On Thu, Jun 04, 2020 at 10:22:27AM -0400, Derrick Stolee wrote: > On 6/4/2020 3:27 AM, Abhishek Kumar wrote: > > The struct commit is used in many contexts. However, members generation > > and graph_pos are only used for commit-graph related operations and > > otherwise waste memory. > > > > This wastage would have been more pronounced as transistion to > > generation number v2, which uses 64-bit generation number instead of > > current 32-bits. > > Thanks! This is an important step, and will already improve > performance in subtle ways. While the reduced memory footprint of each commit object might improve performance, accessing graph position and generation numbers in a commit-slab is more expensive than direct field accesses in 'struct commit' instances. Consequently, these patches increase the runtime of 'git merge-base --is-ancestor HEAD~50000 HEAD' in the linux repository from 0.630s to 0.940s. > > create mode 100644 contrib/coccinelle/generation.cocci > > create mode 100644 contrib/coccinelle/graph_pos.cocci > > I appreciate the Coccinelle scripts to help identify > automatic fixes for other topics in-flight. However, > I wonder if they would be better placed inside the > existing commit.cocci file? We add Coccinelle scripts to avoid undesirable code patterns entering our code base. That, however, is not the case here: this is a one-time conversion, and at the end of this series 'struct commit' won't have a 'generation' field anymore, so once it's merged the compiler will catch any new 'commit->generation' accesses. Therefore I don't think that these Coccinelle scripts should be added at all.
