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//! Workarounds for platform bugs and limitations in switches and loops.
//!
//! In these docs, we use CamelCase links for Naga IR concepts, and ordinary
//! `code` formatting for HLSL or GLSL concepts.
//!
//! ## Avoiding `continue` within `switch`
//!
//! compiler doesn't allow `continue` statements within `switch` statements, but
//! Naga IR does. We work around this by introducing synthetic boolean local
//! variables and branches.
//!
//! Specifically:
//!
//! - We generate code for [`Continue`] statements within [`SwitchCase`]s that
//! sets an introduced `bool` local to `true` and does a `break`, jumping to
//! immediately after the generated `switch`.
//!
//! - When generating code for a [`Switch`] statement, we conservatively assume
//! it might contain such a [`Continue`] statement, so:
//!
//! - If it's the outermost such [`Switch`] within a [`Loop`], we declare the
//! `bool` local ahead of the switch, initialized to `false`. Immediately
//! after the `switch`, we check the local and do a `continue` if it's set.
//!
//! - If the [`Switch`] is nested within other [`Switch`]es, then after the
//! generated `switch`, we check the local (which we know was declared
//! before the surrounding `switch`) and do a `break` if it's set.
//!
//! - As an optimization, we only generate the check of the local if a
//! [`Continue`] statement is encountered within the [`Switch`]. This may
//! help drivers more easily identify that the `bool` is unused.
//!
//! So while we "weaken" the [`Continue`] statement by rendering it as a `break`
//! statement, we also place checks immediately at the locations to which those
//! `break` statements will jump, until we can be sure we've reached the
//! intended target of the original [`Continue`].
//!
//! In the case of nested [`Loop`] and [`Switch`] statements, there may be
//! multiple introduced `bool` locals in scope, but there's no problem knowing
//! which one to operate on. At any point, there is at most one [`Loop`]
//! statement that could be targeted by a [`Continue`] statement, so the correct
//! `bool` local to set and test is always the one introduced for the innermost
//! enclosing [`Loop`]'s outermost [`Switch`].
//!
//! # Avoiding single body `switch` statements
//!
//! front ends miscompile `switch` statements where all cases branch to the same
//! body. Our HLSL and GLSL backends render [`Switch`] statements with a single
//! [`SwitchCase`] as `do {} while(false);` loops.
//!
//! However, this rewriting introduces a new loop that could "capture"
//! `continue` statements in its body. To avoid doing so, we apply the
//! [`Continue`]-to-`break` transformation described above.
//!
//! [`Continue`]: crate::Statement::Continue
//! [`Loop`]: crate::Statement::Loop
//! [`Switch`]: crate::Statement::Switch
//! [`SwitchCase`]: crate::SwitchCase
use crate::proc::Namer;
use std::rc::Rc;
/// A summary of the code surrounding a statement.
enum Nesting {
/// Currently nested in at least one [`Loop`] statement.
///
/// [`Continue`] should apply to the innermost loop.
///
/// When this entry is on the top of the stack:
///
/// * When entering an inner [`Loop`] statement, push a [`Loop`][nl] state
/// onto the stack.
///
/// * When entering a nested [`Switch`] statement, push a [`Switch`][ns]
/// state onto the stack with a new variable name. Before the generated
/// `switch`, introduce a `bool` local with that name, initialized to
/// `false`.
///
/// When exiting the [`Loop`] for which this entry was pushed, pop it from
/// the stack.
///
/// [`Continue`]: crate::Statement::Continue
/// [`Loop`]: crate::Statement::Loop
/// [`Switch`]: crate::Statement::Switch
/// [ns]: Nesting::Switch
/// [nl]: Nesting::Loop
Loop,
/// Currently nested in at least one [`Switch`] that may need to forward
/// [`Continue`]s.
///
/// This includes [`Switch`]es rendered as `do {} while(false)` loops, but
/// doesn't need to include regular [`Switch`]es in backends that can
/// support `continue` within switches.
///
/// [`Continue`] should be forwarded to the innermost surrounding [`Loop`].
///
/// When this entry is on the top of the stack:
///
/// * When entering a nested [`Loop`], push a [`Loop`][nl] state onto the
/// stack.
///
/// * When entering a nested [`Switch`], push a [`Switch`][ns] state onto
/// the stack with a clone of the introduced `bool` variable's name.
///
/// * When encountering a [`Continue`] statement, render it as code to set
/// the introduced `bool` local (whose name is held in [`variable`]) to
/// `true`, and then `break`. Set [`continue_encountered`] to `true` to
/// record that the [`Switch`] contains a [`Continue`].
///
/// * When exiting this [`Switch`], pop its entry from the stack. If
/// [`continue_encountered`] is set, then we have rendered [`Continue`]
/// statements as `break` statements that jump to this point. Generate
/// code to check `variable`, and if it is `true`:
///
/// * If there is another [`Switch`][ns] left on top of the stack, set
/// its `continue_encountered` flag, and generate a `break`. (Both
/// [`Switch`][ns]es are within the same [`Loop`] and share the same
/// introduced variable, so there's no need to set another flag to
/// continue to exit the `switch`es.)
///
/// * Otherwise, `continue`.
///
/// When we exit the [`Switch`] for which this entry was pushed, pop it.
///
/// [`Continue`]: crate::Statement::Continue
/// [`Loop`]: crate::Statement::Loop
/// [`Switch`]: crate::Statement::Switch
/// [`variable`]: Nesting::Switch::variable
/// [`continue_encountered`]: Nesting::Switch::continue_encountered
/// [ns]: Nesting::Switch
/// [nl]: Nesting::Loop
Switch {
variable: Rc<String>,
/// Set if we've generated code for a [`Continue`] statement with this
/// entry on the top of the stack.
