tree 4640bde08baa5ae74e919711b2819fcab01aeca4
parent 1e00bb16f488492198df692ae2e2abc405a138f3
author Mingming Liu <mingmingl@google.com> 1743204706 -0700
committer GitHub <noreply@github.com> 1743204706 -0700
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[CodeGen][StaticDataPartitioning]Place local-linkage global variables in hot or unlikely prefixed sections based on profile information (#125756)

In this PR, static-data-splitter pass finds out the local-linkage global
variables in {`.rodata`, `.data.rel.ro`, `bss`, `.data`} sections by
analyzing machine instruction operands, and aggregates their accesses
from code across functions.

A follow-up item is to analyze global variable initializers and count
for access from data.
* This limitation is demonstrated by `bss2` and `data3` in
`llvm/test/CodeGen/X86/global-variable-partition.ll`.

Some stats of static-data-splitter with this patch:

**section**|**bss**|**rodata**|**data**
:-----:|:-----:|:-----:|:-----:
hot-prefixed section coverage|99.75%|97.71%|91.30%
unlikely-prefixed section size percentage|67.94%|39.37%|63.10%

1. The coverage is defined as `#perf-sample-in-hot-prefixed <data>
section / #perf-sample in <data.*> section` for each <data> section.
* The perf command samples
`MEM_INST_RETIRED.ALL_LOADS:u:pinned:precise=2` events at a high
frequency (`perf -c 2251`) for 30 seconds. The profiled binary is built
as non-PIE so `data.rel.ro` coverage data is not available.
2. The unlikely-prefixed `<data>` section size percentage is defined as
`unlikely <data> section size / the sum size of <data>.* sections` for
each `<data>` section