Re: Null pointer check elimination

[Michael Koch <konqueror at gmx dot de>]

On Fri, Nov 11, 2005 at 02:24:12PM -0800, Anthony Green wrote:
> Our compiler inlines many null pointer tests because the language
> requires that we throw NullPointerExeceptions in certain cases that can
> only be detected through explicit tests.
> 
> What's frustrating is that there are many cases where we're testing the
> nullness of a function return value that we know can never be null.  For
> instance, the result of string appends can never be null.  Neither can
> object allocations.
> 
> The attached experimental patch detects one special instance of a
> non-null function result (gnu.gcj.runtime.StringBuffer.append()), and
> marks it as being non-null.  GCC's value range propagation code then
> magically eliminates any redundant null pointer checks.
> 
> Eventually we should manually mark certain function DECLs as
> not-returning-null instead of my kludgy test for this one case.  I don't
> know if/when I can get to that.  Perhaps somebody else can take it from
> here.
> 
> In any case, here's a patch Diego and I came up with.  It will eliminate
> null-pointer checks related to the string concatenation operator (foo =
> "blah" + bar).

Great work!

In NIO (and probably in other areas too we often have code like this in
instance methods:

... do someting ...
return this;

In this case its known that this method never returns null. Perhaps it
would make sense to eliminate the null-checks for such cases too.


Cheers,
Michael
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