[ python-Bugs-1113244 ] Please add do-while guard to Py_DECREF etc.

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Wed Feb 2 13:28:58 CET 2005


Bugs item #1113244, was opened at 2005-01-31 16:01
Message generated for change (Comment added) made by rjk1002
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Category: Python Interpreter Core
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Richard Kettlewell (rjk1002)
Assigned to: Nobody/Anonymous (nobody)
Summary: Please add do-while guard to Py_DECREF etc.

Initial Comment:
Py_DECREF() is missing do-while macro guards (as found
e.g. in Py_CLEAR), instead relying on being an if-else
to avoid the usual problems associated with such macros.

However if it is used as e.g. "if (newref)
Py_DECREF(obj);" then gcc -Wall still complains about
an ambiguous else clause.  (gcc version is 3.3.5)

Granted it is only a warning but like many people we
build with -Werror, and the code in question is in SWIG
output rather than our own code, so this is quite
painful for us.

At least Py_DECREF, Py_XDECREF and Py_XINCREF need
fixing; I've not checked beyond this.


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>Comment By: Richard Kettlewell (rjk1002)
Date: 2005-02-02 12:28

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It's unambiguous to the compiler.  The warning is there
because humans sometimes get it wrong.

Marginally reduced optimization in a debug build is hardly a
real problem.

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Comment By: Tim Peters (tim_one)
Date: 2005-02-02 02:03

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rjk1002:  it's idiotic complaints about unambigous (in reality) 
if/else structures that are unique to gcc.  Not all compilers 
optimize away the "do {...} while(0)" cruft in debug builds; 
the most important examples for Python are the Microsoft 
compilers.  Because the incref and decref macros are used a 
*lot*, it's unattractive to burden all platforms with this just 
to shut up one compiler.  I don't care about Py_CLEAR() 
(etc), because they're so lightly used.  The incref/decref 
macros are ubiquitous.

I would much rather see SWIG change to insert curlies.  That 
is, generating "if (newref) {Py_DECREF(obj);}" should be just 
as effective at shutting up this gcc nag.  Has anyone asked 
the SWIG project to do this?

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Comment By: Richard Kettlewell (rjk1002)
Date: 2005-02-01 09:51

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Making it compiler-specific seems silly.  It's not like the
do-while idiom for macros is some strange gcc-specific
thing; most people just use it unconditionally on
complicated macros.

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Comment By: Raymond Hettinger (rhettinger)
Date: 2005-02-01 02:53

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Tim, what you think about doing this with a conditional
compile to only redefine for GCC?

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