[llvm] [InstCombine] Retain debug information on store to null instruction (PR #86105)

[Abinaya Saravanan via llvm-commits]

quic-asaravan wrote:

Debug information not attached to the "store to null" instruction inserted in InstCombinePass to mark unreachable code.

cpp code which generates store to null after instcombinePass (test.cpp):

```
void *def;
#define s_node &def

typedef enum {
A,
B,
C,
D,
E
} enum_type;

typedef struct {
    void *field1;
} struct_1;

typedef struct {
  struct_1 *field1;
  void *val;
} struct_2;

typedef struct s {
  struct s *prev, *next;
  struct_2 *r;
  void *val;
} struct_3;

void foo();
void bar();
void foo1();
void bar1(struct_2 *val);

static unsigned int baz( struct_3 *L )
{
  return ( L->val != L->r->field1->field1 );
}

static enum_type baz1( struct_2 *r )
{
  if ( !r )
  {
    return A;
  }
  if ( r->field1 )
  {
    return B;
  }
  if ( s_node == r->val )
  {
    return C;
  }
  if ( r->val )
  {
    return D;
  }

  return E;
}

static unsigned int helper( struct_2     *r,
                                                     enum_type type )
{
  return ( type == baz1( r ) );
}

void F( struct_3 *pool, struct_3 *link);

struct {
    struct_3 *r;
    struct_3 p;
} struct_4;

#define NULL nullptr
#define VALIDATE(x) 

void foo( void )
{
  struct_3 *val;
  struct_3 *next_val;
  struct_3  sen;
  bar();
  val = struct_4.r; 
  sen.next = val;
  sen.prev = NULL;

  if ( val ) 
  { 
    VALIDATE( NULL == val->prev );
    val->prev = &sen;
  }
  
  /* First, remove all the aliases/placeholders */
  while ( val )
  {
    next_val = val->next;
    if ( helper( val->r, B ) ||
         baz( val ) )
    {
      val->prev->next = next_val;
      if ( next_val )
      {
        next_val->prev = val->prev;
      }
      
      F( &struct_4.p, val );
    }
    val = next_val;
  }

  /* Now remove all the remaining links */
  val = sen.next;
  while ( val )
  {
    next_val = val->next;

    val->prev->next = next_val;
    if ( next_val )
    {
      next_val->prev = val->prev;
    }

    bar1( val->r );
    F( &struct_4.p, val );
    
    val = next_val;
  }
  struct_4.r = sen.next;
  VALIDATE( NULL == struct_4.r );
  foo1();
}

```

The above code is compiled with clang (compiled at SHA - https://github.com/llvm/llvm-project/commit/10451ded6d7ef799569652dfce81653d37f167b5)

`clang++ -g -O2 -mllvm -print-after-all -mllvm -filter-print-funcs="_Z3foov" test.cpp -c`

```
.
.
.
*** IR Dump After InstCombinePass on _Z3foov ***
; Function Attrs: mustprogress
define dso_local void @_Z3foov() local_unnamed_addr #0 !dbg !49 {
.
.
lor.lhs.false:                                    ; preds = %_ZL6helperP8struct_29enum_type.exit
  call void @llvm.dbg.value(metadata ptr %val.058, metadata !106, metadata !DIExpression()), !dbg !113
  store i1 true, ptr poison, align 1            **<======== Debug Information missing ========>**
  br i1 poison, label %if.end15, label %if.then7, !dbg !114
.
.

```
Any specific reason not to have debug information attached to "store to null" instruction?

https://github.com/llvm/llvm-project/pull/86105