[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.

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/tools/llvm-cgdata/llvm-cgdata.cpp b/llvm/tools/llvm-cgdata/llvm-cgdata.cpp
index 0931cad..d33459b 100644
--- a/llvm/tools/llvm-cgdata/llvm-cgdata.cpp
+++ b/llvm/tools/llvm-cgdata/llvm-cgdata.cpp
@@ -76,6 +76,7 @@ static StringRef ToolName;
 static StringRef OutputFilename = "-";
 static StringRef Filename;
 static bool ShowCGDataVersion;
+static bool SkipTrim;
 static CGDataAction Action;
 static std::optional<CGDataFormat> OutputFormat;
 static std::vector<std::string> InputFilenames;
@@ -214,7 +215,7 @@ static int merge_main(int argc, const char *argv[]) {
   if (!Result)
     exitWithError("failed to merge codegen data files.");
 
-  GlobalFunctionMapRecord.finalize();
+  GlobalFunctionMapRecord.finalize(SkipTrim);
 
   CodeGenDataWriter Writer;
   if (!GlobalOutlineRecord.empty())
@@ -301,6 +302,7 @@ static void parseArgs(int argc, char **argv) {
   }
 
   ShowCGDataVersion = Args.hasArg(OPT_cgdata_version);
+  SkipTrim = Args.hasArg(OPT_skip_trim);
 
   if (opt::Arg *A = Args.getLastArg(OPT_format)) {
     StringRef OF = A->getValue();