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

[Stephen Tozer via llvm-commits]

================
@@ -1864,181 +1864,56 @@ void at::deleteAll(Function *F) {
     DVR->eraseFromParent();
 }
 
-/// Get the FragmentInfo for the variable if it exists, otherwise return a
-/// FragmentInfo that covers the entire variable if the variable size is
-/// known, otherwise return a zero-sized fragment.
-static DIExpression::FragmentInfo
-getFragmentOrEntireVariable(const DbgVariableRecord *DVR) {
-  DIExpression::FragmentInfo VariableSlice(0, 0);
-  // Get the fragment or variable size, or zero.
-  if (auto Sz = DVR->getFragmentSizeInBits())
-    VariableSlice.SizeInBits = *Sz;
-  if (auto Frag = DVR->getExpression()->getFragmentInfo())
-    VariableSlice.OffsetInBits = Frag->OffsetInBits;
-  return VariableSlice;
-}
-
-static DIExpression::FragmentInfo
-getFragmentOrEntireVariable(const DbgVariableIntrinsic *DVI) {
-  DIExpression::FragmentInfo VariableSlice(0, 0);
-  // Get the fragment or variable size, or zero.
-  if (auto Sz = DVI->getFragmentSizeInBits())
-    VariableSlice.SizeInBits = *Sz;
-  if (auto Frag = DVI->getExpression()->getFragmentInfo())
-    VariableSlice.OffsetInBits = Frag->OffsetInBits;
-  return VariableSlice;
-}
+/// FIXME: Remove this wrapper function and call
+/// DIExpression::calculateFragmentIntersect directly.
 template <typename T>
 bool calculateFragmentIntersectImpl(
     const DataLayout &DL, const Value *Dest, uint64_t SliceOffsetInBits,
     uint64_t SliceSizeInBits, const T *AssignRecord,
     std::optional<DIExpression::FragmentInfo> &Result) {
-  // There are multiple offsets at play in this function, so let's break it
-  // down. Starting with how variables may be stored in allocas:
-  //
-  //   1 Simplest case: variable is alloca sized and starts at offset 0.
-  //   2 Variable is larger than the alloca: the alloca holds just a part of it.
-  //   3 Variable is smaller than the alloca: the alloca may hold multiple
-  //   variables.
-  //
-  // Imagine we have a store to the entire alloca. In case (3) the store
-  // affects bits outside of the bounds of each variable. In case (2), where
-  // the alloca holds the Xth bit to the Yth bit of a variable, the
-  // zero-offset store doesn't represent an assignment at offset zero to the
-  // variable. It is an assignment to offset X.
-  //
-  // # Example 1
-  // Obviously, not all stores are alloca-sized and have zero offset. Imagine
-  // the lower 32 bits of this store are dead and are going to be DSEd:
-  //
-  //    store i64 %v, ptr %dest, !DIAssignID !1
-  //    dbg.assign(..., !DIExpression(fragment, 128, 32), !1, %dest,
-  //               !DIExpression(DW_OP_plus_uconst, 4))
-  //
-  // Goal: Given our dead bits at offset:0 size:32 for the store, determine the
-  // part of the variable, which fits in the fragment expressed by the
-  // dbg.assign, that has been killed, if any.
-  //
-  //     calculateFragmentIntersect(..., SliceOffsetInBits=0,
-  //                 SliceSizeInBits=32, Dest=%dest, Assign=dbg.assign)
-  //
-  // Drawing the store (s) in memory followed by the shortened version ($),
-  // then the dbg.assign (d), with the fragment information on a separate scale
-  // underneath:
-  //
-  // Memory
-  // offset
-  //   from
-  //   dest 0      63
-  //        |      |
-  //       s[######] - Original stores 64 bits to Dest.
-  //       $----[##] - DSE says the lower 32 bits are dead, to be removed.
-  //       d    [##] - Assign's address-modifying expression adds 4 bytes to
-  //       dest.
-  // Variable   |  |
-  // Fragment   128|
-  //  Offsets      159
-  //
-  // The answer is achieved in a few steps:
-  // 1. Add the fragment offset to the store offset:
-  //      SliceOffsetInBits:0 + VarFrag.OffsetInBits:128 = 128
-  //
-  // 2. Subtract the address-modifying expression offset plus difference
-  //    between d.address and dest:
-  //      128 - (expression_offset:32 + (d.address - dest):0) = 96
-  //
-  // 3. That offset along with the store size (32) represents the bits of the
-  //    variable that'd be affected by the store. Call it SliceOfVariable.
-  //    Intersect that with Assign's fragment info:
-  //      SliceOfVariable ∩ Assign_fragment = none
-  //
-  // In this case: none of the dead bits of the store affect Assign.
-  //
-  // # Example 2
-  // Similar example with the same goal. This time the upper 16 bits
-  // of the store are going to be DSE'd.
-  //
-  //    store i64 %v, ptr %dest, !DIAssignID !1
-  //    dbg.assign(..., !DIExpression(fragment, 128, 32), !1, %dest,
-  //               !DIExpression(DW_OP_plus_uconst, 4))
-  //
-  //     calculateFragmentIntersect(..., SliceOffsetInBits=48,
-  //                 SliceSizeInBits=16, Dest=%dest, Assign=dbg.assign)
-  //
-  // Memory
-  // offset
-  //   from
-  //   dest 0      63
-  //        |      |
-  //       s[######] - Original stores 64 bits to Dest.
-  //       $[####]-- - DSE says the upper 16 bits are dead, to be removed.
-  //       d    [##] - Assign's address-modifying expression adds 4 bytes to
-  //       dest.
-  // Variable   |  |
-  // Fragment   128|
-  //  Offsets      159
-  //
-  // Using the same steps in the first example:
-  // 1. SliceOffsetInBits:48 + VarFrag.OffsetInBits:128 = 176
-  // 2. 176 - (expression_offset:32 + (d.address - dest):0) = 144
-  // 3. SliceOfVariable offset = 144, size = 16:
-  //      SliceOfVariable ∩ Assign_fragment = (offset: 144, size: 16)
-  // SliceOfVariable tells us the bits of the variable described by Assign that
-  // are affected by the DSE.
+  // No overlap if this DbgRecord describes a killed location.
   if (AssignRecord->isKillAddress())
     return false;
 
