[Gc] cc under mac os x

Grayson, Daniel R. dan at math.uiuc.edu
Mon Jul 30 16:47:47 PDT 2012


The problem is that the comparison

	(word)(&dummy[0]) > (word)limit

in GC_clear_stack_inner is optimized by -O2 to the constant value FALSE (1).  Same
for any comparison of those two pointers.  The only solution I see is to
pass &dummy[0] through an identity function compiled in another file, so the
optimizer can't see it.  That works.

On Jul 30, 2012, at 5:56 PM, "Boehm, Hans" <hans.boehm at hp.com> wrote:

> Note that GC_clear_stack_inner() is a tiny piece of non-portable code.  It would be nice to understand why this is failing.
> 
> I don't understand the limit value.  It's computed in GC_clear_stack from GC_approx_sp(), which should return the stack pointer.  It should be a few K less than the stack pointer.  It doesn't look like it.  Stepping through GC_clear_stack() the first time it's called might be helpful.
> 
> This is also code that can break with excessively clever whole program analysis.  It recourses until the stack pointer is below a certain value.  If the compiler manages to recognize that the function is tail recursive (which we intentionally make hard), the stack may not actually grow as a result of the recursion, causing this to loop forever.
> 
> Stubbing out GC_clear_stack_inner() completely (just return arg) should only introduce a performance issue.  That may also be worth trying.
> 
> Hans
> 
>> -----Original Message-----
>> From: gc-bounces at linux.hpl.hp.com [mailto:gc-bounces at linux.hpl.hp.com]
>> On Behalf Of Grayson, Daniel R.
>> Sent: Sunday, July 29, 2012 11:59 AM
>> To: gc at linux.hpl.hp.com
>> Subject: [Gc] cc under mac os x
>> 
>> I discovered today that libgc doesn't work with /usr/bin/cc under
>> Mac OS X 10.8.  I usually use gcc, so having it work with cc is not
>> important to me, but it might be to others.
>> 
>> Most of the tests fail with a segmentation fault.  Gdb shows a deep
>> recursion:
>> 
>> #0  0x0000000100011086 in GC_clear_stack_inner (arg=0x0,
>> limit=0x7fff5fbfa0a0 "") at misc.c:306
>> #1  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #2  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #3  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #4  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #5  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #6  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #7  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #8  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #9  0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #10 0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> #11 0x0000000100011096 in GC_clear_stack_inner (arg=0x0, limit=0x6a8
>> <Address 0x6a8 out of bounds>) at misc.c:308
>> 
>> 
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