[llvm] [MergeICmps] Perform dereferenceability check with context (PR #202884)

[Nikita Popov via llvm-commits]

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@@ -734,6 +737,37 @@ static BasicBlock *mergeComparisons(ArrayRef<BCECmpBlock> Comparisons,
   return BB;
 }
 
+// The transform may change the order in which the comparison is performed,
+// in which case we may perform loads that were not performed by the original
+// program. As such, we need to ensure that all the accessed memory is
+// dereferenceable.
+bool BCECmpChain::isDereferenceable() {
+  // We know that there can be no frees inside the merged blocks, so it's
+  // sufficient for dereferenceability to hold at the entry block. One
+  // exception to this is if the entry block performs "other work" and will
+  // get split. In that case, we need to consider frees prior to the splitting
+  // point.
+  Instruction *CxtI = SplitAt ? SplitAt : &EntryBlock_->front();
+
+  for (const auto &Blocks : MergedBlocks_) {
+    const BCECmpBlock &LowestBlock = Blocks.front();
+    const Value *Lhs = LowestBlock.Lhs().LoadI->getPointerOperand();
+    const Value *Rhs = LowestBlock.Rhs().LoadI->getPointerOperand();
+    const DataLayout &DL = LowestBlock.Lhs().LoadI->getDataLayout();
+
+    unsigned SizeInBits = 0;
+    for (const BCECmpBlock &Block : Blocks)
+      SizeInBits += Block.SizeBits();
+
+    APInt Size(64, SizeInBits / 8);
+    SimplifyQuery SQ(DL, CxtI);
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nikic wrote:

In theory AC/DT could be useful to support this transform based on assume_dereferenceable. The pass doesn't use AC/DT currently though (DT is only preserved), and I don't think it's worth it to request them just for this.

https://github.com/llvm/llvm-project/pull/202884