[mlir] Fix exiting OpPatternRewriteDriver::simplifyLocally after first iteration that didn't change the op.

[csigg]

Summary

Before this change, we would run maxIterations if the first iteration changed the op.
After this change, we exit the loop as soon as an iteration hasn't changed the op.
Assuming that we have reached a fixed point when an iteration doesn't change the op, this doesn't affect correctness.

[rriddle]

Can you add a line or two to the description for context on the fix? This just helps reach a fixed point faster and shouldn't affect correctness at all.

Thanks for the fix!

mlir/lib/Transforms/Utils/GreedyPatternRewriteDriver.cpp

302

Can you keep the initialization here anyways? It's a nice to sanity check to always have variables initialized to a good default.

[csigg]

Keep 'changed' initialized.

[sanjoy.google]

mlir/lib/Transforms/Utils/GreedyPatternRewriteDriver.cpp

302

I've heard the opposite advice -- you should leave variables uninitialized when they're not expected to be used, that way asan will let you know if your assumption is violated.

In this trivial case it doesn't matter though.

mlir/lib/Transforms/Utils/GreedyPatternRewriteDriver.cpp

private:
   bool opErasedViaPatternRewrites = false;
 };
 
 } // anonymous namespace
 
 LogicalResult OpPatternRewriteDriver::simplifyLocally(Operation *op,
                                                       int maxIterations,
                                                       bool &erased) {
   bool changed = false;
   erased = false;
   opErasedViaPatternRewrites = false;
   int i = 0;
   // Iterate until convergence or until maxIterations. Deletion of the op as
   // a result of being dead or folded is convergence.
   do {
+    changed = false;
+
     // If the operation is trivially dead - remove it.
     if (isOpTriviallyDead(op)) {
       op->erase();
       erased = true;
       return success();
     }
 
     // Try to fold this op.