Remove few pointer dereferences in dbuf_read().

[amotin]

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
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• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
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• I have added tests to cover my changes.
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[ryao]

Nice catch.

In the past, I would have thought that a good optimizing compiler would have done this for us. However, since there are function calls between where we say to do the pointer dereferences and where the result is used, a compiler might think that the placement was intentional, since a function call might have a side effect that changes the value, which would make moving the dereferences across function calls generate incorrect code. This performance tweak therefore needs to be done manually. This would not have occurred to me unless I were studying the disassembly of this function, which is something I am unlikely to ever do.

[amotin]

This would not have occurred to me unless I were studying the disassembly of this function, which is something I am unlikely to ever do.

@ryao Obviously I am not disassembling everything, but this I've noticed in instruction-level profiles of some benchmarks, which I am sometimes reviewing close for some particularly hot functions.