Index: llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
===================================================================
--- llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -1812,9 +1812,13 @@
   // recorded in the value substitution table. Apply any substitutions to
   // the instruction / operand number in this DBG_INSTR_REF.
   auto Sub = MF.DebugValueSubstitutions.find(std::make_pair(InstNo, OpNo));
+  // Collect any subregister extractions performed during optimization.
+  SmallVector<unsigned, 4> SeenSubregs;
   while (Sub != MF.DebugValueSubstitutions.end()) {
-    InstNo = Sub->second.first;
-    OpNo = Sub->second.second;
+    std::tie(InstNo, OpNo) = Sub->second.Dest;
+    unsigned Subreg = Sub->second.Subreg;
+    if (Subreg)
+      SeenSubregs.push_back(Subreg);
     Sub = MF.DebugValueSubstitutions.find(std::make_pair(InstNo, OpNo));
   }
 
@@ -1847,6 +1851,77 @@
                            MI, InstNo);
   }
 
+  // Apply any subregister extractions, in reverse. We might have seen code
+  // like this:
+  //    CALL64 @foo, implicit-def $rax
+  //    %0:gr64 = COPY $rax
+  //    %1:gr32 = COPY %0.sub_32bit
+  //    %2:gr16 = COPY %1.sub_16bit
+  //    %3:gr8  = COPY %2.sub_8bit
+  // In which case each copy would have been recorded as a substitution with
+  // a subregister qualifier. Apply those qualifiers now.
+  if (NewID && !SeenSubregs.empty()) {
+    unsigned Offset = 0;
+    unsigned Size = 0;
+
+    // Look at each subregister that we passed through, and progressively
+    // narrow in, accumulating any offsets that occur. Substitutions should
+    // only ever be the same or narrower width than what they read from;
+    // iterate in reverse order so that we go from wide to small.
+    for (unsigned Subreg : reverse(SeenSubregs)) {
+      unsigned ThisSize = TRI->getSubRegIdxSize(Subreg);
+      unsigned ThisOffset = TRI->getSubRegIdxOffset(Subreg);
+      Offset += ThisOffset;
+      Size = (Size == 0) ? ThisSize : std::min(Size, ThisSize);
+    }
+
+    // If that worked, look for an appropriate subregister with the register
+    // where the define happens. Don't look at values that were defined during
+    // a stack write: we can't currently express register locations within
+    // spills.
+    LocIdx L = NewID->getLoc();
+    if (NewID && !MTracker->isSpill(L)) {
+      // Find the register class for the register where this def happened.
+      // FIXME: no index for this?
+      Register Reg = MTracker->LocIdxToLocID[L];
+      const TargetRegisterClass *TRC = nullptr;
+      for (auto *TRCI : TRI->regclasses())
+        if (TRCI->contains(Reg))
+          TRC = TRCI;
+      assert(TRC && "Couldn't find target register class?");
+
+      // If the register we have isn't the right size or in the right place,
+      // Try to find a subregister inside it.
+      unsigned MainRegSize = TRI->getRegSizeInBits(*TRC);
+      if (Size != MainRegSize || Offset) {
+        // Enumerate all subregisters, searching.
+        Register NewReg = 0;
+        for (MCSubRegIterator SRI(Reg, TRI, false); SRI.isValid(); ++SRI) {
+          unsigned Subreg = TRI->getSubRegIndex(Reg, *SRI);
+          unsigned SubregSize = TRI->getSubRegIdxSize(Subreg);
+          unsigned SubregOffset = TRI->getSubRegIdxOffset(Subreg);
+          if (SubregSize == Size && SubregOffset == Offset) {
+            NewReg = *SRI;
+            break;
+          }
+        }
+
+        // If we didn't find anything: there's no way to express our value.
+        if (!NewReg) {
+          NewID = None;
+        } else {
+          // Re-state the value as being defined within the subregister
+          // that we found.
