[llvm-bugs] [Bug 38779] New: [DebugInfo at O2][Dexter] Incorrect variable valuations reported from loop

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38779

            Bug ID: 38779
           Summary: [DebugInfo at O2][Dexter] Incorrect variable valuations
                    reported from loop
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Keywords: wrong-debug
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: jeremy.morse.llvm at gmail.com
                CC: chackz0x12 at gmail.com, greg.bedwell at sony.com,
                    international.phantom at gmail.com,
                    llvm-bugs at lists.llvm.org,
                    paul_robinson at playstation.sony.com
            Blocks: 38768

Debuggers report the loop below as having a constant-valued iterator ("i") and
wildly-changing accumulator ("sum"). Testing environment is llvm/clang @
r340912 and options "-O2 -g -fno-inline -fno-unroll-loops" for x86_64. I've
used no-funroll-loops for conciseness, this replicates if one just uses a large
loop bound too. Both lldb and gdb report the same problems.

In the loop below, the iterator "i" is reported as optimised out when stopped
on the loop-header line, or as the value one when stopped on the line that
calls "lul" for every iteration of the loop.

The "sum" variable is more exciting: when stopped on the loop header the true
value of the variable is reported, but when stopped on the line calling "lul",
a constant value of zero is reported.

My feeling here is that the instruction-range for constant initialisation of
"sum" and "i" is encroaching on the loop; as for "i" being optimised-out,
that's likely because the indvars pass makes very little effort to preserve
debug data, for which I'll open a different ticket.

-------->8--------
int
lul(int i, int other)
{
  volatile int foo = i;
  return foo + other;
}

int
somearray[] = {
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

int
main()
{
  int sum = 0;
  int *ptr = somearray;
  for (int i = 1; i < 11; i++, ptr++) {
    sum += lul(i, *ptr);
  }

  return sum;
}
--------8<--------


Referenced Bugs:

https://bugs.llvm.org/show_bug.cgi?id=38768
[Bug 38768] [meta][DebugInfo] Umbrella bug for poor debug experiences
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