[llvm] [NFC] Add DIExpression::calculateFragmentIntersect (PR #97738)

[Stephen Tozer via llvm-commits]

================
@@ -2052,6 +2052,75 @@ std::optional<DIExpression *> DIExpression::createFragmentExpression(
   return DIExpression::get(Expr->getContext(), Ops);
 }
 
+/// See declaration for more info.
+bool DIExpression::calculateFragmentIntersect(
+    const DataLayout &DL, const Value *SliceStart, uint64_t SliceOffsetInBits,
+    uint64_t SliceSizeInBits, const Value *DbgPtr, int64_t DbgPtrOffsetInBits,
+    int64_t DbgExtractOffsetInBits, DIExpression::FragmentInfo VarFrag,
+    std::optional<DIExpression::FragmentInfo> &Result,
+    int64_t &OffsetFromLocationInBits) {
+
+  if (VarFrag.SizeInBits == 0)
+    return false; // Variable size is unknown.
+
+  // Difference between mem slice start and the dbg location start.
+  // 0   4   8   12   16 ...
+  // |       |
+  // dbg location start
+  //         |
+  //         mem slice start
+  // Here MemStartRelToDbgStartInBits is 8. Note this can be negative.
+  int64_t MemStartRelToDbgStartInBits;
+  {
+    auto MemOffsetFromDbgInBytes = SliceStart->getPointerOffsetFrom(DbgPtr, DL);
+    if (!MemOffsetFromDbgInBytes)
+      return false; // Can't calculate difference in addresses.
+    // Difference between the pointers.
+    MemStartRelToDbgStartInBits = *MemOffsetFromDbgInBytes * 8;
+    // Add the difference of the offsets.
+    MemStartRelToDbgStartInBits +=
+        SliceOffsetInBits - (DbgPtrOffsetInBits + DbgExtractOffsetInBits);
+  }
+
+  // Out-param. Invert offset to get offset from debug location.
+  OffsetFromLocationInBits = -MemStartRelToDbgStartInBits;
+
+  // Check if the variable fragment sits outside (before) this memory slice.
+  int64_t MemEndRelToDbgStart = MemStartRelToDbgStartInBits + SliceSizeInBits;
+  if (MemEndRelToDbgStart < 0) {
+    Result = {0, 0}; // Out-param.
+    return true;
+  }
+
+  // Work towards creating SliceOfVariable which is the bits of the variable
+  // that the memory region covers.
+  // 0   4   8   12   16 ...
+  // |       |
+  // dbg location start with VarFrag offset=32
+  //         |
+  //         mem slice start: SliceOfVariable offset=40
+  int64_t MemStartRelToFragInBits =
+      MemStartRelToDbgStartInBits + VarFrag.OffsetInBits;
----------------
SLTozer wrote:

I feel like the term `RelToFrag` is misleading, I feel that implies relative to the current debug fragment rather than to the variable as a whole?

https://github.com/llvm/llvm-project/pull/97738