Re: Stealing ideas from the latest GCC release

22 Mar 2012


      On Thu, Mar 22, 2012 at 5:09 PM, Johan Tibell wrote:
...
Hi all,
While looking at the GCC 4.7 [1] release notes I saw something that's
perhaps worth stealing. Taken from the release notes:
The inter-procedural constant propagation pass has been rewritten. It
   now performs generic function specialization. For example when
   compiling the following:
void foo(bool flag)
   {
     if (flag)
       ... do something ...
     else
       ... do something else ...
   }
   void bar (void)
   {
     foo (false);
     foo (true);
     foo (false);
     foo (true);
     foo (false);
     foo (true);
   }
GCC will now produce two copies of foo. One with flag being true,
   while other with flag being false. This leads to performance
   improvements previously possibly only by inlining all calls. Cloning
   causes a lot less code size growth.
Wait, I thought this is essentially what constructor specialization does?
 I suppose we might then keep around the old body.  Or will these behave
differently in the presence of, say, different constant Int arguments?

-Jan-Willem Maessen