[Mlir-commits] [mlir] [mlir][SCF] Add support for peeling the first iteration out of the loop (PR #74015)

[Lei Zhang]

================
@@ -205,32 +222,84 @@ LogicalResult mlir::scf::peelForLoopAndSimplifyBounds(RewriterBase &rewriter,
   return success();
 }
 
+LogicalResult mlir::scf::peelFirstIterationForLoop(RewriterBase &b, ForOp forOp,
+                                                   ForOp &firstIteration) {
+  RewriterBase::InsertionGuard guard(b);
+  auto lbInt = getConstantIntValue(forOp.getLowerBound());
+  auto ubInt = getConstantIntValue(forOp.getUpperBound());
+  auto stepInt = getConstantIntValue(forOp.getStep());
+
+  // Peeling is not needed if there is one or less iteration.
+  if (lbInt && ubInt && stepInt && (*ubInt - *lbInt) / *stepInt <= 1)
+    return success();
+
+  AffineExpr sym0, sym1, sym2;
+  bindSymbols(b.getContext(), sym0, sym1, sym2);
+  SmallVector<Value> operands{forOp.getLowerBound(), forOp.getUpperBound(),
+                              forOp.getStep()};
+
+  // New lower bound for main loop: %lb + %step
+  auto ubMap = AffineMap::get(0, 2, {sym0 + sym1});
+  b.setInsertionPoint(forOp);
+  auto loc = forOp.getLoc();
+  Value splitBound = b.createOrFold<AffineApplyOp>(
+      loc, ubMap, ValueRange{forOp.getLowerBound(), forOp.getStep()});
+
+  // Peel the first iteration.
+  b.setInsertionPoint(forOp);
+  firstIteration = cast<ForOp>(b.clone(*forOp.getOperation()));
+  firstIteration.getUpperBoundMutable().assign(splitBound);
+
+  // Update main loop with new lower bound.
+  forOp.getInitArgsMutable().assign(firstIteration->getResults());
----------------
antiagainst wrote:

Put this in the `updateRootInPlace` too?

https://github.com/llvm/llvm-project/pull/74015