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Re: computation_cost too blind to hard regs


DJ Delorie wrote:
Am I missing something obvious here?  We're creating a temporary
sequence only to compute its cost, but we're using HARD registers in
ways that they might not support (in my case, my backend aborted due
to a subreg of a register in a mode it didn't support).  In my case,
using FIRST_PSEUDO_REGISTER intead of 0 as the initial regno works
better.

There is similar code in init_expmed in expmed.c that uses a register number of 10000. The number is arbitrary, but high enough to avoid matching hard registers. We could use FIRST_PSEUDO_REGISTER here and it would probably work just as well. In this case, though, we avoid generating any RTL. We create just enough fake RTL to make rtx_cost happy.


In the computation_cost case, a pseudo-reg number is likewise better than a hard reg number, but this one creates RTL. So we need to make sure we don't accidentally create a pseudo-reg conflict, but since gen_raw_REG is used here, it seems safe to use any pseudo register number.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com



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