Missing line information after EarlyCSE and InstCombine pass at Og

[cristianassaiante]

Bugzilla Link	50475
Version	trunk
OS	Linux
CC	@JDevlieghere,@jmorse,@walkerkd,@OCHyams,@pogo59

Extended Description

The last assignment to a variable passed to an external function is not reached in debugging. If we put a breakpoint on line 14, after a next command we end up in line 17 thus skipping lines 15 and 16 where the last assignment to v_0 and v_1 is done. From opt-bisect, the pass after which line 15 is not reached anymore is EarlyCSEPass and the pass after which line 16 is not reached anymore is InstCombinePass.

$ cat a.c
int test_in();
void test_out(int v_0, int v_1, int v_2, int v_3);

int main (void)
{
int v_0, v_1, v_2, v_3;
v_0 = test_in();
v_1 = test_in();
v_2 = test_in();
v_3 = test_in();
v_1 = (!v_1 > (v_2 - (v_3 <= v_0)));
v_0 = (v_1 < (~v_2 <= (v_3 >= v_0)));

v_3 = ((v_2 + v_3) >= v_0);
v_0 = (v_2 | (!(v_3 * v_0) < v_1));
v_1 = (v_3 % !(~!v_0 >= (v_2 + v_1)));
test_out(v_0, v_1, v_2, v_3);
return 0;

}

$ cat lib/test.c
#include <stdio.h>

int ctr = 0;

int test_in() {
return ctr++;
}

void test_out(int v_0, int v_1, int v_2, int v_3) {
printf("%d%d%d%d", v_0, v_1, v_2, v_3);
}

$ 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 revision c2c977c
lldb revision c2c977c

$ clang -g -Og -o opt lib/test.c a.c
$ lldb opt
(lldb) target create "opt"
Current executable set to '/home/stepping/test/opt' (x86_64).
(lldb) b 14
Breakpoint 1: where = opt`main + 83 at a.c:14:14, address = 0x0000000000400553
(lldb) r
Process 155 launched: '/home/stepping/test/opt' (x86_64)
Process 155 stopped

• thread #​1, name = 'opt', stop reason = breakpoint 1.1
  frame #​0: 0x0000000000400553 opt`main at a.c:14:14
  11 v_1 = (!v_1 > (v_2 - (v_3 <= v_0)));
  12 v_0 = (v_1 < (v_2 <= (v_3 >= v_0)));
  13
  -> 14 v_3 = ((v_2 + v_3) >= v_0);
  15 v_0 = (v_2 | (!(v_3 * v_0) < v_1));
  16 v_1 = (v_3 % !(!v_0 >= (v_2 + v_1)));
  17 test_out(v_0, v_1, v_2, v_3);
  (lldb) next
  Process 155 stopped
• thread #​1, name = 'opt', stop reason = step over
  frame #​0: 0x000000000040055c opt`main at a.c:17:2
  14 v_3 = ((v_2 + v_3) >= v_0);
  15 v_0 = (v_2 | (!(v_3 * v_0) < v_1));
  16 v_1 = (v_3 % !(~!v_0 >= (v_2 + v_1)));
  -> 17 test_out(v_0, v_1, v_2, v_3);
  18 return 0;
  19 }

[jmorse]

It looks like early-cse optimises the second argument for test_out to a constant value:

call void @​test_out(i32 %or, i32 0, i32 %call2, i32 %conv13), !dbg !​40

An early-ish invocation of instcombine then manages to optimise the first argument / v_0 to just being the result of the third call to test_in:

call void @​test_out(i32 %call2, i32 0, i32 %call2, i32 %conv13) #​3, !dbg !​31

LLVM naturally deletes the computations on line 15 and 16 as they no longer contribute towards the program, hence they don't appear in the line table. As far as I'm aware these optimisations are legal (I haven't looked too closely), but it's expected behaviour that those lines would disappear as a result of this optimisation.

