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Modifying Insns


Hi,

GCC's optimization passes operate on rtx's 
and alter it.I am trying to figure out if 
there is a standard way to alter rtx 
in an insn so that after substitution it 
is valid.
It seems functions to do the job are

1. validate_update_rtx.
2. validate_replace_rtx_subexp.
3. validate_replace_rtx_group.
  .... 
(all in gcc/recog.c)

Have I got it correct?
It would be nice if someone can enunciate 
any obvious issues, when doing things like 
1. changing reg to mem, 
2. or replacing a reg  by another expression.
   involving regs, ... ).
and semantics of when we
should use validate_replace_rtx_group ?
I am trying to figure in the code, but it 
would be great if someone could help.

Thanks and Regards,
Ranjit.

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