[MachineCopyPropagation] Fix differences in code gen when compiling with -g

Fixes bugs [[ https://bugs.llvm.org/show_bug.cgi?id=50580 | 50580 ]] and [[ https://bugs.llvm.org/show_bug.cgi?id=49446 | 49446  ]]

When compiling with -g "DBG_VALUE <reg>"  instructions are added in the MIR, if such a instruction is inserted between instructions that use <reg> then MachineCopyPropagation invalidates <reg> , this causes some copies  to not be propagated and causes differences in code generation (ex bugs 50580 and 49446 ).  DBG_VALUE instructions should be ignored  since they don't actually modify the register.

Reviewed By: lkail

Differential Revision: https://reviews.llvm.org/D104394

Author: Alexandru Octavian Butiu

llvm/lib/CodeGen/MachineCopyPropagation.cpp

@@ -870,12 +870,32 @@ void MachineCopyPropagation::BackwardCopyPropagateBlock(
       if (MO.isDef())
         Tracker.invalidateRegister(MO.getReg().asMCReg(), *TRI);
 
-      if (MO.readsReg())
-        Tracker.invalidateRegister(MO.getReg().asMCReg(), *TRI);
+      if (MO.readsReg()) {
+        if (MO.isDebug()) {
+          //  Check if the register in the debug instruction is utilized
+          // in a copy instruction, so we can update the debug info if the
+          // register is changed.
+          for (MCRegUnitIterator RUI(MO.getReg().asMCReg(), TRI); RUI.isValid();
+               ++RUI) {
+            if (auto *Copy = Tracker.findCopyDefViaUnit(*RUI, *TRI)) {
+              CopyDbgUsers[Copy].insert(MI);
+            }
+          }
+        } else {
+          Tracker.invalidateRegister(MO.getReg().asMCReg(), *TRI);
+        }
+      }
     }
   }
 
   for (auto *Copy : MaybeDeadCopies) {
+
+    Register Src = Copy->getOperand(1).getReg();
+    Register Def = Copy->getOperand(0).getReg();
+    SmallVector<MachineInstr *> MaybeDeadDbgUsers(CopyDbgUsers[Copy].begin(),
+                                                  CopyDbgUsers[Copy].end());
+
+    MRI->updateDbgUsersToReg(Src.asMCReg(), Def.asMCReg(), MaybeDeadDbgUsers);
     Copy->eraseFromParent();
     ++NumDeletes;
   }

llvm/test/CodeGen/X86/machine-copy-dbgvalue.mir

@@ -0,0 +1,20 @@
+# RUN: llc -mtriple=i686-- -run-pass machine-cp -verify-machineinstrs -o - %s | FileCheck %s
+
+
+---
+# Test that machine copy propagation ignores DBG_VALUE and DBL_VALUE_LIST and updates it.
+# CHECK-LABEL: name: foo
+# CHECK: bb.0:
+# CHECK-NEXT: $rax = MOV64ri 31
+# CHECK-NEXT: DBG_VALUE $rax
+# CHECK-NEXT: DBG_VALUE_LIST 0, !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_LLVM_arg, 1, DW_OP_constu, 4, DW_OP_mul, DW_OP_plus, DW_OP_stack_value), $rax, 0, 0
+# CHECK-NEXT: RETQ implicit killed $rax
+name: foo
+body: |
+  bb.0:
+    renamable $rcx = MOV64ri 31
+    DBG_VALUE $rcx, 0, 0, 0, 0
+    DBG_VALUE_LIST 0, !DIExpression(DW_OP_LLVM_arg, 0, DW_OP_LLVM_arg, 1, DW_OP_constu, 4, DW_OP_mul, DW_OP_plus, DW_OP_stack_value), $rcx, 0, 0
+    $rax = COPY killed renamable $rcx
+    RETQ implicit killed $rax
+...