[DebugInfo] Support for DW_OP_implicit_pointer (for mem2reg optimization)

[alok]

Summary

This patch (8/N) stems from D69787

Summary:

  Just invoked salvaging debug info function before deleting the instructions.

Testing:

  - Added unit tests for validation of LLVM IR and object file thru llvm-dwarfdump
  - check-llvm, and an end-to-end test using gnu GDB to debug an optimized
      program (LLDB need to be enhanced to support).
  - check-debuginfo (the debug info integration tests)

[alok]

Updated to rebase due to changes in trunk and parent patches.

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
     LI->eraseFromParent();
     LBI.deleteValue(LI);
   }
 
   // Finally, after the scan, check to see if the store is all that is left.
   if (!Info.UsingBlocks.empty())
     return false; // If not, we'll have to fall back for the remainder.
 
+  // Try to preserve debug information attached to the dead instruction.
+  salvageDebugInfo(*AI);
+
   // Record debuginfo for the store and remove the declaration's
   // debuginfo.
   for (DbgVariableIntrinsic *DII : Info.DbgDeclares) {
     DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false);
     ConvertDebugDeclareToDebugValue(DII, Info.OnlyStore, DIB);
     DII->eraseFromParent();
   }
   // Remove the (now dead) store and alloca.
if (I == StoresByIndex.begin()) {
 
       LI->replaceAllUsesWith(ReplVal);
     }
 
     LI->eraseFromParent();
     LBI.deleteValue(LI);
   }
 
+  // Try to preserve debug information attached to the dead instruction.
+  salvageDebugInfo(*AI);
+
   // Remove the (now dead) stores and alloca.
   while (!AI->use_empty()) {
     StoreInst *SI = cast<StoreInst>(AI->user_back());
     // Record debuginfo for the store before removing it.
     for (DbgVariableIntrinsic *DII : Info.DbgDeclares) {
       DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false);
       ConvertDebugDeclareToDebugValue(DII, SI, DIB);
     }

llvm/test/DebugInfo/dwarfdump-implicit_pointer_mem2reg.c

+// RUN: clang %s -O2 -gdwarf-5 -o %t.o
+// RUN: llvm-dwarfdump  %t.o | FileCheck %s
+
+// CHECK-LABEL:  DW_AT_name    ("arr1")
+
+// 1. Test if more than one member location list is printed 
+//    (for pointer to scalar)
+// CHECK-LABEL:       DW_AT_location        (
+// CHECK-NEXT:     : DW_OP_lit0, DW_OP_stack_value
+// CHECK-NEXT:     : DW_OP_implicit_pointer {{0x.+}} +0
+// CHECK-NEXT:  DW_AT_name    ("ptr2")
+
+// 2. Test if More than one member location list is printed 
+//    (for pointer to pointer to scalar)
+// CHECK-LABEL:       DW_AT_location        (
+// CHECK-NEXT:     : DW_OP_lit0, DW_OP_stack_value
+// CHECK-NEXT:     : DW_OP_implicit_pointer {{0x.+}} +0
+// CHECK-NEXT:  DW_AT_name    ("ptrptr2")
+
+// 3. Test if one member location list is printed
+//    (for pointer to pointer to array)
+// CHECK-LABEL:       DW_AT_location        (
+// CHECK-NEXT:     : DW_OP_implicit_pointer {{0x.+}} +0
+// CHECK-NEXT:  DW_AT_name    ("ptrptr3")
+//
+// 4. Test if one member location list is printed
+//    (for pointer to pointer to scalar)
+// CHECK-LABEL:       DW_AT_location        (
+// CHECK-NEXT:     : DW_OP_implicit_pointer {{0x.+}} +0
+// CHECK-NEXT:  DW_AT_name    ("ptrptr1")
+
+// 5. Test if one member location list is printed
+//    (for pointer to array)
+// CHECK-LABEL:       DW_AT_location        (
+// CHECK-NEXT:     : DW_OP_implicit_pointer {{0x.+}} +0
+// CHECK-NEXT:  DW_AT_name    ("ptr3")
+
+// 6. Test if one member location list is printed
+//    (for pointer to scalar)
+// CHECK-LABEL:       DW_AT_location        (
+// CHECK-NEXT:     : DW_OP_implicit_pointer {{0x.+}} +0
+// CHECK-NEXT:  DW_AT_name    ("ptr1")
+
+static const char *b = "opq";
+volatile int v;
+int main() {
+  int var1 = 4;
+  int var2 = 5;
+  int arr1[2] = {2, 3};
+  int *ptr1;
+  int *ptr2 = 0;
+  int *ptr3;
+  int **ptrptr1;
+  int **ptrptr2 = 0;
+  int **ptrptr3;
+
+  v++;
+  ptr1 = &var1;
+  ptr2 = &var2;
+  ptr3 = arr1;
+  ptrptr1 = &ptr1;
+  ptrptr2 = &ptr2;
+  ptrptr3 = &ptr3;
+  v++;
+
+  return arr1[0] + arr1[1] + *ptr1 + *ptr2 + **ptrptr1 + **ptrptr2 - 5;
+}

llvm/test/DebugInfo/implicit_pointer_mem2reg.c

+// RUN: clang %s -O2 -gdwarf-5 -S -emit-llvm -o %t.ll
+// RUN: FileCheck %s --input-file=%t.ll
+
+// CHECK:   !DIExpression(DW_OP_LLVM_implicit_pointer, DW_OP_LLVM_arg0, 0)
+// CHECK:   !DIExpression(DW_OP_LLVM_implicit_pointer, DW_OP_LLVM_arg0, 0)
+// CHECK:   !DIExpression(DW_OP_LLVM_implicit_pointer, DW_OP_LLVM_arg0, 0)
+// CHECK:   !DIExpression(DW_OP_LLVM_implicit_pointer, DW_OP_LLVM_arg0, 0)
+// CHECK:   !DIExpression(DW_OP_LLVM_implicit_pointer, DW_OP_LLVM_arg0, 0)
+// CHECK:   !DIExpression(DW_OP_LLVM_implicit_pointer, DW_OP_LLVM_arg0, 0)
+
+static const char *b = "opq";
+volatile int v;
+int main() {
+  int var1 = 4;
+  int var2 = 5;
+  int arr1[2] = {2, 3};
+  int *ptr1;
+  int *ptr2 = 0;
+  int *ptr3;
+  int **ptrptr1;
+  int **ptrptr2 = 0;
+  int **ptrptr3;
+
+  v++;
+  ptr1 = &var1;
+  ptr2 = &var2;
+  ptr3 = arr1;
+  ptrptr1 = &ptr1;
+  ptrptr2 = &ptr2;
+  ptrptr3 = &ptr3;
+  v++;
+
+  return arr1[0] + arr1[1] + *ptr1 + *ptr2 + **ptrptr1 + **ptrptr2 - 5;
+}