GitHub (Jan 13 2020 at 18:07): GitHub (Jan 13 2020 at 18:07):abrown opened Issue #1341:
What is the feature or code improvement you would like to do in Cranelift?
As described in https://github.com/WebAssembly/simd/issues/117#issuecomment-573494583, Wasm shifts do extra runtime work to ensure the shift value is within a certain range. This is extra work that could be removed if we had knowledge that the shift value was constant and in the required range. This knowledge could also be used to lower the Wasm shifts to instructions using an immediate.
What is the value of adding this in Cranelift?
Emit faster code.
Do you have an implementation plan, and/or ideas for data structures or algorithms to use?
I think this type of optimization may apply to more than just Wasm shifts so I hope this approach works for those as well:
- add an
is_constant(v: Value)function toDataFlowGraph; initially this could check if the value passed was created by one of the following instructions:iconst,fconst,bconst,vconst. This could get more complex later but I feel this simple implementation would be a good start.- in
code_translator.rs, check if the shift valueis_constantand if it also inside the expected range (e.g. 0-31), lower the Wasm shift toish*_imm; otherwise, lower toish*as is done currently.Have you considered alternative implementations? If so, how are they better or worse than your proposal?
No, open to suggestions.
GitHub (Jan 13 2020 at 18:12):bjorn3 commented on Issue #1341:
https://github.com/bytecodealliance/cranelift/blob/e6e67154c2cd4b68f1045930097df36783cc748c/cranelift-codegen/src/simple_preopt.rs#L560 already performs
shl/shr->shl_imm/shr_immwhen possible ifopt_level=speedis used.
GitHub (Jan 13 2020 at 18:17):abrown commented on Issue #1341:
Good to know; hadn't seen that!
GitHub (Jan 13 2020 at 18:17):abrown closed Issue #1341:
What is the feature or code improvement you would like to do in Cranelift?
As described in https://github.com/WebAssembly/simd/issues/117#issuecomment-573494583, Wasm shifts do extra runtime work to ensure the shift value is within a certain range. This is extra work that could be removed if we had knowledge that the shift value was constant and in the required range. This knowledge could also be used to lower the Wasm shifts to instructions using an immediate.
What is the value of adding this in Cranelift?
Emit faster code.
Do you have an implementation plan, and/or ideas for data structures or algorithms to use?
I think this type of optimization may apply to more than just Wasm shifts so I hope this approach works for those as well:
- add an
is_constant(v: Value)function toDataFlowGraph; initially this could check if the value passed was created by one of the following instructions:iconst,fconst,bconst,vconst. This could get more complex later but I feel this simple implementation would be a good start.- in
code_translator.rs, check if the shift valueis_constantand if it also inside the expected range (e.g. 0-31), lower the Wasm shift toish*_imm; otherwise, lower toish*as is done currently.Have you considered alternative implementations? If so, how are they better or worse than your proposal?
No, open to suggestions.
GitHub (Jan 13 2020 at 18:20):abrown commented on Issue #1341:
...but not sure that works for SIMD (which I guess I should have mentioned above).
GitHub (Jan 13 2020 at 18:20):abrown reopened Issue #1341:
What is the feature or code improvement you would like to do in Cranelift?
As described in https://github.com/WebAssembly/simd/issues/117#issuecomment-573494583, Wasm shifts do extra runtime work to ensure the shift value is within a certain range. This is extra work that could be removed if we had knowledge that the shift value was constant and in the required range. This knowledge could also be used to lower the Wasm shifts to instructions using an immediate.
What is the value of adding this in Cranelift?
Emit faster code.
Do you have an implementation plan, and/or ideas for data structures or algorithms to use?
I think this type of optimization may apply to more than just Wasm shifts so I hope this approach works for those as well:
- add an
is_constant(v: Value)function toDataFlowGraph; initially this could check if the value passed was created by one of the following instructions:iconst,fconst,bconst,vconst. This could get more complex later but I feel this simple implementation would be a good start.- in
code_translator.rs, check if the shift valueis_constantand if it also inside the expected range (e.g. 0-31), lower the Wasm shift toish*_imm; otherwise, lower toish*as is done currently.Have you considered alternative implementations? If so, how are they better or worse than your proposal?
No, open to suggestions.
GitHub (Jan 13 2020 at 18:22):bjorn3 commented on Issue #1341:
That's probably the condition at https://github.com/bytecodealliance/cranelift/blob/e6e67154c2cd4b68f1045930097df36783cc748c/cranelift-codegen/src/simple_preopt.rs#L571
GitHub (Jan 13 2020 at 18:23):abrown commented on Issue #1341:
Yeah, I guess I could add some code to that function to handle vectors but attempt the same type of translations (instead of mucking around in
code_translator.rs).
GitHub (Feb 28 2020 at 23:28):alexcrichton transferred Issue #1341:
What is the feature or code improvement you would like to do in Cranelift?
As described in https://github.com/WebAssembly/simd/issues/117#issuecomment-573494583, Wasm shifts do extra runtime work to ensure the shift value is within a certain range. This is extra work that could be removed if we had knowledge that the shift value was constant and in the required range. This knowledge could also be used to lower the Wasm shifts to instructions using an immediate.
What is the value of adding this in Cranelift?
Emit faster code.
Do you have an implementation plan, and/or ideas for data structures or algorithms to use?
I think this type of optimization may apply to more than just Wasm shifts so I hope this approach works for those as well:
- add an
is_constant(v: Value)function toDataFlowGraph; initially this could check if the value passed was created by one of the following instructions:iconst,fconst,bconst,vconst. This could get more complex later but I feel this simple implementation would be a good start.- in
code_translator.rs, check if the shift valueis_constantand if it also inside the expected range (e.g. 0-31), lower the Wasm shift toish*_imm; otherwise, lower toish*as is done currently.Have you considered alternative implementations? If so, how are they better or worse than your proposal?
No, open to suggestions.
Last updated: Dec 23 2024 at 13:07 UTC