Missing Variables information at Og

[Quuxplusone]

Bugzilla Link	PR47093
Status	CONFIRMED
Importance	P enhancement
Reported by	Luca Massarelli (massarelli@diag.uniroma1.it)
Reported on	2020-08-10 07:52:23 -0700
Last modified on	2020-08-10 14:16:56 -0700
Version	trunk
Hardware	PC Linux
CC	dblaikie@gmail.com, ditaliano@apple.com, jdevlieghere@apple.com, jeremy.morse.llvm@gmail.com, josh@joshmatthews.net, keith.walker@arm.com, llvm-bugs@lists.llvm.org, paul_robinson@playstation.sony.com, vsk@apple.com
Fixed by commit(s)
Attachments
Blocks	PR38768
Blocked by
See also	PR39787

At line 9 lldb says that variables l_1876 and l_2441 are optimized out, but
they are actually used.

$ cat -n a.c
     1  short a;
     2  int b, d = 4;
     3  int *c = &b;
     4  void func_3() {
     5    int **l_1876 = &c;
     6    int e = 1;
     7    if (d) {
     8      int *l_2441 = a;
     9      (**l_1876) = l_2441;
    10    }
    11    *l_1876 = &e;
    12  }
    13  int main() { func_3(); }

$ cat a.c
short a;
int b, d = 4;
int *c = &b;
void func_3() {
  int **l_1876 = &c;
  int e = 1;
  if (d) {
    int *l_2441 = a;
    (**l_1876) = l_2441;
  }
  *l_1876 = &e;
}
int main() { func_3(); }

$ clang -v
clang version 12.0.0 (https://github.com/llvm/llvm-project.git
bc8be3054067ac822fc6d9f4f8e64c841f530f16)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Candidate multilib: .;@m64
Selected multilib: .;@m64

$ lldb -v
lldb version 12.0.0
  clang revision bc8be3054067ac822fc6d9f4f8e64c841f530f16
  llvm revision bc8be3054067ac822fc6d9f4f8e64c841f530f16

$ clang -g -Og -o opt a.c

$ lldb opt
(lldb) target create "opt"
Current executable set to 'opt' (x86_64).
(lldb) b func_3
Breakpoint 1: where = opt`func_3 at a.c:6:7, address = 0x0000000000400480
(lldb) r
Process 66 launched: '/home/stepping/output/opt' (x86_64)
Process 66 stopped
* thread #1, name = 'opt', stop reason = breakpoint 1.1
    frame #0: 0x0000000000400480 opt`func_3 at a.c:6:7
   3    int *c = &b;
   4    void func_3() {
   5      int **l_1876 = &c;
-> 6      int e = 1;
   7      if (d) {
   8        int *l_2441 = a;
   9        (**l_1876) = l_2441;
(lldb) s
Process 66 stopped
* thread #1, name = 'opt', stop reason = step in
    frame #0: 0x0000000000400488 opt`func_3 at a.c:7:7
   4    void func_3() {
   5      int **l_1876 = &c;
   6      int e = 1;
-> 7      if (d) {
   8        int *l_2441 = a;
   9        (**l_1876) = l_2441;
   10     }
(lldb) s
Process 66 stopped
* thread #1, name = 'opt', stop reason = step in
    frame #0: 0x0000000000400491 opt`func_3 at a.c:8:19
   5      int **l_1876 = &c;
   6      int e = 1;
   7      if (d) {
-> 8        int *l_2441 = a;
   9        (**l_1876) = l_2441;
   10     }
   11     *l_1876 = &e;
(lldb) s
Process 66 stopped
* thread #1, name = 'opt', stop reason = step in
    frame #0: 0x0000000000400498 opt`func_3 at a.c:9:7
   6      int e = 1;
   7      if (d) {
   8        int *l_2441 = a;
-> 9        (**l_1876) = l_2441;
   10     }
   11     *l_1876 = &e;
   12   }
(lldb) p l_2441
error: Couldn't materialize: couldn't get the value of variable l_2441: no
location, value may have been optimized out
error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression
(lldb) p l_1876
error: Couldn't materialize: couldn't get the value of variable l_1876: no
location, value may have been optimized out
error: errored out in DoExecute, couldn't PrepareToExecuteJITExpression
(lldb) di
opt`func_3:
    0x400480 <+0>:  movl   $0x1, -0x4(%rsp)
    0x400488 <+8>:  cmpl   $0x0, 0x200b99(%rip)      ; __dso_handle + 7
    0x40048f <+15>: je     0x4004a1                  ; <+33> at a.c
    0x400491 <+17>: movswl 0x200ba8(%rip), %eax      ; a
->  0x400498 <+24>: movq   0x200b91(%rip), %rcx      ; c
    0x40049f <+31>: movl   %eax, (%rcx)
    0x4004a1 <+33>: leaq   -0x4(%rsp), %rax
    0x4004a6 <+38>: movq   %rax, 0x200b83(%rip)      ; c
    0x4004ad <+45>: retq
(lldb)

[Quuxplusone]

To me, this sounds like something discovered in bug 39787, we're terrible at salvaging anything to do with a global symbol reference. In the reproducer, a bunch of local variables all trace back to global variables, and they all get folded into memory expressions, so never get an in-register location.

I think the biggest problem is that DBG_VALUE instructions are not expected to contain any GlobalValue operands.

It looks like GCC uses DW_OP_addr to describe most of these variables, I imagine that's the correct way, but don't know that part of DWARF well.