htyu wrote:
> @htyu Pretty much a drive-by question if it's convenient for you to share
> more, how ranges are selected out of sampled values? For example, are ranges
> the same for all workloads or dynamically generated based on the distribution
> of size from the per-workload profile data?
We group sampled values by ranges pretty much based on the current
`folly::memcpy` implementation:
```
0
1
[2,3]
[4,7]
[8,16]
[17,32]
[33,64]
[65,128]
[129,inf]
```
Values in a range share the same memcpy code, i.e, a pair of forward and
backward copies.
The range layout is fixed and hardcoded in LLVM. The compiler can choose to
prioritize a specific range based on the value profile it sees. The profile is
just like a LBR profile which is collected per service.
So at compile time we may have such transformation:
memcpy(src, dst, size)
=>
```
if (33 <= size <= 64)
vmovups (%rsi), %ymm0
vmovups -32(%rsi,%rdx), %ymm1
vmovups %ymm0, (%rdi)
vmovups %ymm1, -32(%rdi,%rdx)
else
call memcpy(src, dst, size)
```
https://github.com/llvm/llvm-project/pull/66825
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