Change LittleEndian loads/stores to use memcpy #150
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The existing code uses a series of 8bit loads with shifts and ors to emulate an (unaligned) load of a larger type. These are then expected to become single loads in the compiler, producing optimal assembly. Whilst this is true it happens very late in the compiler, meaning that throughout most of the pipeline it is treated (and cost-modelled) as multiple loads, shifts and ors. This can make the compiler make poor decisions (such as not unrolling loops that should be), or to break up the pattern before it is turned into a single load. For example the loops in CompressFragment do not get unrolled as expected due to a higher cost than the unroll threshold in clang. Instead this patch uses a more conventional methods of loading unaligned data, using a memcpy directly which the compiler will be able to deal with much more straight forwardly, modelling it as a single unaligned load. The old code is left as-is for big-endian systems. This helps improve the performance of the BM_ZFlat benchmarks by up to 10-15% on an Arm Neoverse N1. Change-Id: I986f845ebd0a0806d052d2be3e4dbcbee91713d7
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LGTM from me |
pwnall
approved these changes
Jan 11, 2023
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Thanks! |
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The existing code uses a series of 8bit loads with shifts and ors to
emulate an (unaligned) load of a larger type. These are then expected to
become single loads in the compiler, producing optimal assembly. Whilst
this is true it happens very late in the compiler, meaning that
throughout most of the pipeline it is treated (and cost-modelled) as
multiple loads, shifts and ors. This can make the compiler make poor
decisions (such as not unrolling loops that should be), or to break up
the pattern before it is turned into a single load.
For example the loops in CompressFragment do not get unrolled as
expected due to a higher cost than the unroll threshold in clang.
Instead this patch uses a more conventional methods of loading unaligned
data, using a memcpy directly which the compiler will be able to deal
with much more straight forwardly, modelling it as a single unaligned
load. The old code is left as-is for big-endian systems.
This helps improve the performance of the BM_ZFlat benchmarks by up to
10-15% on an Arm Neoverse N1.