Re: 0006-GCSE-complex-constants.patch

[Jeff Law <law at redhat dot com>]

> Certain architectures (e.g., ARM) cannot easily operate with 
> constants, they need to emit sequences of several instructions to load 
> constants into registers.  The common procedure to do this is to emit 
> a (parallel [(set) (clobber (reg1)) ... (clobber (regN))]) instruction 
> which later splits into several instructions using pseudos (regX) to 
> store intermediate values.
>
> Currently PRE and hoist do not GCSE constants, and there is a good 
> reason for that, to avoid increasing register pressure; interestingly, 
> symbol_refs are allowed to be GCSE'ed, is this intentional or by 
> accident?
It's intentional; a SYMBOL_REF if often be rather expensive.  Some 
CONST_INTs can have that same property.  One could argue that an 
expensive CONST_INT shouldn't be appearing in RTL, but certainly some 
ports have chosen to handle splitting insns with expensive constants 
later in the pipeline.

> In any case, it seems like a good idea to GCSE constants and 
> symbol_refs that need something beyond a simple (set) to get into a 
> register, and not GCSE them otherwise.
Rather than triggering this on the PARALLEL it might be better to 
trigger it on the cost of the RTX.  Triggering on the PARALLEL looks 
like a hack to me -- IMHO we'd be better off fixing the costing 
mechanism and using costing as the trigger.

Jeff