forked from OSchip/llvm-project
7cdeac43e5
This can come up in Bit Stream abstractions. The pattern looks big/scary, but it can't be simplified any further. It only is so simple because a number of my preparatory folds had happened already (shift amount reassociation / shift amount reassociation in bit test, sign bit test detection). Highlights: * There are two main flavors: https://rise4fun.com/Alive/zWi The difference is add vs. sub, and left-shift of -1 vs. 1 * Since we only change the shift opcode, we can preserve the exact-ness: https://rise4fun.com/Alive/4u4 * There can be truncation after high-bit-extraction: https://rise4fun.com/Alive/slHc1 (the main pattern i'm after!) Which means that we need to ignore zext of shift amounts and of NBits. * The sign-extending magic can be extended itself (in add pattern via sext, in sub pattern via zext. not the other way around!) https://rise4fun.com/Alive/NhG (or those sext/zext can be sinked into `select`!) Which again means we should pay attention when matching NBits. * We can have both truncation of extraction and widening of magic: https://rise4fun.com/Alive/XTw In other words, i don't believe we need to have any checks on bitwidths of any of these constructs. This is worsened in general by the fact that we may have `sext` instead of `zext` for shift amounts, and we don't yet canonicalize to `zext`, although we should. I have not done anything about that here. Also, we really should have something to weed out `sub` like these, by folding them into `add` variant. https://bugs.llvm.org/show_bug.cgi?id=42389 llvm-svn: 373964 |
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| .. | ||
| CMakeLists.txt | ||
| InstCombineAddSub.cpp | ||
| InstCombineAndOrXor.cpp | ||
| InstCombineAtomicRMW.cpp | ||
| InstCombineCalls.cpp | ||
| InstCombineCasts.cpp | ||
| InstCombineCompares.cpp | ||
| InstCombineInternal.h | ||
| InstCombineLoadStoreAlloca.cpp | ||
| InstCombineMulDivRem.cpp | ||
| InstCombinePHI.cpp | ||
| InstCombineSelect.cpp | ||
| InstCombineShifts.cpp | ||
| InstCombineSimplifyDemanded.cpp | ||
| InstCombineTables.td | ||
| InstCombineVectorOps.cpp | ||
| InstructionCombining.cpp | ||
| LLVMBuild.txt | ||