[book] describe build flags in the book (#1066)

* ChatGPT here is the `cargo openvm build -h` output please generate the manual chapter about it

* Rename `compile.md` -> `build.md`

* chore: summary

* fixes

---------

Co-authored-by: Jonathan Wang <[email protected]>

book/src/SUMMARY.md

@@ -12,8 +12,7 @@
 
 - [Overview](./writing-apps/overview.md)
 - [Writing a Program](./writing-apps/write-program.md)
-- [Cross-Compilation](./writing-apps/compile.md)
-- [Testing](./writing-apps/testing.md)
+- [Cross-Compilation](./writing-apps/build.md)
 - [Generating Proofs](./writing-apps/prove.md)
 - [Onchain Verification](./writing-apps/verify.md)
 
@@ -29,4 +28,5 @@
 # Advanced Usage
 
 - [Overview](./advanced-usage/overview.md)
-- [Testing the program](./advanced-usage/testing-program.md)
+- [SDK](./advanced-usage/sdk.md)
+- [Testing](./advanced-usage/testing-program.md)

book/src/writing-apps/build.md

@@ -0,0 +1,141 @@
+# Cross-Compilation
+
+First let's define some key terms used in cross-compilation:
+
+- **host** - the machine you're compiling and/or proving on. Note that one can compile and prove on different machines, but they are both called _host_ as they are traditional machine architectures.
+- **guest** - the executable to be run in a different VM architecture (e.g. the OpenVM runtime, or Android app).
+
+There are multiple things happening in the `cargo openvm build` command as in the section [here](./write-program.md). In short, this command compiles on host to an executable for guest target.
+It first compiles the program normally on your _host_ platform with RISC-V and then transpiles it to a different target. See here for some explanation of [cross-compilation](https://rust-lang.github.io/rustup/cross-compilation.html).
+Right now we use `riscv32im-risc0-zkvm-elf` target which is available in the [Rust toolchain](https://doc.rust-lang.org/rustc/platform-support/riscv32im-risc0-zkvm-elf.html), but we will contribute an OpenVM target to Rust in the future.
+
+## Build flags
+
+The following flags are available for the `cargo openvm build` command:
+
+- `--manifest-dir <MANIFEST_DIR>`
+
+  **Description**: Specifies the directory containing the `Cargo.toml` file for the guest code.
+
+  **Default**: The current directory (`.`).
+
+  **Usage Example**: If your `Cargo.toml` is located in `my_project/`, you can run:
+
+  ```bash
+  cargo openvm build --manifest-dir my_project
+  ```
+
+  This ensures the build command is executed in that directory.
+
+- `--features <FEATURES>`
+
+  **Description**: Passes a list of feature flags to the Cargo build process. These flags enable or disable conditional compilation features defined in your `Cargo.toml`.
+
+  **Usage Example**: To enable the `my_feature` feature:
+
+  ```bash
+  cargo openvm build --features my_feature
+  ```
+
+- `--bin`
+
+  **Description**: Restricts the build to binary targets. If your project has multiple target types (binaries, libraries, examples, etc.), using `--bin` ensures only binary targets are considered.
+
+  **Usage Example**:
+
+  ```bash
+  cargo openvm build --bin
+  ```
+
+- `--example`
+
+  **Description**: Restricts the build to example targets. Projects often include code samples or demos under the examples directory, and this flag focuses on compiling those.
+
+  **Usage Example**:
+
+  ```bash
+  cargo openvm build --example
+  ```
+
+- `--name <NAME>`
+
+  **Description**: Filters targets by name. Only targets whose names contain the given substring will be built.
+
+  **Usage Example**: To build only targets that have `client` in their name:
+
+  ```bash
+  cargo openvm build --name client
+  ```
+
+- `--no-transpile`
+
+  **Description**: After building the guest code, doesn't transpile the target ELF into an OpenVM-compatible executable (by default it does).
+
+  **Usage Example**:
+
+  ```bash
+  cargo openvm build --no-transpile
+  ```
+
+- `--config <CONFIG>`
+
+  **Description**: Specifies the path to a .toml configuration file that defines which VM extensions to use.
+
+  **Default**: `./openvm.toml` if `--config` flag is not provided.
+
+  **Usage Example**:
+
+  ```bash
+  cargo openvm build --config path/to/openvm.toml
+  ```
+
+  This allows you to customize the extensions. Currently the CLI only supports known extensions listed in the [Using Existing Extensions](../custom-extensions/overview.md) section. To use other extensions, use the [SDK](../advanced-usage/sdk.md).
+
+- `--exe-output <EXE_OUTPUT>`
+
+  **Description**: Sets the output path for the transpiled program.
+
+  **Default**: `./openvm/app.vmexe` if `--exe-output` flag is not provided.
+
+  **Usage Example**: To specify a custom output filename:
+
+  ```bash
+  cargo openvm build --exe-output ./output/custom_name.vmexe
+  ```
+
+- `--profile <PROFILE>`
+
+  **Description**: Determines the build profile used by Cargo. Common profiles are dev (faster builds, less optimization) and release (slower builds, more optimization).
+
+  **Default**: release
+
+  **Usage Example**:
+
+  ```bash
+  cargo openvm build --profile dev
+  ```
+
+- `--help`
+
+  **Description**: Prints a help message describing the available options and their usage.
+
+  **Usage Example**:
+
+  ```bash
+  cargo openvm build --help
+  ```
+
+## Running a Program
+
+After building and transpiling a program, you can execute it using the `run` command. The `run` command has the following arguments:
+
+```bash
+cargo openvm run
+    --exe <path_to_transpiled_program>
+    --config <path_to_app_config>
+    --input <path_to_input>
+```
+
+If `--exe` and/or `--config` are not provided, the command will search for these files in `./openvm/app.vmexe` and `./openvm.toml` respectively. If `./openvm.toml` is not present, a default configuration will be used.
+
+If your program doesn't require inputs, you can (and should) omit the `--input` flag.