forked from OSchip/llvm-project
55d633bd60
C semantics force sub-int-sized values (e.g. i8, i16) to be promoted to int type (e.g. i32) whenever arithmetic is performed on them. For targets with native i8 or i16 operations, usually InstCombine can shrink the arithmetic type down again. However InstCombine refuses to create illegal types, so for targets without i8 or i16 registers, the lengthening and shrinking remains. Most SIMD ISAs (e.g. NEON) however support vectors of i8 or i16 even when their scalar equivalents do not, so during vectorization it is important to remove these lengthens and truncates when deciding the profitability of vectorization. The algorithm this uses starts at truncs and icmps, trawling their use-def chains until they terminate or instructions outside the loop are found (or unsafe instructions like inttoptr casts are found). If the use-def chains starting from different root instructions (truncs/icmps) meet, they are unioned. The demanded bits of each node in the graph are ORed together to form an overall mask of the demanded bits in the entire graph. The minimum bitwidth that graph can be truncated to is the bitwidth minus the number of leading zeroes in the overall mask. The intention is that this algorithm should "first do no harm", so it will never insert extra cast instructions. This is why the use-def graphs are unioned, so that subgraphs with different minimum bitwidths do not need casts inserted between them. This algorithm works hard to reduce compile time impact. DemandedBits are only queried if there are extends of illegal types and if a truncate to an illegal type is seen. In the general case, this results in a simple linear scan of the instructions in the loop. No non-noise compile time impact was seen on a clang bootstrap build. llvm-svn: 250032 |
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| .. | ||
| BBVectorize.cpp | ||
| CMakeLists.txt | ||
| LLVMBuild.txt | ||
| LoopVectorize.cpp | ||
| Makefile | ||
| SLPVectorizer.cpp | ||
| Vectorize.cpp | ||