[llvm-bugs] [Bug 51943] New: Variable's value late availability after Instruction Selection at Oz

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51943

            Bug ID: 51943
           Summary: Variable's value late availability after Instruction
                    Selection at Oz
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: DebugInfo
          Assignee: unassignedbugs at nondot.org
          Reporter: cristianassaiante at outlook.com
                CC: jdevlieghere at apple.com, keith.walker at arm.com,
                    llvm-bugs at lists.llvm.org,
                    paul_robinson at playstation.sony.com

A variable (l_33) value is visile after its declarations, then its not visible
anymore after a step command and it becomes available at line 7.
>From opt-bisect, it seems that the pass after which this behaviour occurs is: 
DAG->DAG Instruction Selection on function (main)

$ cat a.c
volatile int a, b;
int c;
int main() {
  int l_194 = 0;
  for (; c >= 0; --c) {
    l_194 = a;
    b = 0;
  }
}

$ clang -v
clang version 13.0.0
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 13.0.0

$ clang -g -Oz -o opt a.c
$ lldb opt
(lldb) target create "opt"
Current executable set to
'/home/stepping/debuginfo-analysis/issues/issue16/opt' (x86_64).
(lldb) b main
Breakpoint 1: where = opt`main at a.c:5:10, address = 0x0000000000400478
(lldb) r
Process 27 launched: '/home/stepping/debuginfo-analysis/issues/issue16/opt'
(x86_64)
Process 27 stopped
* thread #1, name = 'opt', stop reason = breakpoint 1.1
    frame #0: 0x0000000000400478 opt`main at a.c:5:10
   2    int c;
   3    int main() {
   4      int l_194 = 0;
-> 5      for (; c >= 0; --c) {
   6        l_194 = a;
   7        b = 0;
   8      }
(lldb) frame var
(int) l_194 = 0
(lldb) n
Process 27 stopped
* thread #1, name = 'opt', stop reason = step over
    frame #0: 0x0000000000400482 opt`main at a.c:6:13
   3    int main() {
   4      int l_194 = 0;
   5      for (; c >= 0; --c) {
-> 6        l_194 = a;
   7        b = 0;
   8      }
   9    }
(lldb) frame var
(int) l_194 = <variable not available>

(lldb) n
Process 27 stopped
* thread #1, name = 'opt', stop reason = step over
    frame #0: 0x0000000000400488 opt`main at a.c:7:7
   4      int l_194 = 0;
   5      for (; c >= 0; --c) {
   6        l_194 = a;
-> 7        b = 0;
   8      }
   9    }
(lldb) frame var
(int) l_194 = 0

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