[Mlir-commits] [mlir] [mlir][scf] Require positive step when uplifting scf.while (PR #226719)

[Ziliang Zhang]

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@@ -165,6 +165,14 @@ FailureOr<scf::ForOp> mlir::scf::upliftWhileToForLoop(RewriterBase &rewriter,
   if (!step || !dom.properlyDominates(step, loop))
     return rewriter.notifyMatchFailure(loop, "Invalid 'addi' form");
 
+  // `scf.for` requires a strictly positive step, and the exit-value
+  // reconstruction below relies on that same precondition.
+  APInt stepValue;
+  if (!matchPattern(step, m_ConstantInt(&stepValue)) ||
+      !stepValue.isStrictlyPositive())
+    return rewriter.notifyMatchFailure(loop,
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ziliangzl wrote:

Yes, this can pessimize cases where a dynamic step happens to be positive at runtime, because such loops will remain as scf.while.
However, without proving that the step is positive, the uplift is not semantics-preserving: scf.while permits zero or negative steps, while scf.for requires a strictly positive step. 

https://github.com/llvm/llvm-project/pull/226719