[Support] Do not use llvm::size in getLoopPreheader

[bnbarham]

BlockT *LoopBase<BlockT, LoopT>::getLoopPreheader() was changed in
7243607 to use llvm::size rather than
the checking that child_begin() + 1 == child_end(). llvm::size
requires that std::distance be O(1) and hence that clients support
random access. Use llvm::hasSingleElement instead.

[llvmbot]

@llvm/pr-subscribers-llvm-support

Author: Ben Barham (bnbarham)

Changes

BlockT *LoopBase<BlockT, LoopT>::getLoopPreheader() was changed in 7243607 to use llvm::size rather than the checking that child_begin() + 1 == child_end(). llvm::size requires that std::distance be O(1) and hence that clients support random access. Switch back to the original check.

---

Full diff: https://github.com/llvm/llvm-project/pull/94540.diff

1 Files Affected:

• (modified) llvm/include/llvm/Support/GenericLoopInfoImpl.h (+4-1)

diff --git a/llvm/include/llvm/Support/GenericLoopInfoImpl.h b/llvm/include/llvm/Support/GenericLoopInfoImpl.h
index 1e0d0ee446fc4..c9c7354937545 100644
--- a/llvm/include/llvm/Support/GenericLoopInfoImpl.h
+++ b/llvm/include/llvm/Support/GenericLoopInfoImpl.h
@@ -208,7 +208,10 @@ BlockT *LoopBase<BlockT, LoopT>::getLoopPreheader() const {
     return nullptr;
 
   // Make sure there is only one exit out of the preheader.
-  if (llvm::size(llvm::children<BlockT *>(Out)) != 1)
+  typedef GraphTraits<BlockT *> BlockTraits;
+  typename BlockTraits::ChildIteratorType SI = BlockTraits::child_begin(Out);
+  ++SI;
+  if (SI != BlockTraits::child_end(Out))
     return nullptr; // Multiple exits from the block, must not be a preheader.
 
   // The predecessor has exactly one successor, so it is a preheader.

[bnbarham]

@kazutakahirata this was a NFC commit, was there an underlying reason for the change or are you happy with reverting this part of it?

[kazutakahirata]

Could we do something like this?

Suggested change

	typedef GraphTraits<BlockT *> BlockTraits;
	typename BlockTraits::ChildIteratorType SI = BlockTraits::child_begin(Out);
	++SI;
	if (SI != BlockTraits::child_end(Out))
	if (!llvm::hasSingleElement(GraphTraits<BlockT *>::children(Out)))

[bnbarham]

Ah nice, much better. Switched to llvm::hasSingleElement, thanks!

[kazutakahirata]

LGTM. Thanks!