///
/// If this is still clear when we finish rendering the [`Switch`], then
/// we know we don't need to generate branch forwarding code. Omitting
/// that may make it easier for drivers to tell that the `bool` we
/// introduced ahead of the [`Switch`] is actually unused.
///
/// [`Continue`]: crate::Statement::Continue
/// [`Switch`]: crate::Statement::Switch
continue_encountered: bool,
},
}
/// A micro-IR for code a backend should generate after a [`Switch`].
///
/// [`Switch`]: crate::Statement::Switch
pub(super) enum ExitControlFlow {
None,
/// Emit `if (continue_variable) { continue; }`
Continue {
variable: Rc<String>,
},
/// Emit `if (continue_variable) { break; }`
///
/// Used after a [`Switch`] to exit from an enclosing [`Switch`].
///
/// After the enclosing switch, its associated check will consult this same
/// variable, see that it is set, and exit early.
///
/// [`Switch`]: crate::Statement::Switch
Break {
variable: Rc<String>,
},
}
/// Utility for tracking nesting of loops and switches to orchestrate forwarding
/// of continue statements inside of a switch to the enclosing loop.
///
/// See [module docs](self) for why we need this.
#[derive(Default)]
pub(super) struct ContinueCtx {
stack: Vec<Nesting>,
}
impl ContinueCtx {
/// Resets internal state.
///
/// Use this to reuse memory between writing sessions.
pub fn clear(&mut self) {
self.stack.clear();
}
/// Updates internal state to record entering a [`Loop`] statement.
///
/// [`Loop`]: crate::Statement::Loop
pub fn enter_loop(&mut self) {
self.stack.push(Nesting::Loop);
}
/// Updates internal state to record exiting a [`Loop`] statement.
///
/// [`Loop`]: crate::Statement::Loop
pub fn exit_loop(&mut self) {
if !matches!(self.stack.pop(), Some(Nesting::Loop)) {
unreachable!("ContinueCtx stack out of sync");
}
}
/// Updates internal state to record entering a [`Switch`] statement.
///
/// Return `Some(variable)` if this [`Switch`] is nested within a [`Loop`],
/// and the caller should introcue a new `bool` local variable named
/// `variable` above the `switch`, for forwarding [`Continue`] statements.
///
/// `variable` is guaranteed not to conflict with any names used by the
/// program itself.
///
/// [`Continue`]: crate::Statement::Continue
/// [`Loop`]: crate::Statement::Loop
/// [`Switch`]: crate::Statement::Switch
pub fn enter_switch(&mut self, namer: &mut Namer) -> Option<Rc<String>> {
match self.stack.last() {
// If the stack is empty, we are not in loop, so we don't need to
// forward continue statements within this `Switch`. We can leave
// the stack empty.
None => None,
Some(&Nesting::Loop { .. }) => {
let variable = Rc::new(namer.call("should_continue"));
self.stack.push(Nesting::Switch {
variable: Rc::clone(&variable),
continue_encountered: false,
});
Some(variable)
}
Some(&Nesting::Switch { ref variable, .. }) => {
self.stack.push(Nesting::Switch {
variable: Rc::clone(variable),
continue_encountered: false,
});
// We have already declared the variable before some enclosing
// `Switch`.
None
}
}
}
/// Update internal state to record leaving a [`Switch`] statement.
///
/// Return an [`ExitControlFlow`] value indicating what code should be
/// introduced after the generated `switch` to forward continues.
///
/// [`Switch`]: crate::Statement::Switch
pub fn exit_switch(&mut self) -> ExitControlFlow {
match self.stack.pop() {
// This doesn't indicate a problem: we don't start pushing entries
// for `Switch` statements unless we have an enclosing `Loop`.
None => ExitControlFlow::None,
Some(Nesting::Loop { .. }) => {
unreachable!("Unexpected loop state when exiting switch");
}
Some(Nesting::Switch {
variable,
continue_encountered: inner_continue,
}) => {
if !inner_continue {
// No `Continue` statement was encountered, so we didn't
// introduce any `break`s jumping to this point.
ExitControlFlow::None
} else if let Some(&mut Nesting::Switch {
continue_encountered: ref mut outer_continue,
..
}) = self.stack.last_mut()
{
// This is nested in another `Switch`. Propagate upwards
// that there is a continue statement present.
*outer_continue = true;
ExitControlFlow::Break { variable }
} else {
ExitControlFlow::Continue { variable }
}
}
}
}
/// Determine what to generate for a [`Continue`] statement.
///
/// If we can generate an ordinary `continue` statement, return `None`.
///
/// Otherwise, we're enclosed by a [`Switch`] that is itself enclosed by a
/// [`Loop`]. Return `Some(variable)` to indicate that the [`Continue`]
/// should be rendered as setting `variable` to `true`, and then doing a
/// `break`.
///
/// This also notes that we've encountered a [`Continue`] statement, so that
/// we can generate the right code to forward the branch following the
/// enclosing `switch`.
///
/// [`Continue`]: crate::Statement::Continue
/// [`Loop`]: crate::Statement::Loop
/// [`Switch`]: crate::Statement::Switch
pub fn continue_encountered(&mut self) -> Option<&str> {
if let Some(&mut Nesting::Switch {
ref variable,
ref mut continue_encountered,
}) = self.stack.last_mut()
{
*continue_encountered = true;
Some(variable)
} else {
None
}
}
}