warning: dereferencing type-punned pointer

Today I compiled PostgreSQL master branch with -fno-strict-aliasing
compile option removed (previous discussions on the $subject [1]). gcc
version is 9.4.0.

There are a few places where $subject warning printed.

In file included from ../../../src/include/nodes/pg_list.h:42,
from ../../../src/include/access/tupdesc.h:19,
from ../../../src/include/access/htup_details.h:19,
from ../../../src/include/access/heaptoast.h:16,
from execExprInterp.c:59:
execExprInterp.c: In function ‘ExecEvalJsonExprPath’:
../../../src/include/nodes/nodes.h:133:29: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
133 | #define nodeTag(nodeptr) (((const Node*)(nodeptr))->type)
| ~^~~~~~~~~~~~~~~~~~~~~~~
../../../src/include/nodes/nodes.h:158:31: note: in expansion of macro ‘nodeTag’
158 | #define IsA(nodeptr,_type_) (nodeTag(nodeptr) == T_##_type_)
| ^~~~~~~
../../../src/include/nodes/miscnodes.h:53:26: note: in expansion of macro ‘IsA’
53 | ((escontext) != NULL && IsA(escontext, ErrorSaveContext) && \
| ^~~
execExprInterp.c:4399:7: note: in expansion of macro ‘SOFT_ERROR_OCCURRED’
4399 | if (SOFT_ERROR_OCCURRED(&jsestate->escontext))
| ^~~~~~~~~~~~~~~~~~~
execExprInterp.c: In function ‘ExecEvalJsonCoercionFinish’:
../../../src/include/nodes/nodes.h:133:29: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
133 | #define nodeTag(nodeptr) (((const Node*)(nodeptr))->type)
| ~^~~~~~~~~~~~~~~~~~~~~~~
../../../src/include/nodes/nodes.h:158:31: note: in expansion of macro ‘nodeTag’
158 | #define IsA(nodeptr,_type_) (nodeTag(nodeptr) == T_##_type_)
| ^~~~~~~
../../../src/include/nodes/miscnodes.h:53:26: note: in expansion of macro ‘IsA’
53 | ((escontext) != NULL && IsA(escontext, ErrorSaveContext) && \
| ^~~
execExprInterp.c:4556:6: note: in expansion of macro ‘SOFT_ERROR_OCCURRED’
4556 | if (SOFT_ERROR_OCCURRED(&jsestate->escontext))
| ^~~~~~~~~~~~~~~~~~~
origin.c: In function ‘StartupReplicationOrigin’:
origin.c:773:16: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
773 | file_crc = *(pg_crc32c *) &disk_state;
| ^~~~~~~~~~~~~~~~~~~~~~~~~

In my understanding from the discussion [1], it would be better to fix
our code to avoid the warning because it *might* point out that there
is something wrong with our code. However the consensus at the time
was, we will not remove -fno-strict-aliasing option for now. It will
take long time before it would happen...

So I think the warnings in ExecEvalJsonExprPath are better fixed
because these are the only places where IsA (nodeTag) macro are used
and the warning is printed. Patch attached.

I am not so sure about StartupReplicationOrigin. Should we fix it now?
For me the code looks sane as long as we keep -fno-strict-aliasing
option. Or maybe better to fix so that someday we could remove the
compiler option?

[1] https://www.postgresql.org/message-id/flat/366.1535731324%40sss.pgh.pa.us#bd93089182d13c79b74593ec70bac435

Best reagards,
--
Tatsuo Ishii
SRA OSS LLC
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp