Customising compilers for customisable processors
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Date
29/11/2012Author
Murray, Alastair Colin
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Abstract
The automatic generation of instruction set extensions to provide application-specific acceleration
for embedded processors has been a productive area of research in recent years. There
have been incremental improvements in the quality of the algorithms that discover and select
which instructions to add to a processor. The use of automatic algorithms, however, result in
instructions which are radically different from those found in conventional, human-designed,
RISC or CISC ISAs. This has resulted in a gap between the hardware’s capabilities and the
compiler’s ability to exploit them.
This thesis proposes and investigates the use of a high-level compiler pass that uses graph-subgraph
isomorphism checking to exploit these complex instructions. Operating in a separate
pass permits techniques to be applied that are uniquely suited for mapping complex instructions,
but unsuitable for conventional instruction selection. The existing, mature, compiler
back-end can then handle the remainder of the compilation. With this method, the high-level
pass was able to use 1965 different automatically produced instructions to obtain an initial average
speed-up of 1.11x over 179 benchmarks evaluated on a hardware-verified cycle-accurate
simulator.
This result was improved following an investigation of how the produced instructions were
being used by the compiler. It was established that the models the automatic tools were using to
develop instructions did not take account of how well the compiler could realistically use them.
Adding additional parameters to the search heuristic to account for compiler issues increased
the speed-up from 1.11x to 1.24x. An alternative approach using a re-designed hardware interface
was also investigated and this achieved a speed-up of 1.26x while reducing hardware and
compiler complexity.
A complementary, high-level, method of exploiting dual memory banks was created to increase
memory bandwidth to accommodate the increased data-processing bandwidth provided
by extension instructions. Finally, the compiler was considered for use in a non-conventional
role where rather than generating code it is used to apply source-level transformations prior to
the generation of extension instructions and thus affect the shape of the instructions that are
generated.