[Bug rtl-optimization/23726] Missed optimizations for divmod

[bjoern dot m dot haase at web dot de]

------- Additional Comments From bjoern dot m dot haase at web dot de  2005-09-06 12:59 -------
I have done some code reading now and come to the following conclusion: 
 
When having an expanded sequence that provides one single result, libcall 
blocks are an appropriate method for making sure that a single-set insn that 
carries a REG_EQUAL note is *not* deleted too early. 
 
Libcall notes, however, do not provide a method so far for dealing with library 
calls or expanded sequences that yield *two* results. E.g. they are no solution 
for both, divmod4 on the one hand and arithmetic expanders doing subreg 
lowering and also yielding a CC on the other hand. 
 
In order to keep changes as small as possible, my suggestion is to change  
 
static void 
rest_of_handle_jump2 (void) 
 
such that it no longer calls "delete_trivially_dead_insns ()" at it's very 
beginning. (I'd have posted a patch, if savannah.gnu.org is down right now.) 
 
IIUC that the trivially dead insn are in first line a performance issue because 
they need memory and handling that would otherwise not be necessary, my 
suggested change would not be too serious. "delete_trivially_dead_insns ()" 
would be among the first things that is called in the pass that immediately 
follows jump2: The first cse pass. 
I'd appreciate comments on whether such trivially dead insn could prevent jump2 
from realizing important optimization steps. 
 
Bjoern. 
 

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23726