[llvm] [GlobalOpt] Add range metadata to loads from constant global variables (PR #127695)

[Nikita Popov via llvm-commits]

================
@@ -2498,6 +2499,102 @@ OptimizeGlobalAliases(Module &M,
   return Changed;
 }
 
+static bool AddRangeMetadata(Module &M) {
+  const DataLayout &DL = M.getDataLayout();
+  bool Changed = false;
+
+  for (GlobalValue &Global : M.global_values()) {
+
+    auto *GV = dyn_cast<GlobalVariable>(&Global);
+    if (!GV || !GV->hasDefinitiveInitializer())
+      continue;
+
+    // To be able to go to the next GlobalVariable with a return
+    [&] {
+      uint64_t GlobalByteSize = DL.getTypeAllocSize(GV->getValueType());
+      unsigned BW = DL.getIndexTypeSizeInBits(GV->getType());
+
+      SmallVector<LoadInst *> ArrayLikeLoads;
+      Type *ElemTy = nullptr;
+
+      for (User *U : GV->users()) {
+        if (auto *GEP = dyn_cast<GetElementPtrInst>(U)) {
+          Type *GEPElemTy = GEP->getResultElementType();
+          if (!GEP->isInBounds() || !GEPElemTy->isIntegerTy())
+            continue;
+
+          // This restriction that all accesses use the same type could be
+          // lifted
+          if (!ElemTy)
+            ElemTy = GEPElemTy;
+          else if (ElemTy != GEPElemTy)
+            return;
+
+          SmallMapVector<Value *, APInt, 4> Index;
+          APInt CstOffset(BW, 0);
+          GEP->collectOffset(DL, BW, Index, CstOffset);
+
+          // This check is needed for correctness of the code below.
+          // Be we could only traverse the range starting at the constant offset
+          if (!CstOffset.isAligned(DL.getPrefTypeAlign(GEPElemTy)))
----------------
nikic wrote:

But shouldn't you be checking against the stride of the GEP, that is Index.front().second?

The preferred type alignment is not necessarily the same as the GEP stride (which is the type alloc size).

https://github.com/llvm/llvm-project/pull/127695