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patterns. This is causing Clang to miscompile itself for 32-bit x86 somehow, and likely also on ARM and PPC. I really don't know how, but reverting now that I've confirmed this is actually the culprit. I have a reproduction as well and so should be able to restore this shortly. This reverts commit r223764. Original commit log follows: Teach instcombine to canonicalize "element extraction" from a load of an integer and "element insertion" into a store of an integer into actual element extraction, element insertion, and vector loads and stores. Previously various parts of LLVM (including instcombine itself) would introduce integer loads and stores into the code as a way of opaquely loading and storing "bits". In some cases (such as a memcpy of std::complex<float> object) we will eventually end up using those bits in non-integer types. In order for SROA to effectively promote the allocas involved, it splits these "store a bag of bits" integer loads and stores up into the constituent parts. However, for non-alloca loads and tsores which remain, it uses integer math to recombine the values into a large integer to load or store. All of this would be "fine", except that it forces LLVM to go through integer math to combine and split up values. While this makes perfect sense for integers (and in fact is critical for bitfields to end up lowering efficiently) it is *terrible* for non-integer types, especially floating point types. We have a much more canonical way of representing the act of concatenating the bits of two SSA values in LLVM: a vector and insertelement. This patch teaching InstCombine to use this representation. With this patch applied, LLVM will no longer introduce integer math into the critical path of every loop over std::complex<float> operations such as those that make up the hot path of ... oh, most HPC code, Eigen, and any other heavy linear algebra library. For the record, I looked *extensively* at fixing this in other parts of the compiler, but it just doesn't work: - We really do want to canonicalize memcpy and other bit-motion to integer loads and stores. SSA values are tremendously more powerful than "copy" intrinsics. Not doing this regresses massive amounts of LLVM's scalar optimizer. - We really do need to split up integer loads and stores of this form in SROA or every memcpy of a trivially copyable struct will prevent SSA formation of the members of that struct. It essentially turns off SROA. - The closest alternative is to actually split the loads and stores when partitioning with SROA, but this has all of the downsides historically discussed of splitting up loads and stores -- the wide-store information is fundamentally lost. We would also see performance regressions for bitfield-heavy code and other places where the integers aren't really intended to be split without seemingly arbitrary logic to treat integers totally differently. - We *can* effectively fix this in instcombine, so it isn't that hard of a choice to make IMO. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223813 91177308-0d34-0410-b5e6-96231b3b80d8 |
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Low Level Virtual Machine (LLVM) ================================ This directory and its subdirectories contain source code for the Low Level Virtual Machine, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you're writing a package for LLVM, see docs/Packaging.rst for our suggestions.