[llvm] ded26bf - [IVDescriptors] Before moving an instruction in SinkAfter checking if it is target of other instructions

[Guozhi Wei via llvm-commits]

Author: Guozhi Wei
Date: 2022-10-03T18:47:51Z
New Revision: ded26bf6b9175201a0627f019e642bf74d2b48b1

URL: https://github.com/llvm/llvm-project/commit/ded26bf6b9175201a0627f019e642bf74d2b48b1
DIFF: https://github.com/llvm/llvm-project/commit/ded26bf6b9175201a0627f019e642bf74d2b48b1.diff

LOG: [IVDescriptors] Before moving an instruction in SinkAfter checking if it is target of other instructions

The attached test case can cause LLVM crash in buildVPlanWithVPRecipes because
invalid VPlan is generated.

  FIRST-ORDER-RECURRENCE-PHI ir<%792> = phi ir<%501>, ir<%806>
  CLONE ir<%804> = fdiv ir<1.000000e+00>, vp<%17>      // use of %17
  CLONE ir<%806> = load ir<%805>
  EMIT vp<%17> = first-order splice ir<%792> ir<%806>   // def of %17
  ...

There is a use before def error on %17.

When vectorizer generates a VPlan, it generates a "first-order splice"
instruction for a loop carried variable after its definition. All related PHI
users are changed to use this "first-order splice" result, and are moved after
it. The move is guided by a MapVector SinkAfter. And the content of SinkAfter is
filled by RecurrenceDescriptor::isFixedOrderRecurrence.

Let's look at the first PHI and related instructions

  %v792 = phi double [ %v806, %Loop ], [ %d1, %Entry ]
  %v802 = fdiv double %v794, %v792
  %v804 = fdiv double 1.000000e+00, %v792
  %v806 = load double, ptr %v805, align 8

%v806 is a loop carried variable, %v792 is related PHI instruction. Vectorizer
will generated a new "first-order splice" instruction for %v806, and it will be
used by %v802 and %v804. So %v802 and %v804 will be moved after %v806 and its
"first-order splice" instruction. So SinkAfter contains

   %v802   ->  %v806
   %v804   ->  %v802

It means %v802 should be moved after %v806 and %v804 will be moved after %v802.
Please pay attention that the order is important.

When isFixedOrderRecurrence processing PHI instruction %v794, related
instructions are

  %v793 = phi double [ %v813, %Loop ], [ %d1, %Entry ]
  %v794 = phi double [ %v793, %Loop ], [ %d2, %Entry ]
  %v802 = fdiv double %v794, %v792
  %v813 = load double, ptr %v812, align 8

This time its related loop carried variable is %v813, its user is %v802. So
%v802 should also be moved after %v813. But %v802 is already in SinkAfter,
because %v813 is later than %v806, so the original %v802 entry in SinkAfter is
deleted, a new %v802 entry is added. Now SinkAfter contains

  %v804   ->  %v802
  %v802   ->  %v813

With these data, %v802 can still be moved after all its operands, but %v804
can't be moved after %v806 and its "first-order splice" instruction. And causes
use before def error.

So when remove/re-insert an instruction I in SinkAfter, we should also
recursively remove instructions targeting I and re-insert them into SinkAfter.
But for simplicity I just bail out in this case.

Differential Revision: https://reviews.llvm.org/D134083

Added: 
    

Modified: 
    llvm/lib/Analysis/IVDescriptors.cpp
    llvm/test/Transforms/LoopVectorize/first-order-recurrence-chains.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/IVDescriptors.cpp b/llvm/lib/Analysis/IVDescriptors.cpp
index d3f6c9e25ae6c..e42512b41aa66 100644
--- a/llvm/lib/Analysis/IVDescriptors.cpp
+++ b/llvm/lib/Analysis/IVDescriptors.cpp
@@ -1025,6 +1025,16 @@ bool RecurrenceDescriptor::isFixedOrderRecurrence(
       // Previous. Nothing left to do.
       if (DT->dominates(Previous, OtherPrev) || Previous == OtherPrev)
         return true;
+
+      // If there are other instructions to be sunk after SinkCandidate, remove
+      // and re-insert SinkCandidate can break those instructions. Bail out for
+      // simplicity.
+      if (any_of(SinkAfter,
+          [SinkCandidate](const std::pair<Instruction *, Instruction *> &P) {
+            return P.second == SinkCandidate;
+          }))
+        return false;
+
       // Otherwise, Previous comes after OtherPrev and SinkCandidate needs to be
       // re-sunk to Previous, instead of sinking to OtherPrev. Remove
       // SinkCandidate from SinkAfter to ensure it's insert position is updated.

diff  --git a/llvm/test/Transforms/LoopVectorize/first-order-recurrence-chains.ll b/llvm/test/Transforms/LoopVectorize/first-order-recurrence-chains.ll
index 6e054809fbb95..2476b272c26a0 100644
--- a/llvm/test/Transforms/LoopVectorize/first-order-recurrence-chains.ll
+++ b/llvm/test/Transforms/LoopVectorize/first-order-recurrence-chains.ll
@@ -637,3 +637,30 @@ loop:
 exit:
   ret void
 }
+
+; Make sure LLVM doesn't generate wrong data in SinkAfter, and causes crash in
+; loop vectorizer.
+define void @test_crash(ptr %p) {
+; CHECK-LABEL: @test_crash
+; CHECK-NOT:   vector.body:
+; CHECK:       ret
+Entry:
+  br label %Loop
+
+Loop:
+  %for.1 = phi double [ %iv1, %Loop ], [ 0.000000e+00, %Entry ]
+  %for.2 = phi double [ %iv2, %Loop ], [ 0.000000e+00, %Entry ]
+  %for.3 = phi double [ %for.2, %Loop ], [ 0.000000e+00, %Entry ]
+  %for.4 = phi i64 [ %count, %Loop ], [ 0, %Entry ]
+  %USE_2_INDVARS = fdiv double %for.3, %for.1
+  %div = fdiv double 0.000000e+00, %for.1
+  %iv1 = load double, ptr null, align 8
+  %count = add nuw nsw i64 %for.4, 1
+  %iv2 = load double, ptr null, align 8
+  store double %div, ptr %p, align 8
+  %cond = icmp eq i64 %count, 0
+  br i1 %cond, label %End, label %Loop
+
+End:
+  ret void
+}