PRE in GCC-3.3.3

[James E Wilson]

Rajesh Babu wrote:
> The target I used is i686-linux.  For the same example gcc-3.4.1 
> eliminated the redundant expression, where as gcc-3.3.3 didn't do it.  I 
> observed it by dumping RTL with -dG switch.  I didnt get abt the flaw 
> you were talking about.  The optimization is done on the pseudo 
> registers, where is the question of forcing into memory there.

Your example has
   int c, d;
   d = c++
which is both a use of c while uninitialized, and a set of c to an 
uninitialized value.

The rest of this isn't very interesting, as gcc-3.3 is way too old to be 
looking at optimizations.  Looking at the RTL, I don't see the 
optimization being performed in gcc-3.4.  I still see two adds for c. 
But since it already works in mainline I don't care.

By the way, x86 is not a good target to use for looking at gcse 
performance, because of all of the condition code clobbers, which make 
it difficult for gcse to move instructions around.  It will only 
eliminate a redundant add if it can convert it to a leal instruction 
which does not clobber the condition code register.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com