[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, 2 insertions, 2 deletions

diff --git a/llvm/unittests/CGData/StableFunctionMapTest.cpp b/llvm/unittests/CGData/StableFunctionMapTest.cpp
index 5e178dc..d551ac8 100644
--- a/llvm/unittests/CGData/StableFunctionMapTest.cpp
+++ b/llvm/unittests/CGData/StableFunctionMapTest.cpp
@@ -108,8 +108,8 @@ TEST(StableFunctionMap, Finalize2) {
 
 TEST(StableFunctionMap, Finalize3) {
   StableFunctionMap Map;
-  StableFunction Func1{1, "Func1", "Mod1", 2, {{{0, 1}, 3}, {{1, 1}, 1}}};
-  StableFunction Func2{1, "Func2", "Mod2", 2, {{{0, 1}, 2}, {{1, 1}, 1}}};
+  StableFunction Func1{1, "Func1", "Mod1", 12, {{{0, 1}, 3}, {{1, 1}, 1}}};
+  StableFunction Func2{1, "Func2", "Mod2", 12, {{{0, 1}, 2}, {{1, 1}, 1}}};
   Map.insert(Func1);
   Map.insert(Func2);