| Issue |
166039
|
| Summary |
ConditionPropagation can be slow
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
wsmoses
|
x/ref:
```
struct ConditionPropagation : public OpRewritePattern<IfOp> {
using OpRewritePattern<IfOp>::OpRewritePattern;
LogicalResult matchAndRewrite(IfOp op,
PatternRewriter &rewriter) const override {
// Early exit if the condition is constant since replacing a constant
// in the body with another constant isn't a simplification.
if (matchPattern(op.getCondition(), m_Constant()))
return failure();
bool changed = false;
mlir::Type i1Ty = rewriter.getI1Type();
// These variables serve to prevent creating duplicate constants
// and hold constant true or false values.
Value constantTrue = nullptr;
Value constantFalse = nullptr;
for (OpOperand &use :
llvm::make_early_inc_range(op.getCondition().getUses())) {
if (op.getThenRegion().isAncestor(use.getOwner()->getParentRegion())) {
changed = true;
if (!constantTrue)
constantTrue = arith::ConstantOp::create(
rewriter, op.getLoc(), i1Ty, rewriter.getIntegerAttr(i1Ty, 1));
rewriter.modifyOpInPlace(use.getOwner(),
[&]() { use.set(constantTrue); });
} else if (op.getElseRegion().isAncestor(
use.getOwner()->getParentRegion())) {
changed = true;
if (!constantFalse)
constantFalse = arith::ConstantOp::create(
rewriter, op.getLoc(), i1Ty, rewriter.getIntegerAttr(i1Ty, 0));
rewriter.modifyOpInPlace(use.getOwner(),
[&]() { use.set(constantFalse); });
}
}
return success(changed);
}
};
```
If the condition has many uses (not just within this if statement), the query of isAncestor can be quite expensive (and in practice is quite expensive when cmopiling large c++ codebases with polygeist/reactant).
We can create a cache where if we've previously proven a region to be within then/else/neither, we don't need to re-query.
cc @ftynse
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