[llvm-dev] CFG manipulation and !llvm.loop metadata

Colin McEwan via llvm-dev
Fri, 20 Mar 2020 17:37:14 -0000

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Hi all,

I have encountered some issues with the preservation of the location of
llvm.loop metadata (containing optimisation hints), and would appreciate some
feedback on the issue.

The IR language description states that llvm.loop metadata is attached to
terminator of a loop latch block, and accordingly Loop::getLoopID() searches for
it in all loop latches (and only successfully finds it if all latches reference
the same metadata)

However, transforms which modify the CFG, for example using SplitCriticalEdge(),
generally don't make any attempt to preserve this property. Some transforms
dealing specifically with loops use getLoopID and setLoopID to preserve and
reset the metadata after transformations, but function transforms such as GVN
and Jump Threading can modify control flow without any attempt to update the
location.

For example:

        preheader:
                ...
        loop.body: ; preds = %preheader, %loop.body
                ...
                br i1 %cmp, label %loop.body, %loop.exit, !llvm.loop !123
        loop.exit:

If a pass needs to split the critical edge from %loop.body -> %loop.body, it
will create something like

        preheader:
                ...
        loop.body: ; preds = %preheader, %loop.body.crit_edge
                ...
                br i1 %cmp, label %loop.body.crit_edge, %loop.exit, !llvm.loop
!123   // No longer a loop latch block
        loop.body.crit_edge:
                ...
                br %loop.body
// Now a loop latch, with no !llvm.loop
        loop.exit:


Now the loop's latch block is %loop.body.crit_edge, and the !llvm.loop will
not be found by Loop::getLoopID(), meaning it is effectively lost. This can
happen in many different places.


I can think of a few approaches to address this, but each has its issues:

    1. Modify the framework's CFG manipulation tools to maintain the
location of the !llvm.loop. A major drawback of this is that LoopInfo is needed
to be able to tell whether a block is a loop latch or not (in the example of
splitting a latch block's back edge, we need LoopInfo to know whether the
edge we're splitting is the latch edge or exit edge) and it's not
necessary available or up to date when we manipulate the CFG.

    2. Have Loop::getLoopID() search other blocks in the loop for metadata. This
has potential compile-time implications, and would change the IR language
definition of the !llvm.loop as potentially existing (in a valid form) anywhere
in the loop.

    3. Fixup utility functions for function passes to use, to search a loop and
move any errant !llvm.loop to the latch block(s) of its loop.


Additionally, it should probably be explicitly stated in the IR language
reference that !llvm.loop preservation is best-effort and may be lost.

Does anyone have any opinions or other insight?


Many thanks,
--
Colin