[llvm-dev] Hexagon, DFAPacketzier and dependency pruning

[Rail Shafigulin via llvm-dev]

On Fri, Nov 20, 2015 at 2:15 PM, Krzysztof Parzyszek via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 11/20/2015 12:52 PM, Rail Shafigulin via llvm-dev wrote:
>
>> DFAPacketizer has a virtual method isLegalToPruneDependencies(). I
>> looked at  the Hexagon code and but wasn't unable to understand the
>> details (unfortunately as of now, I'm not well versed in the Hexagon
>> architecture). Would anyone be able to shed light on what is dependency
>> pruning and how it should be used?
>>
>> Any help is appreciated.
>>
>
> Two instructions can be packetized if
> - "isLegalToPacketizeTogether" returns true, or
> - "isLegalToPruneDependencies" returns true.
>
> The reason why two instructions may not be "legal to packetize together"
> may be that there is some form of dependency between the two.  For example,
> one instruction may define register, and the next one may use it.  In most
> cases on Hexagon, these cannot go into the same packet: all registers used
> in a packet are read before anything gets updated.  There are exception to
> this, and in some cases, instructions can go together despite
> dependencies---this is where these dependencies can get "pruned" (which
> really means "ignored").
>
> The code in Hexagon is somewhat convoluted, in the current shape it's not
> necessarily something to imitate.  As a first approximation, you can just
> leave the "isLegalToPruneDependencies" with the default implementation
> (that returns "false").  This way only "isLegalToPacketizeTogether" will
> determine if two instructions can be in a packet together or not.
>
> -Krzysztof
>
>
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Thanks for the explanation.

-- 
R
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