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

[llvmbot]

Bugzilla Link	43957
Resolution	FIXED
Resolved on	Feb 13, 2020 07:34
Version	trunk
OS	Linux
Blocks	#38116
Reporter	LLVM Bugzilla Contributor
CC	@adrian-prantl,@gregbedwell,@jmorse,@jdm,@OCHyams,@pogo59,@zygoloid,@rnk,@vedantk
Fixed by commit(s)	1e40799

Extended Description

The expected output from lldb should be 36902. However, compiled with "-O3", lldb outputs 36901.

$ 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
$ lldb-trunk -s cmds -b a.out
(lldb) target create "a.out"
Current executable set to '/home/sding/LLDB-testing/reduce/30986/report/a.out' (x86_64).
(lldb) command source -s 0 'cmds'
Executing commands in '/home/sding/LLDB-testing/reduce/30986/report/cmds'.
(lldb) b 6
Breakpoint 1: where = a.out`main + 10 at abc.c:6:7, address = 0x000000000040048a
(lldb) r
Process 10166 stopped

• thread #​1, name = 'a.out', stop reason = breakpoint 1.1
  frame #​0: 0x000000000040048a a.out`main at abc.c:6:7
  3 --b;
  4 unsigned l_801 = 36901;
  5 ++l_801;
  -> 6 if (a) // optimize_me_not0
  7 b = 0;
  8 }

Process 10166 launched: '/home/sding/LLDB-testing/reduce/30986/report/a.out' (x86_64)
(lldb) p l_801
(unsigned int) $0 = 36901
(lldb) kill
Process 10166 exited with status = 9 (0x00000009)
(lldb) q

$ clang-trunk -g abc.c -O0
$ lldb-trunk -s cmds -b a.out
(lldb) target create "a.out"
Current executable set to '/home/sding/LLDB-testing/reduce/30986/report/a.out' (x86_64).
(lldb) command source -s 0 'cmds'
Executing commands in '/home/sding/LLDB-testing/reduce/30986/report/cmds'.
(lldb) b 6
Breakpoint 1: where = a.out`main + 43 at abc.c:6:7, address = 0x00000000004004ab
(lldb) r
Process 18492 stopped

• thread #​1, name = 'a.out', stop reason = breakpoint 1.1
  frame #​0: 0x00000000004004ab a.out`main at abc.c:6:7
  3 --b;
  4 unsigned l_801 = 36901;
  5 ++l_801;
  -> 6 if (a) // optimize_me_not0
  7 b = 0;
  8 }

Process 18492 launched: '/home/sding/LLDB-testing/reduce/30986/report/a.out' (x86_64)
(lldb) p l_801
(unsigned int) $0 = 36902
(lldb) kill
Process 18492 exited with status = 9 (0x00000009)
(lldb) q

$ cat abc.c
char a, b;
int main() {
--b;
unsigned l_801 = 36901;
++l_801;
if (a) // optimize_me_not0
b = 0;
}

$ cat cmds
b 6
r
p l_801
kill
q

[jmorse]

Going into LiveDebugVariables:

DBG_VALUE 36901, $noreg, !"l_801", !DIExpression(), 
DBG_VALUE 36901, $noreg, !"l_801", !DIExpression(DW_OP_plus_uconst, 1, DW_OP_stack_value),

Coming out of VirtRegRewriter:

DBG_VALUE 36901, $noreg, !"l_801", !DIExpression(DW_OP_plus_uconst, 1, DW_OP_stack_value),
DBG_VALUE 36901, $noreg, !"l_801", !DIExpression(),

It looks like LiveDebugVariables loses the original order of locations; it should be only preserving the last location given in each block of DBG_VALUEs.

(CC Orlando, who's wrestling with relevant LiveDebugVariables code in bug 41992)

[jmorse]

I wrote:

(CC Orlando, who's wrestling with relevant LiveDebugVariables code in bug 41992)

Actually, this might be bug 41992. Because LiveDebugVariables incorrectly distinguishes variables by their whole expression, those DBG_VALUEs will be interpreted as two different variables, and LiveDebugVariables probably tries to preserve both.

Orlandos WIP patch should be up today, we can test with that.

[OCHyams]

Actually, this might be bug 41992.

This looks likely. With my upcoming patch, diffing the output of the following commands (pre-patch, patch):

$ clang -O3 -emit-llvm -S -g abc.c
$ llc -stop-after=livedebugvars abc.ll

gives:

bb.0 (%ir-block.0):
renamable $al = MOV8rm $rip, 1, $noreg, @​b, $noreg, debug-location !​20 :: (dereferenceable load 1 from @​b, !tbaa !​21)
renamable $al = ADD8ri killed renamable $al, -1, implicit-def dead $eflags, debug-location !​20
renamable $ecx = XOR32rr undef $ecx, undef $ecx, implicit-def dead $eflags
DBG_VALUE 36901, $noreg, !​18, !DIExpression(DW_OP_plus_uconst, 1, DW_OP_stack_value), debug-location !​24

[- DBG_VALUE 36901, $noreg, !​18, !DIExpression(), debug-location !​24-]

CMP8mi $rip, 1, $noreg, @​a, $noreg, 0, implicit-def $eflags, debug-location !​25 :: (dereferenceable load 1 from @​a, !tbaa !​21)
renamable $eax = MOVZX32rr8 killed renamable $al, debug-location !​27
renamable $eax = CMOV32rr killed renamable $eax, killed renamable $ecx, 5, implicit killed $eflags, debug-location !​27
MOV8mr $rip, 1, $noreg, @​b, $noreg, renamable $al, implicit killed $eax, debug-location !​24 :: (store 1 into @​b)
$eax = XOR32rr undef $eax, undef $eax, implicit-def dead $eflags, debug-location !​28
RETQ $eax, debug-location !​28

[OCHyams]

WIP patch here https://reviews.llvm.org/D70121

[OCHyams]

Fixed with D74053 (1e40799).