deletions build_system/src/build.rs
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Expand Up @@ -44,9 +44,7 @@ impl BuildArg {
fn usage() {
println!(
r#"
`build` command help:

--sysroot : Build with sysroot"#
`build` command help:"#
);
ConfigInfo::show_usage();
println!(" --help : Show this help");
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15 changes: 12 additions & 3 deletions build_system/src/config.rs
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Expand Up @@ -472,13 +472,22 @@ impl ConfigInfo {

pub fn show_usage() {
println!(
"\
"
--features [arg] : Add a new feature [arg]
--target-triple [arg] : Set the target triple to [arg]
--target [arg] : Set the target to [arg]
--out-dir : Location where the files will be generated
--release : Build in release mode
--release-sysroot : Build sysroot in release mode
--release : Build and optimize the backend in release mode
--sysroot : When used on its own, build both
sysroot and backend in dev. mode. Only the backend is optimized.
When used together with --release, build and optimize the backend
in release mode instead of in dev. mode.
When used together with --release-sysroot,
build and optimize the sysroot in release mode instead of in dev. mode
--release-sysroot : When used on its own, build and optimize only the backend in dev. mode.
When combined with --release, it has no effect.
When combined with --sysroot, additionally
build and optimize the sysroot in release mode
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I'll let you update this to reflect the above comments.
Also, most of these flags will only work with the build command, right?
If so, they should only be shown in the help of the build command.

--sysroot-panic-abort : Build the sysroot without unwinding support
--config-file : Location of the config file to be used
--gcc-path : Location of the GCC root folder
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