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March 13, 2024 20:26
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MLIR Left pass combine
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| #include "mlir/IR/Dialect.h" | |
| #include "mlir/InitAllDialects.h" | |
| #include "mlir/InitAllPasses.h" | |
| #include "mlir/Pass/Pass.h" | |
| #include "mlir/Pass/PassRegistry.h" | |
| #include "mlir/Tools/mlir-opt/MlirOptMain.h" | |
| #include "mlir/Transforms/GreedyPatternRewriteDriver.h" | |
| #include <cstddef> | |
| #include <stdio.h> | |
| // docs: https://mlir.llvm.org/docs/PatternRewriter/ | |
| // arith dialect: https://mlir.llvm.org/docs/Dialects/ArithOps/ | |
| // examples: https://github.com/llvm/llvm-project/tree/main/mlir/examples | |
| namespace mlir { | |
| using arith::ConstantIntOp; | |
| using arith::ConstantOp; | |
| using arith::ShLIOp; | |
| // Merge multiple left shifts with constant value to a single left shift | |
| // eg. (x << 4) << 16 => x << 20 | |
| struct LeftShiftPattern : public OpRewritePattern<ShLIOp> { | |
| LeftShiftPattern(mlir::MLIRContext *context) | |
| : OpRewritePattern<ShLIOp>(context, 1) {} | |
| void initialize() { | |
| setDebugName("LeftShift"); | |
| addDebugLabels("ConstantFoldLeftShiftPass"); | |
| } | |
| LogicalResult iterativeMatcher(ShLIOp inst4) const { | |
| // 1. %c4_i32 = arith.constant 4 : i32 | |
| // 2. %c16_i32 = arith.constant 16 : i32 | |
| // 3. %0 = arith.shli %arg0, %c4_i32 : i32 | |
| // 4. %1 = arith.shli %0, %c16_i32 : i32 <--- Match works for this | |
| // Check if the shift left value is a constant | |
| auto inst2 = inst4.getRhs().getDefiningOp<ConstantIntOp>(); | |
| auto inst3 = inst4.getLhs().getDefiningOp<ShLIOp>(); | |
| if (!inst2 || !inst3) { | |
| return failure(); | |
| } | |
| // Check if the shift left's shift left value is a constant shifter | |
| auto inst1 = inst3.getRhs().getDefiningOp<ConstantIntOp>(); | |
| if (!inst1) { | |
| return failure(); | |
| } | |
| return success(); | |
| } | |
| LogicalResult match(ShLIOp inst4) const override { | |
| return iterativeMatcher(inst4); | |
| } | |
| void rewrite(ShLIOp inst4, PatternRewriter &rewriter) const override { | |
| auto inst2 = inst4.getRhs().getDefiningOp<ConstantIntOp>(); | |
| auto inst3 = inst4.getLhs().getDefiningOp<ShLIOp>(); | |
| auto inst1 = inst3.getRhs().getDefiningOp<ConstantIntOp>(); | |
| const auto shiftAmount = inst1.value() + inst2.value(); | |
| const auto width = inst1.getType().getIntOrFloatBitWidth(); | |
| const auto variable = inst3.getLhs(); | |
| // /* | |
| // This if/else block could be removed, or combined with the below | |
| // instruction | |
| if (shiftAmount > width) { | |
| // TODO: If shift amount > int width, use 0 | |
| auto zero = rewriter.create<ConstantOp>( | |
| inst1->getLoc(), rewriter.getIntegerAttr(inst1.getType(), 0)); | |
| ValueRange newZero = {zero}; | |
| rewriter.replaceOp(inst4, newZero); | |
| rewriter.eraseOp(inst1); | |
| rewriter.eraseOp(inst2); | |
| rewriter.eraseOp(inst3); | |
| rewriter.eraseOp(inst4); | |
| return; | |
| } | |
| // */ | |
| ConstantIntOp arithConstantValue = rewriter.create<ConstantIntOp>( | |
| inst1.getLoc(), /*value*/ shiftAmount, | |
| /*width*/ inst1.getType().getIntOrFloatBitWidth()); | |
| // WARN: Why did I have to create ValueRange? | |
| // The rewriter.replace(op, newOp) did not work | |
| ValueRange newConst = {arithConstantValue}; | |
| rewriter.replaceOp(inst1, newConst); | |
| rewriter.eraseOp(inst1); | |
| ShLIOp newShl = | |
| rewriter.create<ShLIOp>(inst4.getLoc(), variable, arithConstantValue); | |
| ValueRange newShlRange = {newShl}; | |
| rewriter.replaceOp(inst4, newShlRange); | |
| rewriter.eraseOp(inst2); | |
| rewriter.eraseOp(inst3); | |
| rewriter.eraseOp(inst4); | |
| } | |
| }; | |
| class MultiToShiftPass | |
| : public PassWrapper<MultiToShiftPass, OperationPass<func::FuncOp>> { | |
| private: | |
| StringRef getArgument() const final { return "instcombine"; } | |
| StringRef getDescription() const final { | |
| return "A simple shift left combiner"; | |
| } | |
| void runOnOperation() override { | |
| RewritePatternSet patterns(&getContext()); | |
| patterns.add<LeftShiftPattern>(&getContext()); | |
| (void)applyPatternsAndFoldGreedily(getOperation(), std::move(patterns)); | |
| } | |
| }; | |
| } // namespace mlir | |
| int main(int argc, char **argv) { | |
| mlir::registerAllPasses(); | |
| mlir::PassRegistration<mlir::MultiToShiftPass>(); | |
| mlir::DialectRegistry registry; | |
| registerAllDialects(registry); | |
| return mlir::asMainReturnCode( | |
| mlir::MlirOptMain(argc, argv, "Custom optimizer driver\n", registry)); | |
| } |
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