match for string slices
#6202
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match for string slices
#6202
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) ## Description This PR adds `string slices` to `CURRENTLY_SUPPORTED_TYPES_MESSAGE` in `match` expressions. The support for string slices is added in #6202. ## Checklist - [x] I have linked to any relevant issues. - [ ] I have commented my code, particularly in hard-to-understand areas. - [ ] I have updated the documentation where relevant (API docs, the reference, and the Sway book). - [ ] If my change requires substantial documentation changes, I have [requested support from the DevRel team](https://github.com/FuelLabs/devrel-requests/issues/new/choose) - [ ] I have added tests that prove my fix is effective or that my feature works. - [ ] I have added (or requested a maintainer to add) the necessary `Breaking*` or `New Feature` labels where relevant. - [x] I have done my best to ensure that my PR adheres to [the Fuel Labs Code Review Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md). - [x] I have requested a review from the relevant team or maintainers.
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Description
This PR implements
matchfor string slices including radix trie optimization and is a task of #5110.For example a simple
matchlikewill generate code following this logic:
In logical terms, this boils down to checking the length and an
O(N)check on the string. Albeit the bytecode will be more complex because of all the branches.Another interesting optimization is the "packed string literal" that coalesces all "match arms string slices" into just one string. In the case above, given that one of the arms contains all the necessary strings for all other comparisons, we will create just one string literal. Saving a lot of bytes in the data section.
The section below describes how
rustcdeals with this desugaring. I think these choices make more sense to us for two reasons:1 - Avoid testing common prefixes multiple times will spend less gas in general (needs more testing);
2 - packing all strings will decrease the data section size.
This is the bytecode generated in this case:
How
rustcdesugarmatchFor comparison, this is the generated ASM with comments on how Rust tackles this.
First, this is the function used:
This is the LLVM IR generated. There is a match on the length of each string slice arms. The valid range is (5, 18), everything outside of this is the wildcard match arm. This range will be important later.
this is how "f" is called
this is
fbody.ja .LBB8_12jumps into a simple return, returning EAX as 6. It is the wildcard return value. The cleverness of this is that whenRSIis smaller than 5, it will become negative (because ofadd rsi, -5, wrapping into huge unsigned ints, and will also triggerJA(which stands forJump Above), effectively jumping when the slice length is outside of the expected range which is (5, 18).After that, it uses a jump table based on the string length minus 5. Everywhere the string length is invalid, the jump address is
LBB8_12., still returningEAXas 6.This is the jump table used:
The interesting entry is entry 5, which has two strings: "get_method" and "get_tokens". Here we can see that
rustactually compares the complete string slice twice. Even though they have an intersection.This is comparable to what
clangis doing: rust-lang/rust#61961Code and Bytecode
This PR also implements code printing when printing bytecode. For now this is only enable for tests. It gnerates something like:
As we can see, not great, but helpful nonetheless. We can (should?) improve this by better "carrying" spans in all transformations and lowerings.
Checklist
Breaking*orNew Featurelabels where relevant.