[llvm] [InstCombine] Fold icmp(constants[x]) when the range of x is given (PR #67093)

[via llvm-commits]

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@@ -187,18 +168,36 @@ Instruction *InstCombinerImpl::foldCmpLoadFromIndexedGlobal(
   // the array, this will fully represent all the comparison results.
   uint64_t MagicBitvector = 0;
 
+  Value *Idx = nullptr;
+
   // Scan the array and see if one of our patterns matches.
-  Constant *CompareRHS = cast<Constant>(ICI.getOperand(1));
-  for (unsigned i = 0, e = ArrayElementCount; i != e; ++i) {
-    Constant *Elt = Init->getAggregateElement(i);
-    if (!Elt) return nullptr;
-
-    // If this is indexing an array of structures, get the structure element.
-    if (!LaterIndices.empty()) {
-      Elt = ConstantFoldExtractValueInstruction(Elt, LaterIndices);
-      if (!Elt)
-        return nullptr;
-    }
+  Constant *ComparedRHS = cast<Constant>(ICI.getOperand(1));
+  APInt OffsetStep = VariableOffsets.front().second;
+  // The longest step we can reach once.
+  uint64_t OffsetStepZExt = OffsetStep.getZExtValue();
+  // Offset from constant pointer where we begin scanning the constant.
+  int64_t BeginOffset = ConstantOffset.getSExtValue();
+
+  // Make BeginOffset the smallest offset >= 0
+  if (BeginOffset % OffsetStepZExt == 0)
+    BeginOffset = 0;
+  else if (BeginOffset < 0)
+    BeginOffset += (-BeginOffset / OffsetStepZExt + 1) * OffsetStepZExt;
+  else if (BeginOffset > 0)
+    BeginOffset -= (BeginOffset / OffsetStepZExt) * OffsetStepZExt;
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XChy wrote:

Simplify it now.

https://github.com/llvm/llvm-project/pull/67093