[PATCH][ISel] Keep matching state consistent when folding during X86 address match

Adam Nemet anemet at apple.com
Fri Oct 3 09:56:47 PDT 2014

Ping.  Can someone please look at this?  It’s getting pretty important for us.


On Sep 26, 2014, at 1:33 PM, Adam Nemet <anemet at apple.com> wrote:

> Hi,
> In the X86 backend, matching an address is initiated by the 'addr' complex
> pattern and its friends.  During this process we may reassociate and-of-shift
> into shift-of-and (FoldMaskedShiftToScaledMask) to allow folding of the
> shift into the scale of the address.
> However as demonstrated by the testcase, this can trigger CSE of not only the
> shift and the AND which the code is prepared for but also the underlying load
> node.  In the testcase this node is sitting in the RecordedNode and MatchScope
> data structures of the matcher and becomes a deleted node upon CSE.  Returning
> from the complex pattern function, we try to access it again hitting an assert
> because the node is no longer a load even though this was checked before.
> Now obviously changing the DAG this late is bending the rules but I think it
> makes sense somewhat.  Outside of addresses we prefer and-of-shift because it
> may lead to smaller immediates (FoldMaskAndShiftToScale is an even better
> example because it create a non-canonical node).  We currently don't recognize
> addresses during DAGCombiner where arguably this canonicalization should be
> performed.  On the other hand, having this in the matcher allows us to cover
> all the cases where an address can be used in an instruction.
> I've also talked a little bit to Dan Gohman on llvm-dev who added the RAUW for
> the new shift node in FoldMaskedShiftToScaledMask.  This RAUW is responsible
> for initiating the recursive CSE on users
> (http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-September/076903.html) but it
> is not strictly necessary since the shift is hooked into the visited user.  Of
> course it's safer to keep the DAG consistent at all times (e.g. for accurate
> number of uses, etc.).
> So rather than changing the fundamentals, I've decided to continue along the
> previous patches and detect the CSE.  This patch installs a very targeted
> DAGUpdateListener for the duration of a complex-pattern match and updates the
> matching state accordingly.  (Previous patches used HandleSDNode to detect the
> CSE but that's not practical here).  The listener is only installed on X86.
> I tested that there is no measurable overhead due to this while running
> through the spec2k BC files with llc.  The only thing we pay for is the
> creation of the listener.  The callback never ever triggers in spec2k since
> this is a corner case.
> Fixes rdar://problem/18206171
> Please let me know if this makes sense.
> Thanks,
> Adam
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