[Canonicalize] Don't call isBeforeInBlock in OperationFolder::tryToFold.

[lattner]

Summary

This patch (e4635e6328c8) fixed a bug where a newly generated/reused
constant wouldn't dominate a folded operation. It did so by calling
isBeforeInBlock to move the constant around on demand. This introduced
a significant compile time regression, because "isBeforeInBlock" is
O(n) in the size of a block the first time it is called, and the cache
is invalidated any time canonicalize changes something big in the block.

This fixes LLVM PR51738 and this CIRCT issue:
https://github.com/llvm/circt/issues/1700

This does affect the order of constants left in the top of a block,
I staged in the testsuite changes in rG42431b8207a5.

[lattner]

River, I think this is what you were asking for?

[lattner]

I was let know off-line that River is out for a week. Given this is basically the patch he prescribes in PR51738, I consider this to be obvious.

[rriddle]

LG, thanks!

[lattner]

Thanks for the confirmation River!

mlir/lib/Transforms/Utils/FoldUtils.cpp

for (unsigned i = 0, e = op->getNumResults(); i != e; ++i) {
     auto res = op->getResult(i);
     Attribute attrRepl = foldResults[i].get<Attribute>();
     if (auto *constOp =
             tryGetOrCreateConstant(uniquedConstants, dialect, builder, attrRepl,
                                    res.getType(), op->getLoc())) {
       // Ensure that this constant dominates the operation we are replacing it
       // with. This may not automatically happen if the operation being folded
       // was inserted before the constant within the insertion block.
-      if (constOp->getBlock() == op->getBlock() &&
-          !constOp->isBeforeInBlock(op)) {
-        constOp->moveBefore(op);
-      }
+      Block *opBlock = op->getBlock();
+      if (opBlock == constOp->getBlock() && &opBlock->front() != constOp)
+        constOp->moveBefore(&opBlock->front());
 
       results.push_back(constOp->getResult(0));
       continue;
     }
     // If materialization fails, cleanup any operations generated for the
     // previous results and return failure.
     for (Operation &op : llvm::make_early_inc_range(
              llvm::make_range(entry.begin(), builder.getInsertionPoint()))) {