From c28e66cd3e0913c5439a7d05f6c106af66ec580a Mon Sep 17 00:00:00 2001 From: Lokathor Date: Mon, 10 Dec 2018 10:58:37 -0700 Subject: [PATCH 1/4] introduction improvements --- book/src/00-introduction/00-index.md | 13 +++++++++---- book/src/00-introduction/05-newtype.md | 22 ++++++++++++++++------ 2 files changed, 25 insertions(+), 10 deletions(-) diff --git a/book/src/00-introduction/00-index.md b/book/src/00-introduction/00-index.md index 6663e2b..0567c77 100644 --- a/book/src/00-introduction/00-index.md +++ b/book/src/00-introduction/00-index.md @@ -10,7 +10,12 @@ of the pages listed in the Table Of Contents. ## Feedback -It's also often hard to tell when you've explained something properly to someone -who doesn't understand the concept yet. Please, if things don't make sense then -[file an issue](https://github.com/rust-console/gba/issues) about it so I know -where things need to improve. +It's very often hard to tell when you've explained something properly. In the +same way that your brain will read over small misspellings and correct things +into the right word, if an explanation for something you already understand +accidentally skips over some small detail then your brain can fill in the gaps +without you realizing it. + +**Please**, if things don't make sense then [file an +issue](https://github.com/rust-console/gba/issues) about it so I know where +things need to improve. diff --git a/book/src/00-introduction/05-newtype.md b/book/src/00-introduction/05-newtype.md index 214250d..02c9676 100644 --- a/book/src/00-introduction/05-newtype.md +++ b/book/src/00-introduction/05-newtype.md @@ -111,12 +111,22 @@ macro_rules! newtype { ``` That seems like enough for all of our examples, so we'll stop there. We could -add more things, such as making the `From` impl optional (because what if you -shouldn't unwrap it for some weird reason?), allowing for more precise -visibility controls (on both the newtype overall and the inner field), and maybe -even other things I can't think of right now. We won't really need those in our -example code for this book, so it's probably nicer to just keep the macro -simpler and quit while we're ahead. +add more things: + +* Making the `From` impl being optional. We'd have to make the newtype + invocation be more complicated somehow, the user puts ", no-unwrap" after the + inner type declaration or something, or something like that. +* Allowing for more precise visibility controls on the wrapping type and on the + inner field. This would add a lot of line noise, so we'll just always have our + newtypes be `pub`. +* Allowing for generic newtypes, which might sound silly but that we'll actually + see an example of soon enough. To do this you might think that we can change + the `:ident` declarations to `:ty`, but then you can't use that captured type + when you declare the new wrapping type. The way you get around this is with a + proc-macro, which is a lot more powerful but which also requires that the + proc-macro be written in an entirely other crate. We don't need that much + power, so for our examples we'll go with the macro_rules version and just do + it by hand in the few cases where we need a generic newtype. **As a reminder:** remember that macros have to appear _before_ they're invoked in your source, so the `newtype` macro will always have to be at the very top of From d201ee899b9d67deb5c3ca59b37d84581f343251 Mon Sep 17 00:00:00 2001 From: Lokathor Date: Mon, 10 Dec 2018 17:01:21 -0700 Subject: [PATCH 2/4] volatile stuff --- book/src/00-introduction/05-newtype.md | 28 +++-- .../01-limitations/03-volatile_destination.md | 115 ++++++++++++++++++ 2 files changed, 133 insertions(+), 10 deletions(-) diff --git a/book/src/00-introduction/05-newtype.md b/book/src/00-introduction/05-newtype.md index 02c9676..d711917 100644 --- a/book/src/00-introduction/05-newtype.md +++ b/book/src/00-introduction/05-newtype.md @@ -120,14 +120,22 @@ add more things: inner field. This would add a lot of line noise, so we'll just always have our newtypes be `pub`. * Allowing for generic newtypes, which might sound silly but that we'll actually - see an example of soon enough. To do this you might think that we can change - the `:ident` declarations to `:ty`, but then you can't use that captured type - when you declare the new wrapping type. The way you get around this is with a - proc-macro, which is a lot more powerful but which also requires that the - proc-macro be written in an entirely other crate. We don't need that much - power, so for our examples we'll go with the macro_rules version and just do - it by hand in the few cases where we need a generic newtype. + see an example of soon enough. To do this you might _think_ that we can change + the `:ident` declarations to `:ty`, but since we're declaring a fresh type not + using an existing type we have to accept it as an `:ident`. The way you get + around this is with a proc-macro, which is a lot more powerful but which also + requires that you write the proc-macro in an entirely other crate that gets + compiled first. We don't need that much power, so for our examples we'll go + with the macro_rules version and just do it by hand in the few cases where we + need a generic newtype. +* Allowing for `Deref` and `DerefMut`, which usually defeats the point of doing + the newtype, but maybe sometimes it's the right thing, so if you were going + for the full industrial strength version with a proc-macro and all you might + want to make that part of your optional add-ons as well the same way you might + want optional `From`. You'd probably want `From` to be "on by default" and + `Deref`/`DerefMut` to be "off by default", but whatever. -**As a reminder:** remember that macros have to appear _before_ they're invoked in -your source, so the `newtype` macro will always have to be at the very top of -your file, or in a module that's declared before other modules and code. +**As a reminder:** remember that `macro_rules` macros have to appear _before_ +they're invoked in your source, so the `newtype` macro will always have to be at +the very top of your file, or if you put it in a module within your project +you'll need to declare the module before anything that uses it. diff --git a/book/src/01-limitations/03-volatile_destination.md b/book/src/01-limitations/03-volatile_destination.md index 693d129..3ce45a9 100644 --- a/book/src/01-limitations/03-volatile_destination.md +++ b/book/src/01-limitations/03-volatile_destination.md @@ -1 +1,116 @@ # Volatile Destination + +There's a reasonable chance that you've never heard of `volatile` before, so +what's that? Well, it's a slightly overloaded term, but basically it means "get +your grubby mitts off my stuff you over-eager compiler". + +## Volatile Memory + +The first, and most common, form of volatile thing is volatile memory. Volatile +memory can change without your program changing it, usually because it's not a +location in RAM, but instead some special location that represents an actual +hardware device, or part of a hardware device perhaps. The compiler doesn't know +what's going on in this situation, but when the program is actually run and the +CPU gets an instruction to read or write from that location, instead of just +accessing some place in RAM like with normal memory, it accesses whatever bit of +hardware and does _something_. The details of that something depend on the +hardware, but what's important is that we need to actually, definitely execute +that read or write instruction. + +This is like the opposite of how normal memory works. Normally when the compiler +sees us write values into variables and read values from variables, it's free to +optimize those expressions and eliminate some of the reads and writes if it can, +and generally try to save us time. Maybe it even knows some stuff about the data +dependencies in our expressions and so it does some of the reads or writes out +of order from what the source says, because the compiler knows that it won't +actually make a difference to the operation of the program. A good and helpful +friend, that compiler. + +Volatile memory works almost the exact opposite way. With volatile memory we +need the compiler to _definitely_ emit an instruction to do a read or write and +they need to happen _exactly_ in the order that we say to do it. Each volatile +read or write might have any sort of unknown side effect that the compiler +doesn't know about and it shouldn't try to be clever about it. Just do what we +say, please. + +In Rust, we don't mark volatile things as being a separate type of thing, +instead we use normal raw pointers and then call the +[read_volatile](https://doc.rust-lang.org/core/ptr/fn.read_volatile.html) and +[write_volatile](https://doc.rust-lang.org/core/ptr/fn.write_volatile.html) +functions (also available as methods, if you like), which then delegate to the +LLVM +[volatile_load](https://doc.rust-lang.org/core/intrinsics/fn.volatile_load.html) +and +[volatile_store](https://doc.rust-lang.org/core/intrinsics/fn.volatile_store.html) +intrinsics. In C and C++ you can tag a pointer as being volatile and then any +normal read and write with it becomes the volatile version, but in Rust we have +to remember to use the correct alternate function instead. + +I'm told by the experts that this makes for a cleaner and saner design from a +_language design_ perspective, but it really kinda screws us when doing low +level code. References, both mutable and shared, aren't volatile, so they +compile into normal