rtlil: Add packed extract implementation for SigSpec
#4350
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Previously `extract` on a `SigSpec` would always unpack it. Since a significant amount of `SigSpec`s have one or few chunks, it's worth having a dedicated implementation. This is especially true, since the RTLIL frontend calls into this for every `wire [lhs:rhs]` slice, making this `extract` take up 40% when profiling `read_rtlil` with one of the largest coarse grained RTLIL designs I had on hand. With this change the `read_rtlil` profile looks like I would expect it to look like, but I noticed that a lot of the other core RTLIL methods also are a bit too eager with unpacking or implementing `SigChunk`/`Const` overloads that just convert to a single chunk `SigSpec` and forward to the implementation for that, when a direct implementation would avoid temporary std::vector allocations. While not relevant for `read_rtlil`, to me it looks like there might be a few easy overall performance gains to be had by addressing this more generally.
povik
reviewed
Apr 22, 2024
I also have the impression from looking at some profiles, and knowing how a lot of operations can trigger unpacking/packing. |
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Your new implementation looks good to me |
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Thanks! |
RCoeurjoly
approved these changes
Apr 24, 2024
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Let me merge it then |
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Previously
extracton aSigSpecwould always unpack it. Since a significant amount ofSigSpecs have one or few chunks, it's worth having a dedicated implementation.This is especially true, since the RTLIL frontend calls into this for every
wire [lhs:rhs]slice, making thisextracttake up 40% when profilingread_rtlilwith one of the largest coarse grained RTLIL designs I had on hand.With this change the
read_rtlilprofile looks like I would expect it to look like, but I noticed that a lot of the other core RTLIL methods also are a bit too eager with unpacking or implementingSigChunk/Constoverloads that just convert to a single chunkSigSpecand forward to the implementation for that, when a direct implementation would avoid temporary std::vector allocations. While not relevant forread_rtlil, to me it looks like there might be a few easy overall performance gains to be had by addressing this more generally.