| commit | d23c5c2d6566fce4380cfa31d438422db19fbce9 | [log] [tgz] |
|---|---|---|
| author | Kyungwoo Lee <kyulee@meta.com> | Wed Nov 13 17:34:07 2024 -0800 |
| committer | GitHub <noreply@github.com> | Wed Nov 13 17:34:07 2024 -0800 |
| tree | a50fe5074a1f87cfb437051b7627093f8a1d7139 | |
| parent | 6e614e11df6a152082b51a1b18332cb8730a4032 [diff] |
[CGData] Global Merge Functions (#112671) This implements a global function merging pass. Unlike traditional function merging passes that use IR comparators, this pass employs a structurally stable hash to identify similar functions while ignoring certain constant operands. These ignored constants are tracked and encoded into a stable function summary. When merging, instead of explicitly folding similar functions and their call sites, we form a merging instance by supplying different parameters via thunks. The actual size reduction occurs when identically created merging instances are folded by the linker. Currently, this pass is wired to a pre-codegen pass, enabled by the `-enable-global-merge-func` flag. In a local merging mode, the analysis and merging steps occur sequentially within a module: - `analyze`: Collects stable function hashes and tracks locations of ignored constant operands. - `finalize`: Identifies merge candidates with matching hashes and computes the set of parameters that point to different constants. - `merge`: Uses the stable function map to optimistically create a merged function. We can enable a global merging mode similar to the global function outliner (https://discourse.llvm.org/t/rfc-enhanced-machine-outliner-part-2-thinlto-nolto/78753/), which will perform the above steps separately. - `-codegen-data-generate`: During the first round of code generation, we analyze local merging instances and publish their summaries. - Offline using `llvm-cgdata` or at link-time, we can finalize all these merging summaries that are combined to determine parameters. - `-codegen-data-use`: During the second round of code generation, we optimistically create merging instances within each module, and finally, the linker folds identically created merging instances. Depends on #112664 This is a patch for https://discourse.llvm.org/t/rfc-global-function-merging/82608.
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