[llvm] [DAGCombine] Fix multi-use miscompile in load combine (PR #81492)

[Nikita Popov via llvm-commits]

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@@ -9245,6 +9252,14 @@ SDValue DAGCombiner::MatchLoadCombine(SDNode *N) {
   if (!Allowed || !Fast)
     return SDValue();
 
+  // calculatebyteProvider() allows multi-use for vector loads. Ensure that
+  // all uses are in vector element extracts that are part of the pattern.
+  for (LoadSDNode *L : Loads)
+    if (L->getMemoryVT().isVector())
+      for (auto It = L->use_begin(); It != L->use_end(); ++It)
+        if (It.getUse().getResNo() == 0 && !ExtractElements.contains(*It))
+          return SDValue();
+
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nikic wrote:

Gah, I've been looking everywhere for a function that does this, and couldn't find it. After looking at a few other uses of that function, it seems like we actually have quite a few folds that are happy to introduce extra loads to avoid other instructions, so it seems like strictly avoiding multi-use loads in this fold may not be desirable. I've opened https://github.com/llvm/llvm-project/pull/81586 to *only* use makeEquivalentMemoryOrdering() instead.

https://github.com/llvm/llvm-project/pull/81492