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Optimize switch sources representation and usage #96838

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May 10, 2022
Merged

Optimize switch sources representation and usage #96838

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fbc3cc1
Avoid constructing switch sources unless necessary
tmiasko May 8, 2022
2be012a
Use sparse representation of switch sources
tmiasko May 8, 2022
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2 changes: 2 additions & 0 deletions compiler/rustc_middle/src/mir/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -580,6 +580,8 @@ impl<'tcx> Body<'tcx> {
self.predecessor_cache.compute(&self.basic_blocks)
}

/// `body.switch_sources()[target][switch]` returns a list of switch values
/// that lead to a `target` block from a `switch` block.
#[inline]
pub fn switch_sources(&self) -> &SwitchSources {
self.switch_source_cache.compute(&self.basic_blocks)
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7 changes: 4 additions & 3 deletions compiler/rustc_mir_dataflow/src/framework/direction.rs
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Expand Up @@ -269,9 +269,9 @@ impl Direction for Backward {

mir::TerminatorKind::SwitchInt { targets: _, ref discr, switch_ty: _ } => {
let mut applier = BackwardSwitchIntEdgeEffectsApplier {
body,
pred,
exit_state,
values: &body.switch_sources()[bb][pred],
bb,
propagate: &mut propagate,
effects_applied: false,
Expand Down Expand Up @@ -305,9 +305,9 @@ impl Direction for Backward {
}

struct BackwardSwitchIntEdgeEffectsApplier<'a, D, F> {
body: &'a mir::Body<'a>,
pred: BasicBlock,
exit_state: &'a mut D,
values: &'a [Option<u128>],
bb: BasicBlock,
propagate: &'a mut F,

Expand All @@ -322,7 +322,8 @@ where
fn apply(&mut self, mut apply_edge_effect: impl FnMut(&mut D, SwitchIntTarget)) {
assert!(!self.effects_applied);

let targets = self.values.iter().map(|&value| SwitchIntTarget { value, target: self.bb });
let values = &self.body.switch_sources()[self.bb][self.pred];
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I don't expect this to affect much, but since we're only interested in self.pred, it could be cheaper to just look at the terminator of basic block with id self.pred and iterate over all targets filtering for ones for self.bb. Caching this map does not seem very useful if "computing" it for specific blocks is just a lookup and an iteration.

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Considering that this is the only user of switch_sources, I'm not sure it is carrying its weight.

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According to the discussion on the original pull request #95120, switch sources were introduced to avoid quadratic time complexity associated with the implementation strategy you are just suggesting.

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I guess if we have a switch with lots of targets... 🤷 added to my todo to do some experiments in the future

let targets = values.iter().map(|&value| SwitchIntTarget { value, target: self.bb });

let mut tmp = None;
for target in targets {
Expand Down