Hi, hackers
"The local variables that do not have the volatile type and have been changed
between the setjmp() invocation and longjmp() call are indeterminate". This is
what the POSIX (and C standard for setjmp) says.
That's fine actually, but if we put the PG_TRY()/CATCH() in a loop, high
version gcc might complain.
Version:
$ gcc-9 --version
gcc-9 (Ubuntu 9.1.0-2ubuntu2~19.04) 9.1.0
(Actually from gcc 7)
Reproducer:
```
#include <setjmp.h>
extern int other(void);
extern void trigger(int *cond1);
extern sigjmp_buf *PG_exception_stack;
void
trigger(int *cond1)
{
while (1)
{
if (*cond1 == 0)
*cond1 = other();
while (*cond1)
{
sigjmp_buf *save_exception_stack = PG_exception_stack;
sigjmp_buf local_sigjmp_buf;
if (sigsetjmp(local_sigjmp_buf, 0) == 0)
PG_exception_stack = &local_sigjmp_buf;
else
PG_exception_stack = (sigjmp_buf *)
save_exception_stack;
PG_exception_stack = (sigjmp_buf *)
save_exception_stack;
}
}
}
```
```
$ gcc-9 -O1 -Werror=uninitialized -fexpensive-optimizations -ftree-pre -c -o
/dev/null reproducer.c
reproducer.c: In function 'trigger':
reproducer.c:17:16: error: 'save_exception_stack' is used uninitialized in this
function [-Werror=uninitialized]
17 | sigjmp_buf *save_exception_stack = PG_exception_stack;
| ^~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors
```
Codes re-ordering matters, when it warns:
```
sigjmp_buf *save_exception_stack = PG_exception_stack;
2f: 48 8b 1d 00 00 00 00 mov 0x0(%rip),%rbx # 36 <trigger+0x36>
36: 48 89 5c 24 18 mov %rbx,0x18(%rsp)
sigjmp_buf local_sigjmp_buf;
if (sigsetjmp(local_sigjmp_buf, 0) == 0)
```
When it doesn't complain:
```
sigjmp_buf *save_exception_stack = PG_exception_stack;
sigjmp_buf local_sigjmp_buf;
if (sigsetjmp(local_sigjmp_buf, 0) == 0)
29: 48 8d 44 24 20 lea 0x20(%rsp),%rax
2e: 48 89 44 24 08 mov %rax,0x8(%rsp)
...
sigjmp_buf *save_exception_stack = PG_exception_stack;
3c: 48 8b 1d 00 00 00 00 mov 0x0(%rip),%rbx # 43 <trigger+0x43>
43: 48 89 5c 24 18 mov %rbx,0x18(%rsp)
```
Greenplum had an issue reporting save_exception_stack and save_context_stack
not initialized.
https://github.com/greenplum-db/gpdb/issues/8262
I filed a bug report for gcc, they think it's expected.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91395
Since high version gcc thinks it's supposed to report warning, we need to make
these two variables volatile? Or have I missed something?
--
Adam Lee
diff --git a/src/include/utils/elog.h b/src/include/utils/elog.h
index dbfd8efd26..97e8405bfa 100644
--- a/src/include/utils/elog.h
+++ b/src/include/utils/elog.h
@@ -300,8 +300,8 @@ extern PGDLLIMPORT ErrorContextCallback
*error_context_stack;
*/
#define PG_TRY() \
do { \
- sigjmp_buf *save_exception_stack = PG_exception_stack; \
- ErrorContextCallback *save_context_stack = error_context_stack;
\
+ sigjmp_buf * volatile save_exception_stack =
PG_exception_stack; \
+ ErrorContextCallback * volatile save_context_stack =
error_context_stack; \
sigjmp_buf local_sigjmp_buf; \
if (sigsetjmp(local_sigjmp_buf, 0) == 0) \
{ \
