[llvm-dev] "Ordered" Instruction Iterator?

Cranmer, Joshua via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 1 09:10:22 PDT 2021

As David points out, there’s some ambiguity as to what “ordered” means with respect to basic blocks. There are three main different traversals that LLVM automatically provides, and which one is appropriate depends on your use case.

The first is regular traversal over all basic blocks in the function:
for (auto &BB : F) {}

This will iterate over every basic block in the function in an arbitrary order (roughly, the order that they are actually laid out during emission), and the only guarantee you have is that the entry block is first.

Alternatively, you can choose to use the depth-first traversal:
for (auto BB : depth_first(F)) {}

This will execute a depth-first preorder, which means every basic block will be visited after one (but not necessarily all) of its predecessors have been visited. This is sufficient to guarantee that you visit every dominator of a block before you visit the block itself.

Another alternative is the reverse postorder traversal (ReversePostOrderTraversal is actually expensive to compute, so you may want to save it to a variable if you intend to do multiple traversals in a pass):
for (auto BB : ReversePostOrderTraversal(F)) {}

Reverse post-order visits every block after all of its predecessors (excluding loops) have been visited. From the short snippet of an explanation you have provided, this may suffice for your use-case. However, it may also be overkill for your needs, and you might instead investigate a cheaper alternative traversal.

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Zhang via llvm-dev
Sent: Monday, May 31, 2021 22:43
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] "Ordered" Instruction Iterator?


In my analysis Pass, I'm trying to visit CallInsts to a special Intrinsic, in the order that they'll be executed, starting from the EntryBlock.
My naive initial assumption was to start from entryBlock and custom handle Br Instructions, but apparently this doesn't work well with more complicated CFGs like loop or Switch.

Is there any LLVM builtin infrastructure that could be used for this? Any hint would be appreciated.

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