Wrong Line Information at Og

[Quuxplusone]

Bugzilla Link	PR46074
Status	CONFIRMED
Importance	P enhancement
Reported by	Luca Massarelli (massarelli@diag.uniroma1.it)
Reported on	2020-05-26 06:01:44 -0700
Last modified on	2020-06-05 11:21:45 -0700
Version	trunk
Hardware	PC Linux
CC	aprantl@apple.com, ditaliano@apple.com, jdevlieghere@apple.com, keith.walker@arm.com, llvm-bugs@lists.llvm.org, paul_robinson@playstation.sony.com, vsk@apple.com
Fixed by commit(s)
Attachments
Blocks
Blocked by
See also

Line 8 should not be hit.

$ cat a.c
typedef int int32_t;
int a, b, c;
static int32_t d;
static char e;
char(f)(char g) { return g >> a; }
int main() {
  for (; d < 9; ++d) {
    int32_t l_273[2][4][1];
    c = 0;
    for (; c < 4; c++)
      l_273[0][c][0] = 0;
    b = f(e--);
    l_273[0][3][0] |= b;
  }
}

$  clang -v
clang version 11.0.0 (https://github.com/llvm/llvm-project.git
268fa40daa151d3b4bff1df12b62e5dae94685d7)
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 11.0.0
  clang revision 268fa40daa151d3b4bff1df12b62e5dae94685d7
  llvm revision 268fa40daa151d3b4bff1df12b62e5dae94685d7

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

$ lldb opt
(lldb) target create "opt"
Current executable set to 'opt' (x86_64).
(lldb) b main
Breakpoint 1: where = opt`main + 7 at a.c:7:3, address = 0x0000000000400497
(lldb) r
Process 65 launched: 'opt' (x86_64)
Process 65 stopped
* thread #1, name = 'opt', stop reason = breakpoint 1.1
    frame #0: 0x0000000000400497 opt`main at a.c:7:3
   4    static char e;
   5    char(f)(char g) { return g >> a; }
   6    int main() {
-> 7      for (; d < 9; ++d) {
   8        int32_t l_273[2][4][1];
   9        c = 0;
   10       for (; c < 4; c++)
(lldb) s
Process 65 stopped
* thread #1, name = 'opt', stop reason = step in
    frame #0: 0x00000000004004b0 opt`main at a.c:8:5
   5    char(f)(char g) { return g >> a; }
   6    int main() {
   7      for (; d < 9; ++d) {
-> 8        int32_t l_273[2][4][1];
   9        c = 0;
   10       for (; c < 4; c++)
   11         l_273[0][c][0] = 0;
(lldb) di
opt`main:
    0x400490 <+0>:   pushq  %r14
    0x400492 <+2>:   pushq  %rbx
    0x400493 <+3>:   subq   $0x28, %rsp
    0x400497 <+7>:   cmpb   $0x0, 0x200b92(%rip)      ; a + 3
    0x40049e <+14>:  jne    0x4004ef                  ; <+95> at a.c:15:1
    0x4004a0 <+16>:  movzbl 0x200b90(%rip), %r14d     ; e
    0x4004a8 <+24>:  xorl   %ebx, %ebx
    0x4004aa <+26>:  nopw   (%rax,%rax)
->  0x4004b0 <+32>:  leal   (%r14,%rbx), %eax
    0x4004b4 <+36>:  xorps  %xmm0, %xmm0
    0x4004b7 <+39>:  movaps %xmm0, (%rsp)
    0x4004bb <+43>:  movl   $0x4, 0x200b6f(%rip)      ; d
    0x4004c5 <+53>:  movsbl %al, %edi
    0x4004c8 <+56>:  callq  0x400480                  ; f at a.c:5
    0x4004cd <+61>:  movsbl %al, %eax
    0x4004d0 <+64>:  movl   %eax, 0x200b66(%rip)      ; b
    0x4004d6 <+70>:  addl   $-0x1, %ebx
    0x4004d9 <+73>:  cmpl   $-0x9, %ebx
    0x4004dc <+76>:  jne    0x4004b0                  ; <+32> at a.c:8:5
    0x4004de <+78>:  addl   %ebx, %r14d
    0x4004e1 <+81>:  movb   %r14b, 0x200b50(%rip)     ; e
    0x4004e8 <+88>:  movb   $0x1, 0x200b41(%rip)      ; a + 3
    0x4004ef <+95>:  xorl   %eax, %eax
    0x4004f1 <+97>:  addq   $0x28, %rsp
    0x4004f5 <+101>: popq   %rbx
    0x4004f6 <+102>: popq   %r14
    0x4004f8 <+104>: retq
(lldb) s
Process 65 stopped
* thread #1, name = 'opt', stop reason = step in
    frame #0: 0x00000000004004b4 opt`main at a.c:11:22
   8        int32_t l_273[2][4][1];
   9        c = 0;
   10       for (; c < 4; c++)
-> 11         l_273[0][c][0] = 0;
   12       b = f(e--);
   13       l_273[0][3][0] |= b;
   14     }
(lldb) s
Process 65 stopped
* thread #1, name = 'opt', stop reason = step in
    frame #0: 0x00000000004004c5 opt`main at a.c:12:9
   9        c = 0;
   10       for (; c < 4; c++)
   11         l_273[0][c][0] = 0;
-> 12       b = f(e--);
   13       l_273[0][3][0] |= b;
   14     }
   15   }

[Quuxplusone]

Confirmed:

Process 24778 launched: '/Users/davide/work/build/bin/a.out' (x86_64)
a.out was compiled with optimization - stepping may behave oddly; variables may
not be available.
Process 24778 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100000f20 a.out`main at a.c:8:5 [opt]
   5    char(f)(char g) { return g >> a; }
   6    int main() {
   7      for (; d < 9; ++d) {
-> 8        int32_t l_273[2][4][1];
   9        c = 0;
   10       for (; c < 4; c++)
   11         l_273[0][c][0] = 0;

I guess this is not a big deal but it's technically incorrect. We should stop
on line 9 instead of the decl when we enter the loop. +Vedant/Adrian.

[Quuxplusone]

It's LSR.

BISECT: running pass (107) Partially inline calls to library functions on
function (f)
BISECT: running pass (108) X86 Partial Reduction on function (f)
BISECT: running pass (109) CodeGen Prepare on function (f)
BISECT: running pass (110) Canonicalize Freeze Instructions in Loops on loop
BISECT: running pass (111) Loop Strength Reduction on loop

[Quuxplusone]

@@ -32,35 +32,39 @@ 4: ; preds = %0 %5 = getelementptr inbounds [2 x [4 x [1 x i32]]], [2 x [4 x [1 x i32]]] %1, i64 0, i64 0, i64 3, i64 0, !dbg !52 %6 = load i8, i8 @e, align 1, !tbaa !53

• br label %7, !dbg !51
• %7 = zext i8 %6 to i32, !dbg !51
• br label %8, !dbg !51

^ presumably when we create the zext that inherits the location of the branch & that causes the funny stepping.