bare1

examples/bare1.rs

+//! bare1.rs
+//!
+//! Inspecting the generated assembly
+//!
+//! What it covers
+//! - tracing over semihosting and ITM
+//! - assembly calls and inline assembly
+//! - more on arithmetics
+
+#![no_main]
+#![no_std]
+
+extern crate panic_halt;
+
+use cortex_m_rt::entry;
+
+#[entry]
+#[inline(never)]
+fn main() -> ! {
+    // Prepend by `x` by _ to avoid warning (never used).
+    // The compiler is smart enough to figure out that
+    // `x` is not used in any menaningful way.
+
+    let mut _x = 0;
+    loop {
+        _x += 1;
+        cortex_m::asm::nop();
+        cortex_m::asm::bkpt();
+        _x -= 1;
+    }
+}
+
+// 0. Setup
+//    For this example we will use the `nightly` compiler
+//    to get inline assembly.
+//    (Inline assembly is currently not stabelized.)
+//
+//    > rustup override set nightly
+//
+//    In the `Corgo.toml` file, uncomment
+//    # features = ["inline-asm"] # <- currently requires nightly compiler
+//
+//    You may need/want to install addititonal components also,
+//    to that end look at the install section in the README.md.
+//    If you change toolchain, exit and re-start `vscode`.
+//
+// 1. Build and run the application
+//
+//    > cargo build --example bare1
+//    (or use the vscode build task)
+//
+//    Look at the `hello.rs` and `itm.rs` examples to setup the tracing.
+//
+//    When debugging the application it should get stuck in the
+//    loop, (press pause/suspend to verify this).
+//    what is the output in the ITM console
+//
+//    ** your answer here **
+//
+//    What is the output in the semihosting (openocd) console
+//    ** your answer here **
+//
+//    Commit your answers (bare1_1)
+//
+// 2. Inspecting the generated assembly code
+//    If in `vcsode` the gdb console in DEBUG CONSOLE
+//
+//    What is the output of:
+//    (gdb) disassemble
+//
+//    ** your answer here **
+//
+//    Commit your answers (bare1_2)
+//
+// 3. Now remove the comment for `cortex_m::asm::nop()`.
+//    Rebuild and debug, pause the program.
+//
+//    What is the output of:
+//    (gdb) disassemble
+//
+//    ** your answer here **
+//
+//    Commit your answers (bare1_3)
+//
+// 4. Now remove the comment for `cortex_m::asm::bkpt()`
+//    Rebuild and debug, let the program run until it halts.
+//
+//    What is the output of:
+//    (gdb) disassemble
+//
+//    ** your answer here **
+//
+//    Commit your answers (bare1_4)
+//
+// 5. Release mode (optimized builds).
+//    Rebuild `bare1.rs` in release (optimized mode).
+//  
+//    > cargo build --example bare1 --release
+//    (or using the vscode build task)
+//
+//    Compare the generated assembly for the loop
+//    between the dev (unoptimized) and release (optimized) build.
+//
+//    ** your answer here **
+//
+//    commit your answers (bare1_5)
+//
+//    Tips: The optimized build should have 3 instructions
+//    while the debug (dev) build should have > 20 instructions
+//    (both counting the inner loop only). The debug build
+//    should have additional code that call panic if the additon
+//    wraps (and in such case call panic).
+//
+//    Discussion:
+//    In release (optimized) mode the addition is unchecked,
+//    so there is a semantic difference here in between
+//    the dev and release modes. This is motivited by:
+//    1) efficiency, unchecked is faster
+//    2) convenience, it would be inconvenient to explicitly use
+//    wrapping arithmetics, and wrapping is what the programmer
+//    typically would expect in any case. So the check
+//    in dev/debug mode is just there for some extra safety
+//    if your intention is NON-wrapping arithmetics.
+//
+// 6. *Optional
+//    You can pass additional flags to the Rust `rustc` compiler.
+//
+//    `-Z force-overflow-checks=off`
+//
+//    Under this flag, code is never generated for oveflow checking.
+//    You can enable this flag (uncomment the corresponding flag in
+//    the `.cargo/config` file.)
+//
+//    What is now the disassembly of the loop (in debug mode):
+//
+//    ** your answer here **
+//
+//    commit your answers (bare1_6)
+//
+//    Now restore the `.cargo/config` to its original state.
+//
+// 7. *Optional
+//    There is another way to conveniently use wrapping arithmetics
+//    without passing flags to the compiler.
+//
+//    https://doc.rust-lang.org/std/num/struct.Wrapping.html
+//
+//    Rewrite the code using this approach.
+//
+//    What is now the disassembly of the code in dev mode?
+//
+//    ** your answer here **
+//
+//    What is now the disassembly of the code in release mode?
+//
+//    ** your answer here **
+//
+//    commit your answers (bare1_7)
+//
+//    Final discussion:
+//
+//    Embedded code typically is performance sensitve, hence
+//    it is important to understand how code is generated
+//    to achieve efficient implementations.
+//
+//    Moreover, arithmetics are key to processing of data,
+//    so its important that we are in control over the
+//    computations. E.g. comupting checksums, hashes, cryptos etc.
+//    all require precise control over wrapping vs. overflow behaviour.
+//
+//    If you write a library depending on wrapping arithmetics
+//    do NOT rely on a compiler flag. (The end user might compile
+//    it without this flag enabled, and thus get erronous results.)
+//
+//    NOTICE:
+//    ------
+//    You are now on a `nightly` release of the compiler for good and bad.
+//    You can chose to switch back to the stable channel. If so you must
+//    restore the `Cargo.toml` (comment out the `features = ["inline-asm"]`)
+//
+//    Pros and cons of nightly:
+//    + Acccess to new Rust features (such as inline assembly)
+//    - No guarantee these features will work, they might change semantics,
+//      or even be revoked.
+//
+//    The compiler itself is the same, the stable release is just a snapchot
+//    of the nightly (released each 6 week). It is the latest nightly
+//    that passed some additional regression test, not a different compiler.
+//    And of course, the stable has the experimental features disabled.
+//
+//    So its up to you to decide if you want to use the stable or nightly.