reads and writes. This means we can't do anything we'd +normally do in Rust that utilizes references of any kind. Volatile blocks of +memory can't use normal `.iter()` or `.iter_mut()` based iteration (which give +`&T` or `&mut T`), and they also can't use normal `Index` and `IndexMut` sugar +like `a + x[i]` or `x[i] = 7`. + +Unlike with normal raw pointers, this pain point never goes away. There's no way +to abstract over the difference with Rust as it exists now, you'd need to +actually adjust the core language by adding an additional pointer type (`*vol +T`) and possibly a reference type to go with it (`&vol T`) to get the right +semantics. And then you'd need an `IndexVol` trait, and you'd need +`.iter_vol()`, and so on for every other little thing. It would be a lot of +work, and the Rust developers just aren't interested in doing all that for such +a limited portion of their user population. We'll just have to deal with not +having any syntax sugar. + +But no syntax sugar doesn't mean we can't at least do a little work for +ourselves. Enter the `VolatilePtr` type, which is a newtype over a `*mut T`: + +```rust +#[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)] +#[repr(transparent)] +pub struct VolatilePtr(*mut T); +``` + +Obviously we'll need some methods go with it. The basic operations are reading +and writing of course: + +```rust +impl VolatilePtr { + /// Performs a `read_volatile`. + pub unsafe fn read(&self) -> T { + self.0.read_volatile() + } + + /// Performs a `write_volatile`. + pub unsafe fn write(&self, data: T) { + self.0.write_volatile(data); + } +``` + +And we want a way to jump around when we do have volatile memory that's in +blocks. For this there's both +[offset](https://doc.rust-lang.org/std/primitive.pointer.html#method.offset) and +[wrapping_offset](https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_offset). +The difference is that `offset` optimizes better, but also it can be Undefined +Behavior if the result is not "in bounds or one byte past the end of the same +allocated object". I asked [ubsan](https://github.com/ubsan) (who is the expert +that you should always listen to on matters like this) what that means for us, +and the answer was that you _can_ use an `offset` in statically memory mapped +situations like this as long as you don't use it to jump to the address of +something that Rust itself allocated at some point. Unfortunately, the downside +to using `offset` instead of `wrapping_offset` is that with `offset`, it's +Undefined Behavior _simply to calculate the out of bounds result_, and with +`wrapping_offset` it's not Undefined Behavior until you _use_ the out of bounds +result. + +```rust + /// Performs a `wrapping_offset`. + pub unsafe fn offset(self, count: isize) -> Self { + VolatilePtr(self.0.offset(count)) + } +``` + +## Volatile ASM \ No newline at end of file From bbd0617602f1ec58127d5b7bbcf8ef9681f64bea Mon Sep 17 00:00:00 2001 From: Lokathor Date: Mon, 10 Dec 2018 21:40:41 -0700 Subject: [PATCH 3/4] volatile work --- .../01-limitations/03-volatile_destination.md | 56 +++++++++++++++---- 1 file changed, 46 insertions(+), 10 deletions(-) diff --git a/book/src/01-limitations/03-volatile_destination.md b/book/src/01-limitations/03-volatile_destination.md index 3ce45a9..cbb14da 100644 --- a/book/src/01-limitations/03-volatile_destination.md +++ b/book/src/01-limitations/03-volatile_destination.md @@ -1,8 +1,8 @@ # Volatile Destination There's a reasonable chance that you've never heard of `volatile` before, so -what's that? Well, it's a slightly overloaded term, but basically it means "get -your grubby mitts off my stuff you over-eager compiler". +what's that? Well, it's a term that can be used in more than one context, but +basically it means "get your grubby mitts off my stuff you over-eager compiler". ## Volatile Memory @@ -17,7 +17,7 @@ hardware and does _something_. The details of that something depend on the hardware, but what's important is that we need to actually, definitely execute that read or write instruction. -This is like the opposite of how normal memory works. Normally when the compiler +This is not how normal memory works. Normally when the compiler sees us write values into variables and read values from variables, it's free to optimize those expressions and eliminate some of the reads and writes if it can, and generally try to save us time. Maybe it even knows some stuff about the data @@ -26,11 +26,11 @@ of order from what the source says, because the compiler knows that it won't actually make a difference to the operation of the program. A good and helpful friend, that compiler. -Volatile memory works almost the exact opposite way. With volatile memory we +Volatile memory works almost the opposite way. With