Re: gcse pass: expression hash table

[James E Wilson <wilson at specifixinc dot com>]

Tarun Kawatra wrote:
> During expression hash table construction in gcse pass(gcc vercion 
> 3.4.1), expressions like a*b does not get included into the expression 
> hash table. Such expressions occur in PARALLEL along with clobbers.

You didn't mention the target, or exactly what the mult looks like.

However, this isn't hard to answer just by using the source. 
hash_scan_set calls want_to_cse_p calls can_assign_to_reg_p calls 
added_clobbers_hard_reg_p which presumably returns true, which prevents 
the optimization.  This makes sense.  If the pattern clobbers a hard 
reg, then we can't safely insert it at any place in the function.  It 
might be clobbering the hard reg at a point where it holds a useful value.

While looking at this, I noticed can_assign_to_reg_p does something 
silly.  It uses "FIRST_PSEUDO_REGISTER * 2" to try to generate a test 
pseudo register, but this can fail if a target has less than 4 
registers, or if the set of virtual registers increases in the future. 
This should probably be LAST_VIRTUAL_REGISTER + 1 as used in another 
recent patch.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com