Re: [tree-ssa] Remove useless null pointer checks

[Andrew Haley <aph at redhat dot com>]

Frank Ch. Eigler writes:
 > 
 > jsturm wrote:
 > 
 > > [...] Java emits tons of useless null pointer checks.
 > > Without this patch, the class:
 > > 
 > > class A { 
 > >   int i;
 > >   final int j() { return i; }
 > >   static int sum(A a) { return a.i + a.j(); }
 > > }
 > 
 > > would optimize to (in method sum):
 > > 
 > > A.sum(A) (a)
 > > {
 > >   [...]
 > >   T.3 = a->i;
 > >   if (a == 0B)
 > >     {
 > >       _Jv_ThrowNullPointerException ()
 > >     }; [...]
 > 
 > Doesn't this look a little late?  If the a reference could be NULL,
 > then it should have been checked before a->i was fetched.

No, this is correct.  Fetching a->i will catch a SEGV and then throw
the exception.

 > Also, can you spell out why exactly you consider this check (assuming
 > its location was corrected) useless?  Can some object not call
 > "A.sum(null);" ?

It's useless because if A were null than accessing a->i would already
have thrown the exception.  There's no need to check again.

Andrew.