[ConstraintElim] Look for row proving Op <= Max - Slack. (#233851)
The constraint system does not record the upper bounds of variables
based on the type, so we often fail to prove that a variable Op <= some
upper bound in doesHoldInRange.
In practice, we frequently have other constraints involving Op and
another variable W, which we can use to prove Op is below some bound.
To check if `Op <= MaxValue - Slack`, we scan the whole constraint
system, looking for rows of the form `Op <= W - Slack`. If W's
bitwidth is <= Op's bitwidth, `W <= MaxValue`.
Similar reasoning is also applied for `Op >= MinValue + Slack`.
This triggers quite widely in practice
(5339 files changed in
https://github.com/dtcxzyw/llvm-opt-benchmark-nightly/pull/1762),
while compile-time impact is in the noise, with a small increase in
ClamAV (+0.04% worst case):
PR: https://github.com/llvm/llvm-project/pull/233851
https://llvm-compile-time-tracker.com/compare.php?from=ce8c2a362e0440be05e7d30129663bd2bf34677f&to=86a0bfaf2610956791d84c91b352582a1bd9b354&stat=instructions:u
As an alternative, we could add bounds based on the type for each
variable, but constraint-system solving is super-linear in the number of
rows, so compile-time impact of that approach is noticeably higher.
llvm-project
807f9b70 - [ConstraintElim] Look for row proving Op <= Max - Slack. (#233851)
