Thread

1. MemSetLoop ignoring the 'val' parameter

   Andres Freund <andres@2ndquadrant.com> — 2012-10-08T20:39:27Z

   Hi,
   
   #define MemSetLoop(start, val, len) \
   	do \
   	{ \
   		long * _start = (long *) (start); \
   		long * _stop = (long *) ((char *) _start + (Size) (len)); \
   	\
   		while (_start < _stop) \
   			*_start++ = 0; \
   	} while (0)
   
   The 'val' parameter is ignored.
   
   Currently it doesn't matter because MemSetLoop is only used with a 0 parameter 
   and only so in mcxt.c but it looks like it should be fixed anyway.
   
   Andres
   -- 
    Andres Freund	                   http://www.2ndQuadrant.com/
    PostgreSQL Development, 24x7 Support, Training & Services
   
   
   

2. Re: MemSetLoop ignoring the 'val' parameter

   Andres Freund <andres@2ndquadrant.com> — 2012-10-08T20:58:56Z

   On Monday, October 08, 2012 10:39:27 PM Andres Freund wrote:
   > The 'val' parameter is ignored.
   Trivial patch attached.
   -- 
    Andres Freund	                   http://www.2ndQuadrant.com/
    PostgreSQL Development, 24x7 Support, Training & Services
   

3. Re: MemSetLoop ignoring the 'val' parameter

   Tom Lane <tgl@sss.pgh.pa.us> — 2012-10-08T22:56:16Z

   Andres Freund <andres@2ndquadrant.com> writes:
   > The 'val' parameter is ignored.
   
   This is not broken.  Read the comments for MemSetTest.
   
   			regards, tom lane
   
   
   

4. Re: MemSetLoop ignoring the 'val' parameter

   Andres Freund <andres@2ndquadrant.com> — 2012-10-08T23:02:57Z

   On Tuesday, October 09, 2012 12:56:16 AM Tom Lane wrote:
   > Andres Freund <andres@2ndquadrant.com> writes:
   > > The 'val' parameter is ignored.
   > 
   > This is not broken.  Read the comments for MemSetTest.
   Ah. I was surprised about that already. The comment says that val has to be 
   constant though, not that it has to be zero. In my understanding 1 is constant 
   as well. Also, why do we even pass in a 'val' parameter in that case?
   
   Feeling a little slow here...
   
   Andres
   -- 
    Andres Freund	                   http://www.2ndQuadrant.com/
    PostgreSQL Development, 24x7 Support, Training & Services
   
   
   

5. Re: MemSetLoop ignoring the 'val' parameter

   Tom Lane <tgl@sss.pgh.pa.us> — 2012-10-08T23:25:56Z

   Andres Freund <andres@2ndquadrant.com> writes:
   > On Tuesday, October 09, 2012 12:56:16 AM Tom Lane wrote:
   >> Andres Freund <andres@2ndquadrant.com> writes:
   >>> The 'val' parameter is ignored.
   
   >> This is not broken.  Read the comments for MemSetTest.
   
   > Ah. I was surprised about that already. The comment says that val has to be 
   > constant though, not that it has to be zero. In my understanding 1 is constant 
   > as well. Also, why do we even pass in a 'val' parameter in that case?
   
   Well, first off, the callers should not be aware of the detail that
   MemSetTest insists on a val of zero, so they have to pass val even
   though it's unused by the current implementation of MemSetLoop.
   
   The callers are responsible for not passing a volatile value there, but
   it's hard to dodge that problem given that we're dealing with macros;
   if the value changes on repeat evaluation we're screwed anyway.
   
   However, "nonvolatile" is not "constant".  For instance, it's perfectly
   fine to pass MemSetTest/Loop a variable for the "val" that is sometimes
   zero and sometimes not.  If we changed the coding as you suggest, the
   compiler would probably generate less efficient code since it wouldn't
   realize (unless it was quite smart) that MemSetLoop is always filling
   with zeroes.
   
   			regards, tom lane