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

Quentin Colombet qcolombet at apple.com
Fri Oct 3 11:18:48 PDT 2014

Hi Adam,

LGTM with some minor fixes.

+    // Some paranoid early-returns here even though we only install this
+    // update listener during matching a complex patterns so none of these
+    // should ever occur.
+    if (!E || E->isMachineOpcode())
+      return;

Should this be an assert then?

+    /// \brief Address-mode matching performs shift-of-and to and-of-shift
+    /// reassocication in order to expose more scaled addressing
+    /// oppportunities.

- reassocication -> reassociation
- oppportunities -> opportunities

+      // If target can modify DAG during matching, keep the matching state
+      // consisent.

typo: consisent -> consistent.

+; CHECK-NOT: and
+; The shift (leal) should be folded into the scale of the address in the load.
+; CHECK-NOT: leal

Add a CHECK-LABEL at the start.

On Oct 3, 2014, at 9:56 AM, Adam Nemet <anemet at apple.com> wrote:

> Ping.  Can someone please look at this?  It’s getting pretty important for us.
> Thanks,
> Adam
> 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
> <ISel-Keep-matching-state-consistent-when-folding-dur.patch>
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