Bug 1506 - Support for JIT circular shifts
Summary: Support for JIT circular shifts
Status: RESOLVED NOT_ON_ROADMAP
Alias: None
Product: Runtime
Classification: Mono
Component: JIT ()
Version: unspecified
Hardware: PC All
: --- enhancement
Target Milestone: ---
Assignee: Bugzilla
URL:
Depends on:
Blocks:
 
Reported: 2011-10-14 21:42 UTC by Stephen Kou
Modified: 2017-07-11 21:12 UTC (History)
2 users (show)

Tags:
Is this bug a regression?: ---
Last known good build:

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Status:
RESOLVED NOT_ON_ROADMAP

Description Stephen Kou 2011-10-14 21:42:31 UTC
This isn't really a bug, but it's a pretty awesome thing to have.  Many modern crypto hash and encryption algorithms make use of rotl and rotr (circular shift) operations.  MurmurHash3 is one example.  In C/C++ compilers, the expression:

(x >> r) | (x << (32 - r)) 

is recognized and is compiled to a single x86 ROR/ROL instruction.  While CIL has no opcode for rotation, it has a distinct signature which the JIT should be able to recognize:

(rotate right)
ldloc x
ldloc r
shr.un
ldloc x
ldc_i4 32
ldloc r
sub
shl
or

Rotate left is pretty similar, with the shr.un / shl flipped.  

X86, ARM, PowerPC, and Itanium all have a rotate instruction, but SPARC does not.
Comment 1 Rodrigo Kumpera 2017-07-11 21:12:04 UTC
Thanks for the feedback. Right now this sort of optimization is left for LLVM to do.