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| author | Roman Lebedev <lebedev.ri@gmail.com> | 2020-02-25 21:51:22 +0300 |
|---|---|---|
| committer | Roman Lebedev <lebedev.ri@gmail.com> | 2020-02-25 23:05:58 +0300 |
| commit | b8793f0dabc974aec74ce09362d8790d77c6acba (patch) | |
| tree | fc73c801270aa15e69acd80679aa042e340ec4d7 /llvm/lib/Transforms/Utils/LoopUtils.cpp | |
| parent | f90973f48645d1d1799d7bdb81cd6873e3a8ab71 (diff) | |
| download | llvm-b8793f0dabc974aec74ce09362d8790d77c6acba.zip llvm-b8793f0dabc974aec74ce09362d8790d77c6acba.tar.gz llvm-b8793f0dabc974aec74ce09362d8790d77c6acba.tar.bz2 | |
[SCEV] SCEVExpander::isHighCostExpansionHelper(): cost-model UDiv by power-of-two as LShr
Summary:
Like with casts, we need to subtract the cost of `lshr` instruction
from budget, and recurse into LHS operand.
Seems "pretty obviously correct" to me?
To be noted, there is a number of other shortcuts we //could// cost-model:
* `... + (-1 * ...)` -> `... - ...` <- likely very frequent case
* `x - (rem x, power-of-2)`, which is currently `(x udiv power-of-2) * power-of-2` -> `x & -log2(power-of-2)`
* `rem x, power-of-2`, which is currently `x - ((x udiv power-of-2) * power-of-2)` -> `x & log2(power-of-2)-1`
* `... * power-of-2` -> `... << log2(power-of-2)` <- likely not very beneficial
Reviewers: reames, mkazantsev, wmi, sanjoy
Reviewed By: mkazantsev
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D73718
Diffstat (limited to 'llvm/lib/Transforms/Utils/LoopUtils.cpp')
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