[SLP][NFC] Assert that tree entry operands completed when scheduler looks for depende...

This change adds an assertion to prevent tricky bug related to recursive
approach of building vectorization tree. For loop below takes number of
operands directly from tree entry rather than from scalars.
If the entry at this moment turns out incomplete (i.e. not all operands set)
then not all the dependencies will be seen by the scheduler.
This can lead to failed scheduling (and thus failed vectorization)
for perfectly vectorizable tree.
Here is code example which is likely to fire the assertion:
for (i : VL0->getNumOperands()) {
  ...
  TE->setOperand(i, Operands);
  buildTree_rec(Operands, Depth + 1,...);
}

Correct way is two steps process: first set all operands to a tree entry
and then recursively process each operand.

Differential Revision: https://reviews.llvm.org/D75296

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index a1134af..8087963 100644 (file)
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -2016,6 +2016,20 @@ private:
         if (TreeEntry *TE = BundleMember->TE) {
           int Lane = BundleMember->Lane;
           assert(Lane >= 0 && "Lane not set");
+
+          // Since vectorization tree is being built recursively this assertion
+          // ensures that the tree entry has all operands set before reaching
+          // this code. Couple of exceptions known at the moment are extracts
+          // where their second (immediate) operand is not added. Since
+          // immediates do not affect scheduler behavior this is considered
+          // okay.
+          auto *In = TE->getMainOp();
+          assert(In &&
+                 (isa<ExtractValueInst>(In) || isa<ExtractElementInst>(In) ||
+                  In->getNumOperands() == TE->getNumOperands()) &&
+                 "Missed TreeEntry operands?");
+          (void)In; // fake use to avoid build failure when assertions disabled
+
           for (unsigned OpIdx = 0, NumOperands = TE->getNumOperands();
                OpIdx != NumOperands; ++OpIdx)
             if (auto *I = dyn_cast<Instruction>(TE->getOperand(OpIdx)[Lane]))