[cristianassaiante]

Similar to #49630
The pass after which the problem appears is: X86 DAG->DAG Instruction Selection on function (main)

• thread #​1, name = 'opt', stop reason = breakpoint 1.1
  frame #​0: 0x0000000000400553 opt`main at a.c:14:14
  11 v_1 = (!v_1 > (v_2 - (v_3 <= v_0)));
  12 v_0 = (v_1 < (v_2 <= (v_3 >= v_0)));
  13
  -> 14 v_3 = ((v_2 + v_3) >= v_0);
  15 v_0 = (v_2 | (!(v_3 * v_0) < v_1));
  16 v_1 = (v_3 % !(!v_0 >= (v_2 + v_1)));
  17 test_out(v_0, v_1, v_2, v_3);
  (lldb) frame var
  (int) v_0 = 1
  (int) v_1 =
  (int) v_2 = 2
  (int) v_3 = 3
  (lldb) n
  Process 31 stopped
• thread #​1, name = 'opt', stop reason = step over
  frame #​0: 0x000000000040055c opt`main at a.c:17:2
  14 v_3 = ((v_2 + v_3) >= v_0);
  15 v_0 = (v_2 | (!(v_3 * v_0) < v_1));
  16 v_1 = (v_3 % !(~!v_0 >= (v_2 + v_1)));
  -> 17 test_out(v_0, v_1, v_2, v_3);
  18 return 0;
  19 }
  (lldb) frame var
  (int) v_0 = 1
  (int) v_1 = 0
  (int) v_2 = 2
  (int) v_3 = 1

Jeremy/Orlando any comment?

[OCHyams]

I think @jmorse is right that the source lines go missing as a result of valid optimisations. Looking at the reproducer with a recent clang, a couple of variable locations go missing unnecessarily. Fixed in #159777. I think we can close this

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: None (34be6015-e4a2-4a0d-bae9-fb7dd711e036)

| | | | --- | --- | | Bugzilla Link | [50475](https://llvm.org/bz50475) | | Version | trunk | | OS | Linux | | CC | @JDevlieghere,@jmorse,@walkerkd,@OCHyams,@pogo59 |

Extended Description

The last assignment to a variable passed to an external function is not reached in debugging. If we put a breakpoint on line 14, after a next command we end up in line 17 thus skipping lines 15 and 16 where the last assignment to v_0 and v_1 is done. From opt-bisect, the pass after which line 15 is not reached anymore is EarlyCSEPass and the pass after which line 16 is not reached anymore is InstCombinePass.

$ cat a.c
int test_in();
void test_out(int v_0, int v_1, int v_2, int v_3);

int main (void)
{
int v_0, v_1, v_2, v_3;
v_0 = test_in();
v_1 = test_in();
v_2 = test_in();
v_3 = test_in();
v_1 = (!v_1 > (v_2 - (v_3 <= v_0)));
v_0 = (v_1 < (~v_2 <= (v_3 >= v_0)));

v_3 = ((v_2 + v_3) >= v_0);
v_0 = (v_2 | (!(v_3 * v_0) < v_1));
v_1 = (v_3 % !(~!v_0 >= (v_2 + v_1)));
test_out(v_0, v_1, v_2, v_3);
return 0;

}

$ cat lib/test.c
#include <stdio.h>

int ctr = 0;

int test_in() {
return ctr++;
}

void test_out(int v_0, int v_1, int v_2, int v_3) {
printf("%d%d%d%d", v_0, v_1, v_2, v_3);
}

$ 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 revision c2c977c
lldb revision c2c977c

$ clang -g -Og -o opt lib/test.c a.c
$ lldb opt
(lldb) target create "opt"
Current executable set to '/home/stepping/test/opt' (x86_64).
(lldb) b 14
Breakpoint 1: where = opt`main + 83 at a.c:14:14, address = 0x0000000000400553
(lldb) r
Process 155 launched: '/home/stepping/test/opt' (x86_64)
Process 155 stopped

• thread #&#8203;1, name = 'opt', stop reason = breakpoint 1.1
  frame #&#8203;0: 0x0000000000400553 opt`main at a.c:14:14
  11 v_1 = (!v_1 > (v_2 - (v_3 <= v_0)));
  12 v_0 = (v_1 < (v_2 <= (v_3 >= v_0)));
  13
  -> 14 v_3 = ((v_2 + v_3) >= v_0);
  15 v_0 = (v_2 | (!(v_3 * v_0) < v_1));
  16 v_1 = (v_3 % !(!v_0 >= (v_2 + v_1)));
  17 test_out(v_0, v_1, v_2, v_3);
  (lldb) next
  Process 155 stopped
• thread #&#8203;1, name = 'opt', stop reason = step over
  frame #&#8203;0: 0x000000000040055c opt`main at a.c:17:2
  14 v_3 = ((v_2 + v_3) >= v_0);
  15 v_0 = (v_2 | (!(v_3 * v_0) < v_1));
  16 v_1 = (v_3 % !(~!v_0 >= (v_2 + v_1)));
  -> 17 test_out(v_0, v_1, v_2, v_3);
  18 return 0;
  19 }