llvm dev - Jun 2021 - "Ordered" Instruction Iterator?

Hi:


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.




Zhang
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Hi,

It's probably worth stepping back a bit and explaining a bit more what 
you mean by 'ordered' and why that's what you want.  Instructions
within
a basic block that do not have side effects are guaranteed to be 
executed in some order that satisfies their dependencies but there is no 
guarantee within code generation that the order in the IR is preserved.

Once you get to three basic blocks, there's no single execution order of 
basic blocks within the function.  The entry block will be executed 
first, but then the next block to execute depends on the branch 
conditions and some blocks may be executed multiple times.

You can trivially visit all blocks in an undefined order.  You can do a 
breadth-first traversal by looking at the successors found in the 
terminator in every block at a given depth (thought it's a graph, so you 
need to maintain a set of BBs you've visited before), but for any 
non-trivial function that will only very loosely approximate execution 
order.

If you want to track dependencies between invocations of an intrinsic, 
then you might want to consider having that intrinsic return and consume 
a token.  You can then walk def-use chains to get a set of paths through 
the function.

David

On 01/06/2021 03:42, Zhang via llvm-dev wrote:
> Hi:
> 
> 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.
> 
> 
> Zhang
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

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?

Hi:

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.


Zhang
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