https://bugs.llvm.org/show_bug.cgi?id=51941
Bug ID: 51941
Summary: Variable's value late availability after Instruction
Selection at O3
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: DebugInfo
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected],
[email protected]
A variable (l_33) value is visile after its declarations, then its not visible
anymore after a next command and comes back in after another next command.
>From opt-bisect, it seems that the pass after which this behaviour occurs is:
X86 DAG->DAG Instruction Selection on function (main)
$ cat a.c
short a;
int b ;
int main()
{
{int l_33 = 0;
int *c = &l_33;
*c = a
;
*c ^= 2
;
b = l_33 ;
}
}
$ 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 -g -O3 -o opt a.c
$ lldb opt
(lldb) target create "opt"
Current executable set to
'/home/stepping/debuginfo-analysis/issues/issue14/opt' (x86_64).
(lldb) b main
Breakpoint 1: where = opt`main at a.c:7:12, address = 0x0000000000400480
(lldb) run
Process 28 launched: '/home/stepping/debuginfo-analysis/issues/issue14/opt'
(x86_64)
Process 28 stopped
* thread #1, name = 'opt', stop reason = breakpoint 1.1
frame #0: 0x0000000000400480 opt`main at a.c:7:12
4 {
5 {int l_33 = 0;
6 int *c = &l_33;
-> 7 *c = a
8 ;
9 *c ^= 2
10 ;
(lldb) frame var
(int) l_33 = 0
(int *) c = <no location, value may have been optimized out>
(lldb) n
Process 28 stopped
* thread #1, name = 'opt', stop reason = step over
frame #0: 0x0000000000400487 opt`main at a.c:9:12
6 int *c = &l_33;
7 *c = a
8 ;
-> 9 *c ^= 2
10 ;
11 b = l_33 ;
12 }
(lldb) frame var
(int) l_33 = <variable not available>
(int *) c = <no location, value may have been optimized out>
(lldb) n
Process 28 stopped
* thread #1, name = 'opt', stop reason = step over
frame #0: 0x000000000040048a opt`main at a.c:11:11
8 ;
9 *c ^= 2
10 ;
-> 11 b = l_33 ;
12 }
13 }
(lldb) frame var
(int) l_33 = 2
(int *) c = <no location, value may have been optimized out>
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