Bug 19550 – [REG 2.078] Massive compiler backend slowdown

Test case: --- # timeout 5 dmd -O bug.d struct Buf { char[7_000] buffer = void; } void main() { uint len; auto b = Buf(); b.buffer[len] = 'a'; } --- In case you thought that the timeout of 5 seconds is too short, I compared build times of the two releases in full. Compiler | Build time ---------------------- 2.077.1 | 0m0.527s 2.078.0 | 1m3.694s

First bad commit: https://github.com/dlang/dmd/pull/7386

Commenting out the call to blassertsplit() fixes this issue.

This bug is still there.

I ran the test code through callgrind and `accumaecpx` gets called 6.155 million times. Adding issues #6401 and #7157 to the see also list.

Ok I believe to have found the culprit: There is a special case for OPcomma here, which traverses the entire list of elems: https://github.com/dlang/dmd/blob/793635ea7ad3cf30c02bf1b5d51eefdc166c8b82/src/dmd/backend/gflow.d#L624 But `accumaecpx`already recurses over the list of elems: https://github.com/dlang/dmd/blob/793635ea7ad3cf30c02bf1b5d51eefdc166c8b82/src/dmd/backend/gflow.d#L1102 at `accumaecpx(n.EV.E2);`

Created attachment 1781 preliminary patch Attached a preliminary patch. It doesn't help the other related bugzilla issues, but takes the test case from this issue from 1m5.368s -> 0m0.603s.

@WalterWaldron created dlang/dmd pull request #11085 "fix Issue 19550 - [REG 2.078] Massive compiler backend slowdown" fixing this issue: - fix Issue 19550 - [REG 2.078] Massive compiler backend slowdown Fixes typo which caused a redundant O(N^2) slowdown. https://github.com/dlang/dmd/pull/11085

dlang/dmd pull request #11085 "fix Issue 19550 - [REG 2.078] Massive compiler backend slowdown" was merged into master: - 8e4338627e5be46e79770289bb0948a9712a5330 by WalterWaldron: fix Issue 19550 - [REG 2.078] Massive compiler backend slowdown Fixes typo which caused a redundant O(N^2) slowdown. https://github.com/dlang/dmd/pull/11085