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//===- WrapInZeroTripCheck.cpp - Loop transforms to add zero-trip-check ---===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "mlir/Dialect/SCF/IR/SCF.h"
#include "mlir/Dialect/SCF/Transforms/Transforms.h"
#include "mlir/IR/IRMapping.h"
#include "mlir/IR/PatternMatch.h"
using namespace mlir;
/// Create zero-trip-check around a `while` op and return the new loop op in the
/// check. The while loop is rotated to avoid evaluating the condition twice.
///
/// Given an example below:
///
/// scf.while (%arg0 = %init) : (i32) -> i64 {
/// %val = .., %arg0 : i64
/// %cond = arith.cmpi .., %arg0 : i32
/// scf.condition(%cond) %val : i64
/// } do {
/// ^bb0(%arg1: i64):
/// %next = .., %arg1 : i32
/// scf.yield %next : i32
/// }
///
/// First clone before block to the front of the loop:
///
/// %pre_val = .., %init : i64
/// %pre_cond = arith.cmpi .., %init : i32
/// scf.while (%arg0 = %init) : (i32) -> i64 {
/// %val = .., %arg0 : i64
/// %cond = arith.cmpi .., %arg0 : i32
/// scf.condition(%cond) %val : i64
/// } do {
/// ^bb0(%arg1: i64):
/// %next = .., %arg1 : i32
/// scf.yield %next : i32
/// }
///
/// Create `if` op with the condition, rotate and move the loop into the else
/// branch:
///
/// %pre_val = .., %init : i64
/// %pre_cond = arith.cmpi .., %init : i32
/// scf.if %pre_cond -> i64 {
/// %res = scf.while (%arg1 = %va0) : (i64) -> i64 {
/// // Original after block
/// %next = .., %arg1 : i32
/// // Original before block
/// %val = .., %next : i64
/// %cond = arith.cmpi .., %next : i32
/// scf.condition(%cond) %val : i64
/// } do {
/// ^bb0(%arg2: i64):
/// %scf.yield %arg2 : i32
/// }
/// scf.yield %res : i64
/// } else {
/// scf.yield %pre_val : i64
/// }
FailureOr<scf::WhileOp> mlir::scf::wrapWhileLoopInZeroTripCheck(
scf::WhileOp whileOp, RewriterBase &rewriter, bool forceCreateCheck) {
// If the loop is in do-while form (after block only passes through values),
// there is no need to create a zero-trip-check as before block is always run.
if (!forceCreateCheck && isa<scf::YieldOp>(whileOp.getAfterBody()->front())) {
return whileOp;
}
OpBuilder::InsertionGuard insertion_guard(rewriter);
IRMapping mapper;
Block *beforeBlock = whileOp.getBeforeBody();
// Clone before block before the loop for zero-trip-check.
for (auto [arg, init] :
llvm::zip_equal(beforeBlock->getArguments(), whileOp.getInits())) {
mapper.map(arg, init);
}
rewriter.setInsertionPoint(whileOp);
for (auto &op : *beforeBlock) {
if (isa<scf::ConditionOp>(op)) {
break;
}
// Safe to clone everything as in a single block all defs have been cloned
// and added to mapper in order.
rewriter.insert(op.clone(mapper));
}
scf::ConditionOp condOp = whileOp.getConditionOp();
Value clonedCondition = mapper.lookupOrDefault(condOp.getCondition());
SmallVector<Value> clonedCondArgs = llvm::map_to_vector(
condOp.getArgs(), [&](Value arg) { return mapper.lookupOrDefault(arg); });
// Create rotated while loop.
auto newLoopOp = scf::WhileOp::create(
rewriter, whileOp.getLoc(), whileOp.getResultTypes(), clonedCondArgs,
[&](OpBuilder &builder, Location loc, ValueRange args) {
// Rotate and move the loop body into before block.
auto newBlock = builder.getBlock();
rewriter.mergeBlocks(whileOp.getAfterBody(), newBlock, args);
auto yieldOp = cast<scf::YieldOp>(newBlock->getTerminator());
rewriter.mergeBlocks(whileOp.getBeforeBody(), newBlock,
yieldOp.getResults());
rewriter.eraseOp(yieldOp);
},
[&](OpBuilder &builder, Location loc, ValueRange args) {
// Pass through values.
scf::YieldOp::create(builder, loc, args);
});
// Create zero-trip-check and move the while loop in.
auto ifOp = scf::IfOp::create(
rewriter, whileOp.getLoc(), clonedCondition,
[&](OpBuilder &builder, Location loc) {
// Then runs the while loop.
rewriter.moveOpBefore(newLoopOp, builder.getInsertionBlock(),
builder.getInsertionPoint());
scf::YieldOp::create(builder, loc, newLoopOp.getResults());
},
[&](OpBuilder &builder, Location loc) {
// Else returns the results from precondition.
scf::YieldOp::create(builder, loc, clonedCondArgs);
});
rewriter.replaceOp(whileOp, ifOp);
return newLoopOp;
}
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