Add a version of the tiny example which prints hello world. (#137)

Author: sunfishcode

example-crates/tiny-hello/.cargo/config.toml

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+[build]
+# Disable traps on unreachable code.
+rustflags = ["-Z", "trap-unreachable=no"]

example-crates/tiny-hello/Cargo.toml

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+[package]
+name = "tiny-hello"
+version = "0.0.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+# Origin can be depended on just like any other crate. For no_std, disable
+# the default features, and add the desired features.
+rustix = { version = "0.38.35", default-features = false, features = ["stdio"] }
+origin = { path = "../..", default-features = false, features = ["origin-start", "eh-personality-continue", "panic-handler-abort", "nightly", "optimize_for_size"] }
+
+# This is just an example crate, and not part of the origin workspace.
+[workspace]
+
+# Let's optimize for small size!
+[profile.release]
+# Give the optimizer more lattitude to optimize and delete unneeded code.
+lto = true
+# "abort" is smaller than "unwind".
+panic = "abort"
+# Tell the optimizer to optimize for size.
+opt-level = "z"
+# Delete the symbol table from the executable.
+strip = true

example-crates/tiny-hello/README.md

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+This crate is similar to the [tiny example], except that it prints
+"Hello, world!".
+
+[tiny example]: https://github.com/sunfishcode/origin/blob/main/example-crates/origin-start/README.md

example-crates/tiny-hello/build.rs

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+fn main() {
+    // Pass -nostartfiles to the linker. In the future this could be obviated
+    // by a `no_entry` feature: <https://github.com/rust-lang/rfcs/pull/2735>
+    println!("cargo:rustc-link-arg=-nostartfiles");
+
+    // The following options optimize for code size!
+
+    // Tell the linker to exclude the .eh_frame_hdr section.
+    println!("cargo:rustc-link-arg=-Wl,--no-eh-frame-hdr");
+    // Tell the linker to make the text and data readable and writable. This
+    // allows them to occupy the same page.
+    println!("cargo:rustc-link-arg=-Wl,-N");
+    // Tell the linker to exclude the `.note.gnu.build-id` section.
+    println!("cargo:rustc-link-arg=-Wl,--build-id=none");
+    // Disable PIE, which adds some code size.
+    println!("cargo:rustc-link-arg=-Wl,--no-pie");
+    // Disable the `GNU-stack` segment, if we're using lld.
+    println!("cargo:rustc-link-arg=-Wl,-z,nognustack");
+}

example-crates/tiny-hello/src/main.rs

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+//! Going for minimal size!
+
+#![no_std]
+#![no_main]
+
+extern crate origin;
+
+#[no_mangle]
+unsafe fn origin_main(_argc: usize, _argv: *mut *mut u8, _envp: *mut *mut u8) -> i32 {
+    let message = b"Hello, world!\n";
+    let mut remaining = &message[..];
+    while !remaining.is_empty() {
+        match rustix::io::write(rustix::stdio::stdout(), message) {
+            Ok(n) => remaining = &remaining[n as usize..],
+            Err(rustix::io::Errno::INTR) => continue,
+            Err(_) => origin::program::abort(),
+        }
+    }
+    origin::program::exit_immediately(0);
+}