* Dependence between Control Register and instruction
@ 2002-08-13 5:33 umar janjua
0 siblings, 0 replies; only message in thread
From: umar janjua @ 2002-08-13 5:33 UTC (permalink / raw)
To: gcc, gcc-help
> Let us suppose that we have an add instruction that depends on certain
> bits in the control register. Whenever i need to
> perfom the add operation, I am required to set those bits before the
> operation and clear them afterwards. So that they
> remain unchanged for later operations. In this case, it is necessary that
> we have an optimised output. for example if we
> have the assembly instructions like
>
> clear_control_register
> add_insn1
> set_control_register // these two
> clear_control_register // can be removed
> add_insn2
> set_control_register
> ........
> ......
> ......
> should be optimized as
>
> clear_control_register
> add_insn1
> add_insn2
> set_control_register
>
> Similarly, if the add operation is loop invariant, then the compiler
> should also move the set and clear instructions along the add operation
> outside the loop.
>
> Solution 1)
> I can hardcode the set and clearing of bits with in the define_expand
> addm3 pattern. Then define the peephole patterns for eliminating the
> useless and extra set and clear instructions. However, this method
> involves lot of overhead for large number of instructions and fails to
> work when we have other instructions in between .
>
> Solution 2)
> I can generate the RTL for set and clear operation , but here I do not
> know how to create dependency between the add insn and the insn for set
> and clear, so that compiler knows that add insn is dependent on the bits
> in control register set by earlier instruction. Though I have used ( use
> (reg:CC X) ) to show that add insn is using the control register bits, but
> that does not seem to work.
>
> I need urgent suggestions
>
> Regards
> S Rauf
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