Reapply D124184, [DebugInfo][InstrRef] Add a size operand to DBG_PHI

This was reverted twice, in 987cd7c3ed75b and 13815e8cbf8d4. The latter
stemed from not accounting for rare register classes in a pre-allocated
array, and the former from an array not being completely initialized,
leading to asan complaining.

1 files changed, 19 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugVariables.cpp b/llvm/lib/CodeGen/LiveDebugVariables.cpp
index ce350b7..320e682 100644
--- a/llvm/lib/CodeGen/LiveDebugVariables.cpp
+++ b/llvm/lib/CodeGen/LiveDebugVariables.cpp
@@ -1850,16 +1850,33 @@ void LDVImpl::emitDebugValues(VirtRegMap *VRM) {
       const TargetRegisterClass *TRC = MRI.getRegClass(Reg);
       unsigned SpillSize, SpillOffset;
 
-      // Test whether this location is legal with the given subreg.
+      unsigned regSizeInBits = TRI->getRegSizeInBits(*TRC);
+      if (SubReg)
+        regSizeInBits = TRI->getSubRegIdxSize(SubReg);
+
+      // Test whether this location is legal with the given subreg. If the
+      // subregister has a nonzero offset, drop this location, it's too complex
+      // to describe. (TODO: future work).
       bool Success =
           TII->getStackSlotRange(TRC, SubReg, SpillSize, SpillOffset, *MF);
 
-      if (Success) {
+      if (Success && SpillOffset == 0) {
         auto Builder = BuildMI(*OrigMBB, OrigMBB->begin(), DebugLoc(),
                                TII->get(TargetOpcode::DBG_PHI));
         Builder.addFrameIndex(VRM->getStackSlot(Reg));
         Builder.addImm(InstNum);
+        // Record how large the original value is. The stack slot might be
+        // merged and altered during optimisation, but we will want to know how
+        // large the value is, at this DBG_PHI.
+        Builder.addImm(regSizeInBits);
+      }
+
+      LLVM_DEBUG(
+      if (SpillOffset != 0) {
+        dbgs() << "DBG_PHI for Vreg " << Reg << " subreg " << SubReg <<
+                  " has nonzero offset\n";
       }
+      );
     }
     // If there was no mapping for a value ID, it's optimized out. Create no
     // DBG_PHI, and any variables using this value will become optimized out.