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[mlir][spirv] Update mergeInfo of blocks nested in regions #137789

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Merged
merged 1 commit into from
Apr 30, 2025
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[mlir][spirv] Update mergeInfo of blocks nested in regions #137789

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86 changes: 62 additions & 24 deletions mlir/lib/Target/SPIRV/Deserialization/Deserializer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2105,37 +2105,75 @@ LogicalResult ControlFlowStructurizer::structurize() {
// selection/loop. If so, they will be recorded within blockMergeInfo.
// We need to update the pointers there to the newly remapped ones so we can
// continue structurizing them later.
//
// We need to walk each block as constructBlocks do not include blocks
// internal to ops already structured within those blocks. It is not
// fully clear to me why the mergeInfo of blocks (yet to be structured)
// inside already structured selections/loops get invalidated and needs
// updating, however the following example code can cause a crash (depending
// on the structuring order), when the most inner selection is being
// structured after the outer selection and loop have been already
// structured:
//
// spirv.mlir.for {
// // ...
// spirv.mlir.selection {
// // ..
// // A selection region that hasn't been yet structured!
// // ..
// }
// // ...
// }
//
// If the loop gets structured after the outer selection, but before the
// inner selection. Moving the already structured selection inside the loop
// will invalidate the mergeInfo of the region that is not yet structured.
// Just going over constructBlocks will not check and updated header blocks
// inside the already structured selection region. Walking block fixes that.
//
// TODO: If structuring was done in a fixed order starting with inner
// most constructs this most likely not be an issue and the whole code
// section could be removed. However, with the current non-deterministic
// order this is not possible.
//
// TODO: The asserts in the following assumes input SPIR-V blob forms
// correctly nested selection/loop constructs. We should relax this and
// support error cases better.
auto it = blockMergeInfo.find(block);
if (it != blockMergeInfo.end()) {
// Use the original location for nested selection/loop ops.
Location loc = it->second.loc;

Block *newHeader = mapper.lookupOrNull(block);
if (!newHeader)
return emitError(loc, "failed control flow structurization: nested "
"loop header block should be remapped!");

Block *newContinue = it->second.continueBlock;
if (newContinue) {
newContinue = mapper.lookupOrNull(newContinue);
if (!newContinue)
auto updateMergeInfo = [&](Block *block) -> WalkResult {
auto it = blockMergeInfo.find(block);
if (it != blockMergeInfo.end()) {
// Use the original location for nested selection/loop ops.
Location loc = it->second.loc;

Block *newHeader = mapper.lookupOrNull(block);
if (!newHeader)
return emitError(loc, "failed control flow structurization: nested "
"loop continue block should be remapped!");
"loop header block should be remapped!");

Block *newContinue = it->second.continueBlock;
if (newContinue) {
newContinue = mapper.lookupOrNull(newContinue);
if (!newContinue)
return emitError(loc, "failed control flow structurization: nested "
"loop continue block should be remapped!");
}

Block *newMerge = it->second.mergeBlock;
if (Block *mappedTo = mapper.lookupOrNull(newMerge))
newMerge = mappedTo;

// The iterator should be erased before adding a new entry into
// blockMergeInfo to avoid iterator invalidation.
blockMergeInfo.erase(it);
blockMergeInfo.try_emplace(newHeader, loc, it->second.control, newMerge,
newContinue);
}

Block *newMerge = it->second.mergeBlock;
if (Block *mappedTo = mapper.lookupOrNull(newMerge))
newMerge = mappedTo;
return WalkResult::advance();
};

// The iterator should be erased before adding a new entry into
// blockMergeInfo to avoid iterator invalidation.
blockMergeInfo.erase(it);
blockMergeInfo.try_emplace(newHeader, loc, it->second.control, newMerge,
newContinue);
}
if (block->walk(updateMergeInfo).wasInterrupted())
return failure();

// The structured selection/loop's entry block does not have arguments.
// If the function's header block is also part of the structured control
Expand Down