+          LocIdx NewLoc = MTracker->lookupOrTrackRegister(NewReg);
+          NewID = ValueIDNum(NewID->getBlock(), NewID->getInst(), NewLoc);
+        }
+      }
+    } else {
+      // If we can't handle subregisters, unset the new value.
+      NewID = None;
+    }
+  }
+
   // We, we have a value number or None. Tell the variable value tracker about
   // it. The rest of this LiveDebugValues implementation acts exactly the same
   // for DBG_INSTR_REFs as DBG_VALUEs (just, the former can refer to values that
Index: llvm/test/DebugInfo/MIR/InstrRef/livedebugvalues_subreg_substitutions.mir
===================================================================
--- /dev/null
+++ llvm/test/DebugInfo/MIR/InstrRef/livedebugvalues_subreg_substitutions.mir
@@ -0,0 +1,106 @@
+# RUN: llc %s -march=x86-64 -run-pass=livedebugvalues -experimental-debug-variable-locations -o - 2>&1 | FileCheck %s
+#
+# Test that when we have a subregister qualifiers in substitutions, that
+# InstrRefBasedLDV correctly applies them to the variable location. Below, a
+# call defines all of $rax, but the variable locations should only apply to
+# the low order 8 bits.
+--- |
+  define i8 @test(i32 %bar) local_unnamed_addr !dbg !7 {
+  entry:
+    ret i8 0, !dbg !12
+  }
+
+  declare dso_local void @ext(i64)
+
+  !llvm.dbg.cu = !{!0}
+  !llvm.module.flags = !{!3, !4, !5, !6}
+  !0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+  !1 = !DIFile(filename: "foo.cpp", directory: ".")
+  !2 = !DIBasicType(name: "int", size: 8, encoding: DW_ATE_signed)
+  !3 = !{i32 2, !"Dwarf Version", i32 4}
+  !4 = !{i32 2, !"Debug Info Version", i32 3}
+  !5 = !{i32 1, !"wchar_size", i32 2}
+  !6 = !{i32 7, !"PIC Level", i32 2}
+  !7 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: !1, file: !1, line: 6, type: !8, scopeLine: 6, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !10)
+  !8 = !DISubroutineType(types: !9)
+  !9 = !{!2, !2}
+  !10 = !{!11}
+  !11 = !DILocalVariable(name: "baz", scope: !7, file: !1, line: 7, type: !2)
+  !12 = !DILocation(line: 10, scope: !7)
+...
+---
+name: test
+tracksRegLiveness: true
+liveins:
+  - { reg: '$rdi', virtual-reg: '' }
+debugValueSubstitutions:
+  - { srcinst: 1, srcop: 0, dstinst: 2, dstop: 0, subreg: 1 } # sub_8bit
+  - { srcinst: 2, srcop: 0, dstinst: 3, dstop: 0, subreg: 4 } # sub_16bit
+  - { srcinst: 3, srcop: 0, dstinst: 4, dstop: 5, subreg: 6 } # sub_32bit
+  # Substitution involving sub_8bit_hi, should land in $ah
+  - { srcinst: 5, srcop: 0, dstinst: 6, dstop: 0, subreg: 2 } # sub_8bit_hi
+  - { srcinst: 6, srcop: 0, dstinst: 7, dstop: 0, subreg: 4 } # sub_16bit
+  - { srcinst: 7, srcop: 0, dstinst: 4, dstop: 5, subreg: 6 } # sub_32bit
+  # Several redundant substitutions, representing extractions from a small
+  # register, followed by larger spurious ones, for example:
+  # %0:gr64 = COPY $rax
+  # %1:gr32 = COPY %0.sub_32bit
+  # %2:gr16 = COPY %1.sub_16bit
+  # %3:gr64 = SUBREG_TO_REG %2, sub_8bit_hi
+  # %4:gr32 = COPY %3.sub_32bit
+  # %5:gr16 = COPY %2.sub_16bit
+  # Should still come out as ah.
