[racket-dev] SGC as default

Apparently, when gcc 4.9.1 sees

 memcpy(x, y, n);
 if (y)
   ....

then it believes `y` can be assumed to be NULL --- even if `n` turns
out to be zero --- and so the conditional can be optimized away.

I'm surprised by that rule, but it's easy enough to move the test
before the memcpy().

At Tue, 12 Aug 2014 02:00:04 -0400, Asumu Takikawa wrote:
> On 2014-08-12 06:06:51 +0100, Matthew Flatt wrote:
> > What platform are you using?
> > 
> > I imagine that running `./racketcgc` within the "racket" subdirectory
> > of your build directory will similarly crash. Can you get any
> > information from running `gdb racketcgc`?
> 
> This is on Linux. Running the built executable immediately produces a segfault.
> 
> Here's a backtrace from gdb:
>   (gdb) bt
>   #0  0x00000000005e78db in free_managed (s=s at entry=0x0) at 
> ../../../racket/sgc/sgc.c:1535
>   #1  0x00000000005e7f63 in GC_add_roots (start=start at entry=0x881b70 
> <collects_path>, end=end at entry=0x881b79 <run_cmd+1>) at 
> ../../../racket/sgc/sgc.c:1657
>   #2  0x0000000000576f9a in scheme_register_static (ptr=ptr at entry=0x881b70 
> <collects_path>, size=size at entry=8) at ../../../racket/src/salloc.c:720
>   #3  0x000000000045b87f in scheme_set_collects_path (p=p at entry=0x7ffff7f701a0) 
> at ../../../racket/src/file.c:6841
>   #4  0x0000000000436673 in run_from_cmd_line (mk_basic_env=<optimized out>, 
> cont_run=0x435a10 <cont_run>, _argv=<optimized out>, argc=0) at 
> ../../racket/cmdline.inc:1444
>   #5  main_after_stack (data=data at entry=0x7fffffffdc30) at 
> ../../racket/main.c:450
>   #6  0x000000000057694d in do_main_stack_setup (data=0x7fffffffdc30, 
> _main=0x435a80 <main_after_stack>, no_auto_statics=1) at 
> ../../../racket/src/salloc.c:198
>   #7  scheme_main_stack_setup (no_auto_statics=no_auto_statics at entry=1, 
> _main=_main at entry=0x435a80 <main_after_stack>, data=data at entry=0x7fffffffdc30)
>       at ../../../racket/src/salloc.c:310
>   #8  0x00000000004346ee in main_after_dlls (argv=<optimized out>, 
> argc=<optimized out>) at ../../racket/main.c:381
>   #9  main (argc=<optimized out>, argv=<optimized out>) at 
> ../../racket/main.c:341
> 
> Cheers,
> Asumu