Re: [PATCH][rtl-optimization] Fix PR23726

[BjÃrn Haase <bjoern dot m dot haase at web dot de>]

David Edelsohn wrote on Dienstag, 6. September 2005 17:25 :
>  Shouldn't CSE look inside the PARALLELs or
> the generated code for the PARALLEL automatically add REG_EQUAL notes?
I forgot one point. Unfortunately, I think that it's impossible to add the 
correct REG_EQUAL notes for condition codes automatically. For an example 
let's look at the issue I think to be most relevant: a condition code as 
second result.

Which kind of condition code is set is *very* target and operation dependent. 
E.g. on avr, we will be able to generate the full condition code information 
for expanded minus operations (i.e. tests for eq lt, lte, gte etc. ). For 
plus, however, we will not be able to test for equality. We will only be able 
to test wether the result is positive or negative. For some expanded logic 
operations, we will not be able to extract *any* useful condition code 
information, e.g. for some class of immediate operands.
So I don't see how to avoid hand-coding of the machine description with 
respect to such notes.

Yours,

Bjoern.