Redundant logical operations left after early splitting

[Jeff Law]

hutchinsonandy@aim.com wrote:
> 
> I am stumped and hope someone more skilled can give me some clues to 
> solve this problem I have  4.3/4.4 gcc.
> 
> I have created RTL define/splits for AVR port logical operation (AND,IOR 
> etc). These split larger mode operations in to QImode operations. I also 
> created similar splitters for zero_extend and some combined 
> zero_extended shift operations.
> 
> All of these create QImode moves of subregs and all split before reload 
> (ie unconditional).
> 
> They all split as expected at first oppertunity and produce expected 
> RTL. Whoppee!
> 
> When code involves zero constants (created by zero_extend, or shift) I 
> can see propagation of constant into following logical operations. (Os 
> or O3 optimizations). Such as:
> 
> Rx = 0
> OR Ry,Rx
> 
> becoming
> 
> OR Ry,0
> 
> The propagation is fine, BUT the creation of OR Ry,0  is a totally 
> redundant operation and  remains intact thru all further passes into 
> final code - apparently not being removed by any optimisations after 
> split1 pass!
> 
> I created RTL pattern to remove these (splitting  OR Rx,0 into NOP) and 
> that removes them - but surely this workaround should not be needed. I 
> am stumped by what could be causing the problem. Help!
> 
> There also seem to be cases where zero constants are not propagated into 
> instructions. Yet testcase only involves simple operands, no loops or 
> conditionals or any other side effects that might be reason to block 
> this. Can anyone suggest some non-obvious reasons for this?
You'll need to look at the RTL dumps to determine why these redundant
operations are not being removed or why some propagations aren't being
performed.

GCC certainly has code to do things like remove X IOR 0, so you just
need to figure out why it isn't triggering.  If your code had lots of
SUBREGs, then that's definitely worth investigating -- many of GCC's
optimizers aren't particularly adept at dealing with SUBREGs.

jeff