forked from OSchip/llvm-project
c34f789e38
The core idea is to (re-)introduce some redundancies where their cost is hidden by the cost of materializing immediates for constant operands of PHI nodes. When the cost of the redundancies is covered by this, avoiding materializing the immediate has numerous benefits: 1) Less register pressure 2) Potential for further folding / combining 3) Potential for more efficient instructions due to immediate operand As a motivating example, consider the remarkably different cost on x86 of a SHL instruction with an immediate operand versus a register operand. This pattern turns up surprisingly frequently, but is somewhat rarely obvious as a significant performance problem. The pass is entirely target independent, but it does rely on the target cost model in TTI to decide when to speculate things around the PHI node. I've included x86-focused tests, but any target that sets up its immediate cost model should benefit from this pass. There is probably more that can be done in this space, but the pass as-is is enough to get some important performance on our internal benchmarks, and should be generally performance neutral, but help with more extensive benchmarking is always welcome. One awkward part is that this pass has to be scheduled after *everything* that can eliminate these kinds of redundancies. This includes SimplifyCFG, GVN, etc. I'm open to suggestions about better places to put this. We could in theory make it part of the codegen pass pipeline, but there doesn't really seem to be a good reason for that -- it isn't "lowering" in any sense and only relies on pretty standard cost model based TTI queries, so it seems to fit well with the "optimization" pipeline model. Still, further thoughts on the pipeline position are welcome. I've also only implemented this in the new pass manager. If folks are very interested, I can try to add it to the old PM as well, but I didn't really see much point (my use case is already switched over to the new PM). I've tested this pretty heavily without issue. A wide range of benchmarks internally show no change outside the noise, and I don't see any significant changes in SPEC either. However, the size class computation in tcmalloc is substantially improved by this, which turns into a 2% to 4% win on the hottest path through tcmalloc for us, so there are definitely important cases where this is going to make a substantial difference. Differential revision: https://reviews.llvm.org/D37467 llvm-svn: 319164 |
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| CMakeLists.txt | ||
| LLVMBuild.txt | ||
| PassBuilder.cpp | ||
| PassRegistry.def | ||