ReOptimizeLayer was building LLVM IR to define a precomputed,
SPS-serialized argument buffer, then inserting calls directly to
__orc_rt_jit_dispatch, passing the address of the precomputed buffer and
an __orc_rt_reoptimize_tag defined by the ORC runtime. This design is
non-canonical, requiring the ORC runtime to be loaded (or an extra
definition for __orc_rt_reoptimize_tag to be inserted) while not using
the runtime to perform the serialization.
This commit updates ReOptimizeLayer to instead insert calls to an
__orc_rt_reoptimize function implemented in the ORC runtime. This
function will perform serialization and call __orc_rt_jit_dispatch,
similar to other functions in the ORC runtime.
To maintain support for in-process JITs that don't use the ORC runtime,
this commit adds a ReOptimizeLayer::addOrcRTLiteSupport method which
injects IR to define __orc_rt_reoptimize (calling through to an
orc_rt_lite_reoptimize_helper function defined in LLVM) and
__orc_rt_reoptimize_tag. The ReOptimizeLayerTest is updated to use
addOrcRTLiteSupport.
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