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Re: MIPS elimate trap-if-zero instruction if possible for divisions


Graham Stott <graham.stott@btinternet.com> writes:
> Hi Richard,
>  
> This patch attemps to elimate the TEQ instruction div DIV/MOD instructions
> if possible (i.e the numerator is known to be non-zero)
>  
> I have introduced and seperated UNSPEC UNSPEC_SET_HILO_NOTRAP
> which is generation by a peephole2 when the trap is known not to be required.
>  
> The peephole's work by checking for a REG_EQUAL or/REG_EQUIV on the
> instruction which sets the numerator for the DIV/MOD this doesn't catch all
> possible cases but does catch the common cases where ths numerator is
> set set in a insn immediately preceeding the DIV/MOD.

Nice idea, but TBH, I think I'd prefer to expose the trap code at expand time
and let the rtl optimisers try to remove it.  I can't remember any correctness
reason why that wouldn't work.  I think the only reason we don't do it yet is
because (before gimple) there was a preference for keeping the division as
a unit during early rtl optimisations.

Thanks,
Richard


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