mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-23 11:13:28 +01:00
Mirror of https://github.com/RPCS3/llvm-mirror
233ed83478
Summary: Targets often have instructions that can sign-extend certain cases faster than the equivalent shift-left/arithmetic-shift-right. Such cases can be identified by matching a shift-left/shift-right pair but there are some issues with this in the context of combines. For example, suppose you can sign-extend 8-bit up to 32-bit with a target extend instruction. %1:_(s32) = G_SHL %0:_(s32), i32 24 # (I've inlined the G_CONSTANT for brevity) %2:_(s32) = G_ASHR %1:_(s32), i32 24 %3:_(s32) = G_ASHR %2:_(s32), i32 1 would reasonably combine to: %1:_(s32) = G_SHL %0:_(s32), i32 24 %2:_(s32) = G_ASHR %1:_(s32), i32 25 which no longer matches the special case. If your shifts and extend are equal cost, this would break even as a pair of shifts but if your shift is more expensive than the extend then it's cheaper as: %2:_(s32) = G_SEXT_INREG %0:_(s32), i32 8 %3:_(s32) = G_ASHR %2:_(s32), i32 1 It's possible to match the shift-pair in ISel and emit an extend and ashr. However, this is far from the only way to break this shift pair and make it hard to match the extends. Another example is that with the right known-zeros, this: %1:_(s32) = G_SHL %0:_(s32), i32 24 %2:_(s32) = G_ASHR %1:_(s32), i32 24 %3:_(s32) = G_MUL %2:_(s32), i32 2 can become: %1:_(s32) = G_SHL %0:_(s32), i32 24 %2:_(s32) = G_ASHR %1:_(s32), i32 23 All upstream targets have been configured to lower it to the current G_SHL,G_ASHR pair but will likely want to make it legal in some cases to handle their faster cases. To follow-up: Provide a way to legalize based on the constant. At the moment, I'm thinking that the best way to achieve this is to provide the MI in LegalityQuery but that opens the door to breaking core principles of the legalizer (legality is not context sensitive). That said, it's worth noting that looking at other instructions and acting on that information doesn't violate this principle in itself. It's only a violation if, at the end of legalization, a pass that checks legality without being able to see the context would say an instruction might not be legal. That's a fairly subtle distinction so to give a concrete example, saying %2 in: %1 = G_CONSTANT 16 %2 = G_SEXT_INREG %0, %1 is legal is in violation of that principle if the legality of %2 depends on %1 being constant and/or being 16. However, legalizing to either: %2 = G_SEXT_INREG %0, 16 or: %1 = G_CONSTANT 16 %2:_(s32) = G_SHL %0, %1 %3:_(s32) = G_ASHR %2, %1 depending on whether %1 is constant and 16 does not violate that principle since both outputs are genuinely legal. Reviewers: bogner, aditya_nandakumar, volkan, aemerson, paquette, arsenm Subscribers: sdardis, jvesely, wdng, nhaehnle, rovka, kristof.beyls, javed.absar, hiraditya, jrtc27, atanasyan, Petar.Avramovic, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D61289 llvm-svn: 368487 |
||
|---|---|---|
| benchmarks | ||
| bindings | ||
| cmake | ||
| docs | ||
| examples | ||
| include | ||
| lib | ||
| projects | ||
| resources | ||
| runtimes | ||
| test | ||
| tools | ||
| unittests | ||
| utils | ||
| .arcconfig | ||
| .clang-format | ||
| .clang-tidy | ||
| .gitattributes | ||
| .gitignore | ||
| CMakeLists.txt | ||
| CODE_OWNERS.TXT | ||
| configure | ||
| CREDITS.TXT | ||
| LICENSE.TXT | ||
| llvm.spec.in | ||
| LLVMBuild.txt | ||
| README.txt | ||
| RELEASE_TESTERS.TXT | ||
The LLVM Compiler Infrastructure ================================ This directory and its subdirectories contain source code for LLVM, 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 are writing a package for LLVM, see docs/Packaging.rst for our suggestions.