[NFC] Add DIExpression::calculateFragmentIntersect

Patch [3/x] to fix structured bindings debug info in SROA.

This function computes a fragment, bit-extract operation if needed, and new
constant offset to describe a part of a variable covered by some memory.

This generalises, simplifies, and replaces at::calculateFragmentIntersect. That
version is still used as a wrapper for now though to keep this change NFC.

The new version takes doesn't have a DbgRecord parameter, instead using an
explicit address and address offset. The old version only operates on
dbg_assigns and this change means it can also operate on dbg_declare records
easily, which it will do in a subsequent patch.

The new version has a new out-param OffsetFromLocationInBits which is set to the
difference between the first bit of the variable location and the first bit of
the memory slice. This will be used in a subsequent patch in SROA to determine
the new offset to use in the address expression after splitting an alloca.

Author: OCHyams

llvm/include/llvm/IR/DebugInfoMetadata.h

@@ -3074,6 +3074,43 @@ class DIExpression : public MDNode {
       return 0;
   }
 
+  /// Computes a fragment, bit-extract operation if needed, and new constant
+  /// offset to describe a part of a variable covered by some memory.
+  ///
+  /// The memory region starts at:
+  ///   \p SliceStart + \p SliceOffsetInBits
+  /// And is size:
+  ///   \p SliceSizeInBits
+  ///
+  /// The location of the existing variable fragment \p VarFrag is:
+  ///   \p DbgPtr + \p DbgPtrOffsetInBits + \p DbgExtractOffsetInBits.
+  ///
+  /// It is intended that these arguments are derived from a debug record:
+  /// - \p DbgPtr is the (single) DIExpression operand.
+  /// - \p DbgPtrOffsetInBits is the constant offset applied to \p DbgPtr.
+  /// - \p DbgExtractOffsetInBits is the offset from a
+  ///   DW_OP_LLVM_bit_extract_[sz]ext operation.
+  ///
+  /// Results and return value:
+  /// - Return false if the result can't be calculated for any reason.
+  /// - \p Result is set to nullopt if the intersect equals \p VarFarg.
+  /// - \p Result contains a zero-sized fragment if there's no intersect.
+  /// - \p OffsetFromLocationInBits is set to the difference between the first
+  ///   bit of the variable location and the first bit of the slice. The
+  ///   magnitude of a negative value therefore indicates the number of bits
+  ///   into the variable fragment that the memory region begins.
+  ///
+  /// We don't pass in a debug record directly to get the constituent parts
+  /// and offsets because different debug records store the information in
+  /// different places (dbg_assign has two DIExpressions - one contains the
+  /// fragment info for the entire intrinsic).
+  static bool 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);
+
   using ExtOps = std::array<uint64_t, 6>;
 
   /// Returns the ops for a zero- or sign-extension in a DIExpression.

llvm/lib/IR/DebugInfo.cpp

@@ -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;
 
-    int64_t PointerOffsetInBytes = *DestOffsetInBytes + ExprOffsetInBytes;
-    PointerOffsetInBits = PointerOffsetInBytes * 8;
-  }
+  Value *Addr = AssignRecord->getAddress();
+  // FIXME: It may not always be zero.
+  int64_t BitExtractOffsetInBits = 0;
+  DIExpression::FragmentInfo VarFrag =
+      AssignRecord->getFragmentOrEntireVariable();
 
-  // Adjust the slice offset so that we go from describing the a slice
-  // of memory to a slice of the variable.
-  int64_t NewOffsetInBits =
-      SliceOffsetInBits + VarFrag.OffsetInBits - PointerOffsetInBits;
-  if (NewOffsetInBits < 0)
-    return false; // Fragment offsets can only be positive.
-  DIExpression::FragmentInfo SliceOfVariable(SliceSizeInBits, NewOffsetInBits);
-  // Intersect the variable slice with AssignRecord's fragment to trim it down
-  // to size.
-  DIExpression::FragmentInfo TrimmedSliceOfVariable =
-      DIExpression::FragmentInfo::intersect(SliceOfVariable, VarFrag);
-  if (TrimmedSliceOfVariable == VarFrag)
-    Result = std::nullopt;
-  else
-    Result = TrimmedSliceOfVariable;
-  return true;
+  int64_t OffsetFromLocationInBits; //< Unused.
+  return DIExpression::calculateFragmentIntersect(
+      DL, Dest, SliceOffsetInBits, SliceSizeInBits, Addr, AddrOffsetInBits,
+      BitExtractOffsetInBits, VarFrag, Result, OffsetFromLocationInBits);
 }
+
+/// FIXME: Remove this wrapper function and call
+/// DIExpression::calculateFragmentIntersect directly.
 bool at::calculateFragmentIntersect(
     const DataLayout &DL, const Value *Dest, uint64_t SliceOffsetInBits,
     uint64_t SliceSizeInBits, const DbgAssignIntrinsic *DbgAssign,
     std::optional<DIExpression::FragmentInfo> &Result) {
+
   return calculateFragmentIntersectImpl(DL, Dest, SliceOffsetInBits,
                                         SliceSizeInBits, DbgAssign, Result);
 }
+
+/// FIXME: Remove this wrapper function and call
+/// DIExpression::calculateFragmentIntersect directly.
 bool at::calculateFragmentIntersect(
     const DataLayout &DL, const Value *Dest, uint64_t SliceOffsetInBits,
     uint64_t SliceSizeInBits, const DbgVariableRecord *DVRAssign,
     std::optional<DIExpression::FragmentInfo> &Result) {
+  // FIXME: Remove this wrapper function and call
+  // DIExpression::calculateFragmentIntersect directly.
   return calculateFragmentIntersectImpl(DL, Dest, SliceOffsetInBits,
                                         SliceSizeInBits, DVRAssign, Result);
 }

llvm/lib/IR/DebugInfoMetadata.cpp

@@ -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;
+  int64_t MemEndRelToFragInBits = MemStartRelToFragInBits + SliceSizeInBits;
+  // If the memory region starts before the debug location the fragment
+  // offset would be negative, which we can't encode. Limit those to 0. This
+  // is fine becausethose bits necessarily don't overlap with the variable
+  // fragment.
+  int64_t MemFragStart = std::max<int64_t>(0, MemStartRelToFragInBits);
+  int64_t MemFragSize =
+      std::max<int64_t>(0, MemEndRelToFragInBits - MemFragStart);
+  DIExpression::FragmentInfo SliceOfVariable(MemFragSize, MemFragStart);
+
+  // Intersect the memory region fragment with the variable location fragment.
+  DIExpression::FragmentInfo TrimmedSliceOfVariable =
+      DIExpression::FragmentInfo::intersect(SliceOfVariable, VarFrag);
+  if (TrimmedSliceOfVariable == VarFrag)
+    Result = std::nullopt; // Out-param.
+  else
+    Result = TrimmedSliceOfVariable; // Out-param.
+  return true;
+}
+
 std::pair<DIExpression *, const ConstantInt *>
 DIExpression::constantFold(const ConstantInt *CI) {
   // Copy the APInt so we can modify it.