[llvm-bugs] [Bug 52238] New: Variable's value late availability and incorrect at Og

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52238

            Bug ID: 52238
           Summary: Variable's value late availability and incorrect at Og
           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

Variable l_5 value is not available on line 4 even if its assignment has been
done and it becomes available at line 6.
Moreover, the value of the variable seems to be wrong since lldb shows 1 but
the assignment at line 7 has not been reached yet.
>From bisect, this behavior may be caused by Branch Probability Basic Block
Placement on function (main).

$ cat a.c
volatile int a, b;
void  c() {
    int l_5 = 5;
    for (;
        b >= 0;
        b --) {
            l_5 = 1;
        }
}
int main () {
    c();
}

$ 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 -Og -o opt a.c
$ lldb opt
(lldb) target create "opt"
Current executable set to '/home/stepping/40/reduce/opt' (x86_64).
(lldb) b 3
Breakpoint 1: 2 locations.
(lldb) run
Process 197 launched: '/home/stepping/40/reduce/opt' (x86_64)
Process 197 stopped
* thread #1, name = 'opt', stop reason = breakpoint 1.2
    frame #0: 0x00000000004004b7 opt`main [inlined] c at a.c:4:5
   1    volatile int a, b;
   2    void  c() {
   3        int l_5 = 5;
-> 4        for (;
   5            b >= 0;
   6            b --) {
   7                l_5 = 1;
(lldb) frame var
(int) l_5 = <variable not available>

(lldb) n
Process 197 stopped
* thread #1, name = 'opt', stop reason = step over
    frame #0: 0x00000000004004c0 opt`main [inlined] c at a.c:6:11
   3        int l_5 = 5;
   4        for (;
   5            b >= 0;
-> 6            b --) {
   7                l_5 = 1;
   8            }
   9    }
(lldb) frame var
(int) l_5 = 1

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