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spv/lift: elide empty targets of an if-else/switch, as allowed by SPIR-V. #28

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spv/lift: elide empty targets of an if-else/switch, as allowed by SPIR-V. #28

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62 changes: 56 additions & 6 deletions src/spv/lift.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -863,22 +863,72 @@ impl<'a> FuncLifting<'a> {
// HACK(eddyb) this takes advantage of `blocks` being an `IndexMap`,
// to iterate at the same time as mutating other entries.
for block_idx in (0..blocks.len()).rev() {
let BlockLifting { terminator: original_terminator, .. } = &blocks[block_idx];
// HACK(eddyb) elide empty cases of an `if`-`else`/`switch`, as
// SPIR-V allows their targets to just be the whole merge block
// (the same one that `OpSelectionMerge` describes).
let block = &blocks[block_idx];
if let (cfg::ControlInstKind::SelectBranch(_), Some(Merge::Selection(merge_point))) =
(&*block.terminator.kind, block.terminator.merge)
{
for target_idx in 0..block.terminator.targets.len() {
let block = &blocks[block_idx];
let target = block.terminator.targets[target_idx];
if !block
.terminator
.target_phi_values
.get(&target)
.copied()
.unwrap_or_default()
.is_empty()
{
continue;
}

let target_is_trivial_branch = {
let BlockLifting {
phis,
insts,
terminator:
Terminator { attrs, kind, inputs, targets, target_phi_values, merge },
} = &blocks[&target];

(phis.is_empty()
&& insts.iter().all(|insts| insts.is_empty())
&& *attrs == AttrSet::default()
&& matches!(**kind, cfg::ControlInstKind::Branch)
&& inputs.is_empty()
&& targets.len() == 1
&& target_phi_values.is_empty()
&& merge.is_none())
.then(|| targets[0])
};
if let Some(target_of_target) = target_is_trivial_branch {
// FIXME(eddyb) what does it mean for this to not be true?
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Let's put an error or panic in here so if we hit it we are aware.

// (can it even happen?)
if target_of_target == merge_point {
blocks[block_idx].terminator.targets[target_idx] = target_of_target;
*use_counts.get_mut(&target).unwrap() -= 1;
*use_counts.get_mut(&target_of_target).unwrap() += 1;
}
}
}
}

let block = &blocks[block_idx];
let is_trivial_branch = {
let Terminator { attrs, kind, inputs, targets, target_phi_values, merge } =
original_terminator;
&block.terminator;

*attrs == AttrSet::default()
(*attrs == AttrSet::default()
&& matches!(**kind, cfg::ControlInstKind::Branch)
&& inputs.is_empty()
&& targets.len() == 1
&& target_phi_values.is_empty()
&& merge.is_none()
&& merge.is_none())
.then(|| targets[0])
};

if is_trivial_branch {
let target = original_terminator.targets[0];
if let Some(target) = is_trivial_branch {
let target_use_count = use_counts.get_mut(&target).unwrap();

if *target_use_count == 1 {
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