+  - { srcinst: 8, srcop: 0, dstinst: 9, dstop: 0, subreg: 4 } # sub_16bit
+  - { srcinst: 9, srcop: 0, dstinst: 10,dstop: 0, subreg: 6 } # sub_32bit
+  - { srcinst: 10,srcop: 0, dstinst: 11,dstop: 0, subreg: 2 } # sub_8bit_hi
+  - { srcinst: 11,srcop: 0, dstinst: 12,dstop: 0, subreg: 4 } # sub_16bit
+  - { srcinst: 12,srcop: 0, dstinst: 4, dstop: 5, subreg: 6 } # sub_32bit
+  # If some kind of really mal-formed code appears that extracts the high bits
+  # out of a too-small location, we should drop it. It's not clear whether this
+  # scenario could ever happen; but if it did, best to not emit a known bad
+  # variable location. Should generate a DBG_VALUE $noreg.
+  - { srcinst: 13, srcop: 0, dstinst: 14,dstop: 0, subreg: 5 } # sub_16bit_hi
+  - { srcinst: 14, srcop: 0, dstinst: 15,dstop: 0, subreg: 6 } # sub_32bit
+  - { srcinst: 15, srcop: 0, dstinst: 4, dstop: 5, subreg: 1 } # sub_8bit
+stack:
+  - { id: 0, name: '', type: spill-slot, offset: -16, size: 8, alignment: 8,
+      stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+body:  |
+  bb.0:
+  liveins: $rdi, $rax
+    CALL64pcrel32 @ext, csr_64, implicit $rsp, implicit $ssp, implicit $edi, implicit-def $rax, debug-instr-number 4, debug-location !12
+    ; CHECK:      CALL64pcrel32
+    DBG_INSTR_REF 1, 0, !11, !DIExpression(), debug-location !12
+    ; CHECK-NEXT: DBG_INSTR_REF 1, 0
+    ; CHECK-NEXT: DBG_VALUE $al
+    DBG_INSTR_REF 5, 0, !11, !DIExpression(), debug-location !12
+    ; CHECK-NEXT: DBG_INSTR_REF 5, 0
+    ; CHECK-NEXT: DBG_VALUE $ah
+    DBG_INSTR_REF 8, 0, !11, !DIExpression(), debug-location !12
+    ; CHECK-NEXT: DBG_INSTR_REF 8, 0
+    ; CHECK-NEXT: DBG_VALUE $ah
+    DBG_INSTR_REF 13, 0, !11, !DIExpression(), debug-location !12
+    ; CHECK-NEXT: DBG_INSTR_REF 13, 0
+    ; CHECK-NEXT: DBG_VALUE $noreg
+    MOV64mr $rsp, 1, $noreg, 16, $noreg, $rax :: (store 8 into %stack.0)
+    $rax = MOV64ri 0, debug-location !12
+    ; CHECK:      $rax = MOV64ri 0
+    ; The value is now located in a spill slot; currently InstrRefBasedLDV
+    ; can't express subregister locations inside spills. These should all
+    ; end up being $noreg, but could be improved in the future.
+    DBG_INSTR_REF 1, 0, !11, !DIExpression(), debug-location !12
+    ; CHECK-NEXT: DBG_INSTR_REF 1, 0
+    ; CHECK-NEXT: DBG_VALUE $noreg
+    DBG_INSTR_REF 5, 0, !11, !DIExpression(), debug-location !12
+    ; CHECK-NEXT: DBG_INSTR_REF 5, 0
+    ; CHECK-NEXT: DBG_VALUE $noreg
+    DBG_INSTR_REF 8, 0, !11, !DIExpression(), debug-location !12
+    ; CHECK-NEXT: DBG_INSTR_REF 8, 0
+    ; CHECK-NEXT: DBG_VALUE $noreg
+    DBG_INSTR_REF 13, 0, !11, !DIExpression(), debug-location !12
+    ; CHECK-NEXT: DBG_INSTR_REF 13, 0
+    ; CHECK-NEXT: DBG_VALUE $noreg
+    $rax = MOV64rm $rsp, 1, $noreg, 8, $noreg :: (load 8 from %stack.0)
+    RETQ $rax, debug-location !12
+...