GCSE considers read only memory clobbered by function calls.

[Paolo Bonzini]

>>My wild guess is that this was not possible when MEM_READONLY_P was
>>RTX_UNCHANGING_P, and now it is.
>>    
>>
>What wasn't possible? the fact that GCSE considers MEM_READONLY_P call
>clobbered or the fact that CSE does not consider it such that?
>  
>
I think that it was not so easy as with your patch when RTX_UNCHANGING_P 
meant that a location was "write once".  Indeed, CSE had some tricks in 
the paths that handled RTX_UNCHANGING_P. The patch that killed that flag 
(replacing it with MEM_READONLY_P) did some simplification of the code 
that handled it:

http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cse.c.diff?r1=1.307&r2=1.308

RTX_UNCHANGING_P was never mentioned in gcse.c so probably it was not 
easy at all to teach gcse.c about it.

(This was not a remark on your patch, but only a half-hearted attempt at 
archaeology).

Paolo