[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, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/TargetPassConfig.cpp b/llvm/lib/CodeGen/TargetPassConfig.cpp
index 782ed60..5f3fe12 100644
--- a/llvm/lib/CodeGen/TargetPassConfig.cpp
+++ b/llvm/lib/CodeGen/TargetPassConfig.cpp
@@ -141,6 +141,9 @@ static cl::opt<RunOutliner> EnableMachineOutliner(
                           "Disable all outlining"),
                // Sentinel value for unspecified option.
                clEnumValN(RunOutliner::AlwaysOutline, "", "")));
+static cl::opt<bool> EnableGlobalMergeFunc(
+    "enable-global-merge-func", cl::Hidden,
+    cl::desc("Enable global merge functions that are based on hash function"));
 // Disable the pass to fix unwind information. Whether the pass is included in
 // the pipeline is controlled via the target options, this option serves as
 // manual override.
@@ -489,6 +492,7 @@ CGPassBuilderOption llvm::getCGPassBuilderOption() {
 
   SET_BOOLEAN_OPTION(EarlyLiveIntervals)
   SET_BOOLEAN_OPTION(EnableBlockPlacementStats)
+  SET_BOOLEAN_OPTION(EnableGlobalMergeFunc)
   SET_BOOLEAN_OPTION(EnableImplicitNullChecks)
   SET_BOOLEAN_OPTION(EnableMachineOutliner)
   SET_BOOLEAN_OPTION(MISchedPostRA)
@@ -884,6 +888,9 @@ void TargetPassConfig::addIRPasses() {
   // Convert conditional moves to conditional jumps when profitable.
   if (getOptLevel() != CodeGenOptLevel::None && !DisableSelectOptimize)
     addPass(createSelectOptimizePass());
+
+  if (EnableGlobalMergeFunc)
+    addPass(createGlobalMergeFuncPass());
 }
 
 /// Turn exception handling constructs into something the code generators can