[llvm] [ExpandMemCmp] Improve memcmp optimisation for boolean results (PR #71221)

[Nikita Popov via llvm-commits]

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@@ -656,6 +658,40 @@ Value *MemCmpExpansion::getMemCmpOneBlock() {
 
   const LoadPair Loads = getLoadPair(LoadSizeType, BSwapSizeType, MaxLoadType,
                                      /*Offset*/ 0);
+
+  // If a user of memcmp cares only about two outcomes, for example:
+  //    bool result = memcmp(a, b, NBYTES) > 0;
+  // We can generate more optimal code with a smaller number of operations
+  if (auto *U = CI->getUniqueUndroppableUser()) {
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nikic wrote:

You are looking for hasOneUse(). If you use getUniqueUndroppableUser() you actually need to drop droppable users.

https://github.com/llvm/llvm-project/pull/71221