-  DIExpression::FragmentInfo VarFrag =
-      getFragmentOrEntireVariable(AssignRecord);
-  if (VarFrag.SizeInBits == 0)
-    return false; // Variable size is unknown.
-
-  // Calculate the difference between Dest and the dbg.assign address +
-  // address-modifying expression.
-  int64_t PointerOffsetInBits;
-  {
-    auto DestOffsetInBytes =
-        AssignRecord->getAddress()->getPointerOffsetFrom(Dest, DL);
-    if (!DestOffsetInBytes)
-      return false; // Can't calculate difference in addresses.
-
-    int64_t ExprOffsetInBytes;
-    if (!AssignRecord->getAddressExpression()->extractIfOffset(
-            ExprOffsetInBytes))
-      return false;
+  int64_t AddrOffsetInBytes;
+  SmallVector<uint64_t> PostOffsetOps; //< Unused.
+  // Bail if we can't find a constant offset (or none) in the expression.
+  if (!AssignRecord->getAddressExpression()->extractLeadingOffset(
+          AddrOffsetInBytes, PostOffsetOps))
+    return false;
+  int64_t AddrOffsetInBits = AddrOffsetInBytes * 8;
----------------
SLTozer wrote:

```suggestion
  int64_t AddrOffsetInBits;
  {
    int64_t AddrOffsetInBytes;
    SmallVector<uint64_t> PostOffsetOps; //< Unused.
    // Bail if we can't find a constant offset (or none) in the expression.
    if (!AssignRecord->getAddressExpression()->extractLeadingOffset(
            AddrOffsetInBytes, PostOffsetOps))
      return false;
    AddrOffsetInBits = AddrOffsetInBytes * 8;
  }
```
Minor style suggestion, feel free to ignore.

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