Re: unoptimized dead store

[Stuart Hastings <stuart@xxxxxxxxx>]

On Wednesday, May 9, 2001, at 04:05  PM, Chan, Sun C wrote:

> The other possibility is that since "a" is global, it could be modified
> by a shared library. If you mark "a" as static, the statement should go
> away. If not, it is back to your conjecture.
> BTW, thx for pointing out the obvious.
> Sun

O.K., if 'a' is declared static, then it is immune from tampering by a 
shared library. However, I was hoping that IPA would be able to tell 
that neither glarp() nor any of its callees reference variable 'a'. If a 
shared library was going to reference 'a', then glarp() or one of its 
callees would be invoking an external function somewhere; they don't, 
and IPA should know that.

  % sgicc -O3 -IPA:inline=no -v -o test test.c >& log ; objdump -S 
test > test.dump ; more test.dump

Please recall the commandline; I have explicitly disabled the inliner. 
If I allow SGICC to inline everything, functions glarp() and bling() are 
inlined, their bodies disappear, and the store to 'a' is deleted. If 
SGICC was worried about a dynamic library referencing 'a', then why is 
it so bold to delete global functions glarp() and bling()? By the same 
reasoning, functions glarp() and bling() could be invoked by a dynamic 
library.

The current test program is a simplified case; the original program was 
three different modules. If I split the program into three files, keep 
variable 'a' as a global, and allow SGICC to inline everything, 
functions glarp() and bling() still disappear, and so does the dead 
store to 'a'.

By-the-way, declaring 'a' static in the given testcase makes no 
difference; SGICC still doesn't delete the store to 'a'.

Is there anything I can do to allow IPA to delete this store? (Other 
than enabling the inliner :-)

stuart hastings

-----------------------------------------
int a, b ;

main( int xx)
{
  a = 2 ;
  glarp( xx) ;
  a++ ;
  return a + b ;
}
glarp( int j)
{
  extern int a ;

  if (j < 10)
    bling( j-1) ;
}

bling( int k)
{
  extern int b ;

  b = k ;
}