volatile memory we need the compiler to _definitely_ emit an instruction to do a read or write and they need to happen _exactly_ in the order that we say to do it. Each volatile -read or write might have any sort of unknown side effect that the compiler -doesn't know about and it shouldn't try to be clever about it. Just do what we +read or write might have any sort of side effect that the compiler +doesn't know about, and it shouldn't try to be clever about the optimization. Just do what we say, please. In Rust, we don't mark volatile things as being a separate type of thing, @@ -80,12 +80,12 @@ and writing of course: ```rust impl VolatilePtr { /// Performs a `read_volatile`. - pub unsafe fn read(&self) -> T { + pub unsafe fn read(self) -> T { self.0.read_volatile() } /// Performs a `write_volatile`. - pub unsafe fn write(&self, data: T) { + pub unsafe fn write(self, data: T) { self.0.write_volatile(data); } ``` @@ -107,10 +107,46 @@ Undefined Behavior _simply to calculate the out of bounds result_, and with result. ```rust - /// Performs a `wrapping_offset`. + /// Performs a normal `offset`. pub unsafe fn offset(self, count: isize) -> Self { VolatilePtr(self.0.offset(count)) } ``` -## Volatile ASM \ No newline at end of file +Now, one thing of note is that doing the `offset` isn't `const`. If we wanted to have a `const` function for +finding the correct spot within a volatile block of memory we'd have to do all the math using `usize` values +and then cast that value into being a pointer once we were done. In the future Rust might be +able to do it without a goofy work around, but `const` is quite limited at the moment. +It'd look something like this: + +```rust +const fn address_index(address: usize, index: usize) -> usize { + address + (index * std::mem::size_of::()) +} +``` + +We will sometimes want to be able to cast a `VolatilePtr` between pointer types. Since we +won't be able to do that with `as`, we'll have to write a method for that: + +```rust + /// Performs a cast into some new pointer type. + pub fn cast(self) -> VolatilePtr { + VolatilePtr(self.0 as *mut Z) + } +``` + +TODO: iterator stuff + +Also, just as a little bonus that we probably won't use, we could enable our new pointer type +to be formatted as a pointer value. + +```rust +impl core::fmt::Pointer for VolatilePtr { + /// Formats exactly like the inner `*mut T`. + fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result { + write!(f, "{:p}", self.0) + } +} +``` + +## Volatile ASM From 8e4299b0ad7d463fb8f12f2aa1610fd47068682a Mon Sep 17 00:00:00 2001 From: Lokathor Date: Tue, 11 Dec 2018 01:27:10 -0700 Subject: [PATCH 4/4] volatile complete --- .../01-limitations/03-volatile_destination.md | 239 ++++++++++++++++-- 1 file changed, 215 insertions(+), 24 deletions(-) diff --git a/book/src/01-limitations/03-volatile_destination.md b/book/src/01-limitations/03-volatile_destination.md index cbb14da..d635e88 100644 --- a/book/src/01-limitations/03-volatile_destination.md +++ b/book/src/01-limitations/03-volatile_destination.md @@ -65,17 +65,19 @@ work, and the Rust developers just aren't interested in doing all that for such a limited portion of their user population. We'll just have to deal with not having any syntax sugar. -But no syntax sugar doesn't mean we can't at least do a little work for -ourselves. Enter the `VolatilePtr` type, which is a newtype over a `*mut T`: +### VolatilePtr + +No syntax sugar doesn't mean we can't at least make things a little easier for +ourselves. Enter the `VolatilePtr` type, which is a newtype over a `*mut T`. +One of those "manual" newtypes I mentioned where we can't use our nice macro. ```rust #[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)] #[repr(transparent)] -pub struct VolatilePtr(*mut T); +pub struct VolatilePtr(pub *mut T); ``` -Obviously we'll need some methods go with it. The basic operations are reading -and writing of course: +Obviously we want to be able to read and write: ```rust impl VolatilePtr { @@ -91,20 +93,23 @@ impl VolatilePtr { ``` And we want a way to jump around when we do have volatile memory that's in -blocks. For this there's both +blocks. This is where we can get ourselves into some trouble if we're not +careful. We have to decide between [offset](https://doc.rust-lang.org/std/primitive.pointer.html#method.offset) and [wrapping_offset](https://doc.rust-lang.org/std/primitive.pointer.html#method.wrapping_offset). The difference is that `offset` optimizes better, but also it can be Undefined Behavior if the result is not "in bounds or one byte past the end of the same allocated object". I asked [ubsan](https://github.com/ubsan) (who is the expert -that you should always listen to on matters like this) what that means for us, -and the answer was that you _can_ use an `offset` in statically memory mapped +that you should always listen to on matters like this) what that means exactly +when memory mapped hardware is involved (since we never allocated anything), and +the answer was that you _can_ use an `offset` in statically memory mapped situations like this as long as you don't use it to jump to the address of -something that Rust itself allocated at some point. Unfortunately, the downside -to using `offset` instead of `wrapping_offset` is that with `offset`, it's -Undefined Behavior _simply to calculate the out of bounds result_, and with -`wrapping_offset` it's not Undefined Behavior until you _use_ the out of bounds -result. +something that Rust itself allocated at some point. Cool, we all like being able +to use the one that optimizes better. Unfortunately, the downside to using +`offset` instead of `wrapping_offset` is that with `offset`, it's Undefined +Behavior _simply to calculate the out of bounds result_ (with `wrapping_offset` +it's not Undefined Behavior until you _use_ the out of bounds result). We'll +have to be quite careful when we're using `offset`. ```rust /// Performs a normal `offset`. @@ -113,20 +118,25 @@ result. } ``` -Now, one thing of note is that doing the `offset` isn't `const`. If we wanted to have a `const` function for -finding the correct spot within a volatile block of memory we'd have to do all the math using `usize` values -and then cast that value into being a pointer once we were done. In the future Rust might be -able to do it without a goofy work around, but `const` is quite limited at the moment. -It'd look something like this: +Now, one thing of note is that doing the `offset` isn't `const`. The math for it +is something that's possible to do in a `const` way of course, but Rust +basically doesn't allow you to fiddle raw pointers much during `const` right +now. Maybe in the future that will improve. + +If we did want to have a `const` function for finding the correct address within +a volatile block of memory we'd have to do all the math using `usize` values, +and then cast that value into being a pointer once we were done. It'd look +something like this: ```rust const fn address_index(address: usize, index: usize) -> usize { - address + (index * std::mem::size_of::()) + address + (index * std::mem::size_of::()) } ``` -We will sometimes want to be able to cast a `VolatilePtr` between pointer types. Since we -won't be able to do that with `as`, we'll have to write a method for that: +But, back to methods for `VolatilePtr`, well we sometimes want to be able to +cast a `VolatilePtr` between pointer types. Since we won't be able to do that +with `as`, we'll have to write a method for it: ```rust /// Performs a cast into some new pointer type. @@ -135,10 +145,116 @@ won't be able to do that with `as`, we'll have to write a method for that: } ``` -TODO: iterator stuff +### Volatile Iterating -Also, just as a little bonus that we probably won't use, we could enable our new pointer type -to be formatted as a pointer value. +How about that `Iterator` stuff I said we'd be missing? We can actually make +_an_ Iterator available, it's just not the normal "iterate by shared reference +or unique reference" Iterator. Instead, it's more like a "throw out a series of +`VolatilePtr` values" style Iterator. Other than that small difference it's +totally normal, and we'll be able to use map and skip and take and all those +neat methods. + +So how do we make this thing we need? First we check out the [Implementing +Iterator](https://doc.rust-lang.org/core/iter/index.html#implementing-iterator) +section in the core documentation. It says we need a struct for holding the +iterator state. Right-o, probably something like this: + +```rust +#[derive(Debug, Clone, Hash, PartialEq, Eq)] +pub struct VolatilePtrIter { + vol_ptr: VolatilePtr, + slots: usize, +} +``` + +And then we just implement +[core::iter::Iterator](https://doc.rust-lang.org/core/iter/trait.Iterator.html) +on that struct. Wow, that's quite the trait though! Don't worry, we only need to +implement two small things and then the rest of it comes free as a bunch of +default methods. + +So, the code that we _want_ to write looks like this: + +```rust +impl Iterator for VolatilePtrIter { + type Item = VolatilePtr; + + fn next(&mut self) -> Option> { + if self.slots > 0 { + let out = Some(self.vol_ptr); + self.slots -= 1; + self.vol_ptr = unsafe { self.vol_ptr.offset(1) }; + out + } else { + None + } + } +} +``` + +Except we _can't_ write that code. What? The problem is that we used +`derive(Clone, Copy` on `VolatilePtr`. Because of a quirk in how `derive` works, +this makes `VolatilePtr` will only be `Copy` if the `T` is `Copy`, _even +though the pointer itself is always `Copy` regardless of what it points to_. +Ugh, terrible. We've got three basic ways to handle this: + +* Make the `Iterator` implementation be for ``, and then hope that we + always have types that are `Clone`. +* Hand implement every trait we want `VolatilePtr` (and `VolatilePtrIter`) to + have so that we can override the fact that `derive` is basically broken in + this case. +* Make `VolatilePtr` store a `usize` value instead of a pointer, and then cast + it to `*mut T` when we actually need to read and write. This would require us + to also store a `PhantomData` so that the type of the address is tracked + properly, which would make it a lot more verbose to construct a `VolatilePtr` + value. + +None of those options are particularly appealing. I guess we'll do the first one +because it's the least amount of up front trouble, and I don't _think_ we'll +need to be iterating non-Clone values. All we do to pick that option is add the +bound to the very start of the `impl` block, where we introduce the `T`: + +```rust +impl Iterator for VolatilePtrIter { + type Item = VolatilePtr; + + fn next(&mut self) -> Option> { + if self.slots > 0 { + let out = Some(self.vol_ptr.clone()); + self.slots -= 1; + self.vol_ptr = unsafe { self.vol_ptr.clone().offset(1) }; + out + } else { + None + } + } +} +``` + +What's going on here? Okay so our iterator has a number of slots that it'll go +over, and then when it's out of slots it starts producing `None` forever. That's +actually pretty simple. We're also masking some unsafety too. In this case, +we'll rely on the person who made the `VolatilePtrIter` to have selected the +correct number of slots. This gives us a new method for `VolatilePtr`: + +```rust + pub unsafe fn iter_slots(self, slots: usize) -> VolatilePtrIter { + VolatilePtrIter { + vol_ptr: self, + slots, + } + } +``` + +With this design, making the `VolatilePtrIter` at the start is `unsafe` (we have +to trust the caller that the right number of slots exists), and then using it +after that is totally safe (if the right number of slots was given we'll never +screw up our end of it). + +### VolatilePtr Formatting + +Also, just as a little bonus that we probably won't use, we could enable our new +pointer type to be formatted as a pointer value. ```rust impl core::fmt::Pointer for VolatilePtr { @@ -149,4 +265,79 @@ impl core::fmt::Pointer for VolatilePtr { } ``` +Neat! + +### VolatilePtr Complete + +That was a lot of small code blocks, let's look at it all put together: + +```rust +#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)] +#[repr(transparent)] +pub struct VolatilePtr(pub *mut T); +impl VolatilePtr { + pub unsafe fn read(self) -> T { + self.0.read_volatile() + } + pub unsafe fn write(self, data: T) { + self.0.write_volatile(data); + } + pub unsafe fn offset(self, count: isize) -> Self { + VolatilePtr(self.0.offset(count)) + } + pub fn cast(self) -> VolatilePtr { + VolatilePtr(self.0 as *mut Z) + } + pub unsafe fn iter_slots(self, slots: usize) -> VolatilePtrIter { + VolatilePtrIter { + vol_ptr: self, + slots, + } + } +} +impl core::fmt::Pointer for VolatilePtr { + fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result { + write!(f, "{:p}", self.0) + } +} + +#[derive(Debug, Clone, Hash, PartialEq, Eq)] +pub struct VolatilePtrIter { + vol_ptr: VolatilePtr, + slots: usize, +} +impl Iterator for VolatilePtrIter { + type Item = VolatilePtr; + fn next(&mut self) -> Option> { + if self.slots > 0 { + let out = Some(self.vol_ptr.clone()); + self.slots -= 1; + self.vol_ptr = unsafe { self.vol_ptr.clone().offset(1) }; + out + } else { + None + } + } +} +``` + ## Volatile ASM + +In addition to some memory locations being volatile, it's also possible for +inline assembly to be declared volatile. This is basically the same idea, "hey +just do what I'm telling you, don't get smart about it". + +Normally when you have some `asm!` it's basically treated like a function, +there's inputs and outputs and the compiler will try to optimize it so that if +you don't actually use the outputs it won't bother with doing those +instructions. However, `asm!` is basically a pure black box, so the compiler +doesn't know what's happening inside at all, and it can't see if there's any +important side effects going on. + +An example of an important side effect that doesn't have output values would be +putting the CPU into a low power state while we want for the next VBlank. This +lets us save quite a bit of battery power. It requires some setup to be done +safely (otherwise the GBA won't ever actually wake back up from the low power +state), but the `asm!` you use once you're ready is just a single instruction +with no return value. The compiler can't tell what's going on, so you just have +to say "do it anyway".