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Chandler CarruthandJon Ross-Perkins
97e98bcc5a
First, this replaces the separate line index and column index in the token information with a single 32-bit byte offset of the token. This is then used to compute line and column numbers with a binary search of the line structure and then using that to compute the column within the line. In practice, this is _much_ more efficient: - Smaller token data structure. This will hopefully combine with a subsequent optimization PR that shrinks the token data structure still further. - Fewer stores to form each token's information in the tight hot loop of the lexer. - Less state to maintain while lexing, fewer computations while lexing. We only have to search to build the line and column information off the hot lexing path, and so this ends up being a significant win and shrinks some of the more significant data structures. Second, this shrinks the line start to a 32-bit integer and removes the line length. Our source buffer already ensures we only have 2 GiB of source with a nice diagnostic. I've just added a check to help document this in the lexer. The line length can be avoided in all of the cases it was being used, largely by looking at the next line's start and working from there. This also precipitated cleaning up some code that dated from when lines were only built during lexing rather than being pre-built, which resulted in nice simplifications. With this PR, I think it makes sense to re-name a bunch of methods on `TokenizedBuffer`, but to an extent that was already needed as these methods somewhat predate the more pervasive style conventions. I avoided that here to keep this PR focused on the implementation change, I'll create a subsequent PR to update the API to both better nomenclature and remove deviations from our conventions. There may also be a way to de-duplicate the binary search in the diagnostic location conversion and the main line accessor binary search, but it wasn't obvious to me that it would be a net savings, so left it alone for now. The performance impact of this varies quite a bit... The lexer's benchmark improves pretty consistent across the board on both x86 and Arm. For x86, where I have nice comparison tools, it appears 3% to 20% faster depending on the specific pattern. For Arm server CPUs at least it seems a much smaller but still an improvement. The overall compilation benchmarks however don't improve much with these changes alone on x86. Significant reduction in instruction count required for lexing, but the overall performance is bottlenecked elsewhere in the overall compilation it seems. However, on Arm, despite the more modest gains in special cases of lexing, this shows fairly consistent 1-2% improvements in overall lexing performance on our compilation benchmark. And the expectaiton is these improvements will compound with subsequent work to further compact our representation. --------- Co-authored-by: Jon Ross-Perkins <jperkins@google.com>