LLVM Clang 14 Lands An "Amazing" Performance Optimization
Written by Michael Larabel in LLVM on 6 December 2021 at 01:54 PM EST. 33 Comments
LLVM --
While the performance of LLVM/Clang has improved a lot over the years and for x86_64 and AArch64 can be neck-and-neck with the GCC compiler, the fierce performance battle is not over. With LLVM/Clang 14.0 due out in the early months of 2022 will be more performance work with one recent commit in particular showing a lot of promise.

LLVM developer Djordje Todorovic recently landed an improvement to LLVM's Loop Invariant Code Motion (LICM) Pass for being able to hoist a LOAD without STORE. The patch explains, "When doing load/store promotion within LICM, if we cannot prove that it is safe to sink the store we won't hoist the load, even though we can prove the load could be dereferenced and moved outside the loop. This patch implements the load promotion by moving it in the loop preheader by inserting proper PHI in the loop. The store is kept as is in the loop. By doing this, we avoid doing the load from a memory location in each iteration." The improvement to this pass helps to address this bug report around missed opportunities for register promotion.

But for those not into compiler internals and just interested in the net gain, Todorovic shared some benchmark results and commentary:

In our PostgreSQL benchmark he is seeing around ~12% higher performance with this load hoisting patch, and a variety of other workloads from XZ compression to C-Ray to MrBayes and others are seeing improvements by generally a few percent.

This improvement and countless other patches will be part of LLVM Clang 14.0 that going by their usual release cadence should surface as stable around March.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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