[DebugInfo][InstrRef] Add a size operand to DBG_PHI

DBG_PHI instructions can refer to stack slots, to indicate that multiple
values merge together on control flow joins in that slot. This is fine --
however the slot might be merged at a later date with a slot of a different
size. In doing so, we lose information about the size the eliminated PHI.
Later analysis passes have to guess.

Improve this by attaching an optional "bit size" operand to DBG_PHI, which
only gets added for stack slots, to let us know how large a size the value
on the stack is.

Differential Revision: https://reviews.llvm.org/D124184

1 files changed, 7 insertions, 32 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
index 74f6e6e8..72d025e 100644
--- a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -1293,41 +1293,16 @@ bool InstrRefBasedLDV::transferDebugPHI(MachineInstr &MI) {
     if (!SpillNo)
       return EmitBadPHI();
 
-    // Problem: what value should we extract from the stack? LLVM does not
-    // record what size the last store to the slot was, and it would become
-    // sketchy after stack slot colouring anyway. Take a look at what values
-    // are stored on the stack, and pick the largest one that wasn't def'd
-    // by a spill (i.e., the value most likely to have been def'd in a register
-    // and then spilt.
-    std::array<unsigned, 4> CandidateSizes = {64, 32, 16, 8};
-    Optional<ValueIDNum> Result = None;
-    Optional<LocIdx> SpillLoc = None;
-    for (unsigned CS : CandidateSizes) {
-      unsigned SpillID = MTracker->getLocID(*SpillNo, {CS, 0});
-      SpillLoc = MTracker->getSpillMLoc(SpillID);
-      ValueIDNum Val = MTracker->readMLoc(*SpillLoc);
-      // If this value was defined in it's own position, then it was probably
-      // an aliasing index of a small value that was spilt.
-      if (Val.getLoc() != SpillLoc->asU64()) {
-        Result = Val;
-        break;
-      }
-    }
+    // Any stack location DBG_PHI should have an associate bit-size.
+    assert(MI.getNumOperands() == 3 && "Stack DBG_PHI with no size?");
+    unsigned slotBitSize = MI.getOperand(2).getImm();
 
-    // If we didn't find anything, we're probably looking at a PHI, or a memory
-    // store folded into an instruction. FIXME: Take a guess that's it's 64
-    // bits. This isn't ideal, but tracking the size that the spill is
-    // "supposed" to be is more complex, and benefits a small number of
-    // locations.
-    if (!Result) {
-      unsigned SpillID = MTracker->getLocID(*SpillNo, {64, 0});
-      SpillLoc = MTracker->getSpillMLoc(SpillID);
-      Result = MTracker->readMLoc(*SpillLoc);
-    }
+    unsigned SpillID = MTracker->getLocID(*SpillNo, {slotBitSize, 0});
+    LocIdx SpillLoc = MTracker->getSpillMLoc(SpillID);
+    ValueIDNum Result = MTracker->readMLoc(SpillLoc);
 
     // Record this DBG_PHI for later analysis.
-    auto DbgPHI =
-        DebugPHIRecord({InstrNum, MI.getParent(), *Result, *SpillLoc});
+    auto DbgPHI = DebugPHIRecord({InstrNum, MI.getParent(), Result, SpillLoc});
     DebugPHINumToValue.push_back(DbgPHI);
   } else {
     // Else: if the operand is neither a legal register or a stack slot, then