[fpc-devel] Peephole Optimizer suggestions

[J. Gareth Moreton]

Hi everyone,

Now that I'm back in my element and looking for improvements in the 
generated assembly language, I'm pondering about refactoring something 
in the Peephole Optimizer when it comes to removing instrucitons.

During the development of the optimisations over at #37389, I was 
struggling to deal with an internal error that was cropping up for no 
apparent reason, and the reason for that is because I forgot to call 
"asml.remove" on an instruction before freeing it, therefore causing a 
dangling pointer to remain in the linked list.  To avoid this situation 
from happening in the future, I would like to introduce a new method 
called RemoveInstruction that ensures that the instruction is removed 
from AsmL before being freed.  While RemoveCurrentP already exists, it 
is designed only for the current instruction and not ones that appear 
later, especially if the instruction that appears after it is unknown.

For a more experimental one, I want to see if it's possible to run the 
Peephole Optimizer (with the possible exception of the Post-Peephole 
stage due to the nature of its optimisations) before all the virtual 
registers are turned to real ones, because a number of optimisations 
involve the removal of registers holding temporary values and hence 
might allow a simpler function prologue and epilogue to be generated.  
Also, some potential new optimisations would require rearranging 
registers, an overly complex task that can be avoided completely if 
virtual registers are used.  Theoretically, the only changes that would 
be needed is to ensure the TUsedRegs objects can handle virtual 
registers (trying to use it now will cause an out of bounds error or 
access violation on account of the virtual registers having starting 
indices of 2^24).

Gareth aka. Kit