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authorJakob Stoklund Olesen <stoklund@2pi.dk>2012-04-19 16:46:26 +0000
committerJakob Stoklund Olesen <stoklund@2pi.dk>2012-04-19 16:46:26 +0000
commit6b6c81e6b21e946d587c0078a36f92ff8cf1df72 (patch)
tree66e78647e856154da70378f63c6d957ce7fa68cd /lldb/source/Plugins/Process/gdb-remote/GDBRemoteRegisterContext.cpp
parent902c9b243d869b1d28b351db9e4d76e124a4d12c (diff)
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Defer some shl transforms to DAGCombine.
The shl instruction is used to represent multiplication by a constant power of two as well as bitwise left shifts. Some InstCombine transformations would turn an shl instruction into a bit mask operation, making it difficult for later analysis passes to recognize the constsnt multiplication. Disable those shl transformations, deferring them to DAGCombine time. An 'shl X, C' instruction is now treated mostly the same was as 'mul X, C'. These transformations are deferred: (X >>? C) << C --> X & (-1 << C) (When X >> C has multiple uses) (X >>? C1) << C2 --> X << (C2-C1) & (-1 << C2) (When C2 > C1) (X >>? C1) << C2 --> X >>? (C1-C2) & (-1 << C2) (When C1 > C2) The corresponding exact transformations are preserved, just like div-exact + mul: (X >>?,exact C) << C --> X (X >>?,exact C1) << C2 --> X << (C2-C1) (X >>?,exact C1) << C2 --> X >>?,exact (C1-C2) The disabled transformations could also prevent the instruction selector from recognizing rotate patterns in hash functions and cryptographic primitives. I have a test case for that, but it is too fragile. llvm-svn: 155136
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