GCC hacking questions (peepholes etc.)

[Marek Michalkiewicz <marekm at linux dot org dot pl>]

Hello,

I have a few questions about GCC internals (I'm hacking the AVR port,
which is quite a learning experience, but I think these questions are
general enough to ask here).

 - is it allowed to mix both define_peephole and define_peephole2 in
   one md file?  The manual says define_peephole is deprecated, but
   the AVR port currently uses it, and I see some possible optimizations
   that would be easier done with define_peephole2 - but I'm running
   into various strange bugs if I do it: operands that don't satisfy
   HARD_REGNO_MODE_OK resulting in invalid asm output, abort() in
   insn-extract.c, segfault in final.c cleanup_subreg_operands() etc.
   (If I use -fno-peephole, or remove all define_peephole2 added,
   everything is stable again; but just making the conditions of all
   define_peephole2 always 0 doesn't help - this means I can't make
   the unsafe changes optional, enabled with -m...)

 - if the answer to the above is "no", is it possible for define_peephole
   to request a scratch register (as documented for define_peephole2)?
   What I'm trying to do is to optimize loading immediate constants
   to registers - can be done directly only to r16-r31, not to r0-r15,
   so currently there are no such alternatives in movM patterns, and
   the constants are loaded from memory, which is expensive.  Even for
   loading SImode constants (to 4 registers), only one QImode register
   from r16-r31 is enough, and likely available even if 4 consecutive
   registers (to hold SImode) are not.  (I tried secondary reload, but
   that is not generally a win, as we unconditionally lose a register.
   It seems better to optimize only if a scratch register is available.)

 - how to tell GCC that it is cheaper to load constants to a certain
   register class (r16-r31) than to other registers (r0-r15)?
   It often loads a constant to a low register (from memory) only to
   copy it to a high register in the next insn.  Using a peephole to
   detect this sequence (and change it so that the constant is loaded
   to the high register, then copied to the low one) would not catch
   cases with unrelated insns between the two - is there a better way
   to do this?  (The CONST_COSTS macro doesn't know which register is
   going to be loaded with the constant, which makes a big difference.)

 - is it safe to use a constraint letter for which REG_CLASS_FROM_LETTER
   returns either GENERAL_REGS or NO_REGS depending on target_flags, to
   optionally enable some alternatives in a define_insn?
   (It seems to work correctly, but I'm just making sure.)

Thanks,
Marek