Wrong debug info generated at -O3 (-O0 is correct)

[llvmbot]

Bugzilla Link	43955
Version	trunk
OS	Linux
Blocks	#38116
Reporter	LLVM Bugzilla Contributor
CC	@adrian-prantl,@gregbedwell,@jmorse,@jdm,@pogo59,@zygoloid,@vedantk

Extended Description

The expected output from lldb should be 5. However, compiled with "-O3", lldb outputs 0.

$ clang-trunk -v
clang version 10.0.0 (trunk 375507)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8.5
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.4.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@MX32
Selected multilib: .;@m64

$ clang-trunk -g abc.c -O3
abc.c:6:13: warning: implicitly declaring library function 'printf' with type 'int (const char *, ...)' [-Wimplicit-function-declaration]
void f(g) { printf("%X\n", g); }
^
abc.c:6:13: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
1 warning generated.
$ lldb-trunk -s cmds -b a.out
(lldb) target create "a.out"
Current executable set to '/home/sding/LLDB-testing/reduce/30972/report/a.out' (x86_64).
(lldb) command source -s 0 'cmds'
Executing commands in '/home/sding/LLDB-testing/reduce/30972/report/cmds'.
(lldb) b 25
Breakpoint 1: where = a.out`main + 781 at abc.c:25:7, address = 0x000000000040083d
(lldb) r
Process 21519 stopped

• thread #​1, name = 'a.out', stop reason = breakpoint 1.1
  frame #​0: 0x000000000040083d a.out`main at abc.c:25:7
  22 for (; i < 5; i++)
  23 h(e[i]);
  24 h(1);
  -> 25 f(b ^ 5); // optimize_me_not0
  26 }

Process 21519 launched: '/home/sding/LLDB-testing/reduce/30972/report/a.out' (x86_64)
(lldb) p i
(int) $0 = 0

$ clang-trunk -g abc.c -O0
abc.c:6:13: warning: implicitly declaring library function 'printf' with type 'int (const char *, ...)' [-Wimplicit-function-declaration]
void f(g) { printf("%X\n", g); }
^
abc.c:6:13: note: include the header <stdio.h> or explicitly provide a declaration for 'printf'
1 warning generated.
$ lldb-trunk -s cmds -b a.out
(lldb) target create "a.out"
Current executable set to '/home/sding/LLDB-testing/reduce/30972/report/a.out' (x86_64).
(lldb) command source -s 0 'cmds'
Executing commands in '/home/sding/LLDB-testing/reduce/30972/report/cmds'.
(lldb) b 25
Breakpoint 1: where = a.out`main + 164 at abc.c:25:5, address = 0x0000000000400634
(lldb) r
Process 29588 stopped

• thread #​1, name = 'a.out', stop reason = breakpoint 1.1
  frame #​0: 0x0000000000400634 a.out`main at abc.c:25:5
  22 for (; i < 5; i++)
  23 h(e[i]);
  24 h(1);
  -> 25 f(b ^ 5); // optimize_me_not0
  26 }

Process 29588 launched: '/home/sding/LLDB-testing/reduce/30972/report/a.out' (x86_64)
(lldb) p i
(int) $0 = 5

$ cat abc.c
int a[56];
int b;
short c;
static int d;
static int e[] = {4, 0, 4, 0, 4};
void f(g) { printf("%X\n", g); }
void h(g) {
b = b & 4095 ^ a[(b ^ g) & 255];
b = b & 4095 ^ a[(b ^ 8) & 255];
b = b & 4095 ^ a[(b ^ 6) & 5];
}
int main() {
int i, j;
i = 0;
h(c);
for (; i < 3; i++) {
j = 0;
for (; j < 2; j++)
h(d);
}
i = 0;
for (; i < 5; i++)
h(e[i]);
h(1);
f(b ^ 5); // optimize_me_not0
}

$ cat cmds
b 25
r
p i
kill
q

[jmorse]

Much like bug 43949 this reproducer has some exciting stepping behaviour, including two gratuitous backwards steps (23->15, 24->19)

Temporary breakpoint 2, main () at abc.c:15
15 h(c);
(gdb) n
19 h(d);
(gdb) n
23 h(e[i]);
(gdb) n
15 h(c);
(gdb) n
24 h(1);
(gdb) n
19 h(d);
(gdb) n
25 f(b ^ 5); // optimize_me_not0
(gdb)

Much like bug 43949, this too appears to be the fault of the machine instruction scheduler. The location list tail:

DW_OP_consts +3, DW_OP_stack_value
DW_OP_consts +4, DW_OP_stack_value
DW_OP_consts +0, DW_OP_stack_value
DW_OP_consts +5, DW_OP_stack_value
DW_OP_consts +0, DW_OP_stack_value)

clearly has some zeros in the wrong place (the entire program has been inlined & unrolled). The re-ordering appears to happen in machine instruction scheduler.

Technically this problem is a duplicate of 43949, but I think the stepping behaviour is worth looking at as well.