[BOLT][NFCI] Use heuristic for matching split global functions

[aaupov]

This change speeds up fragment matching for large BOLTed binaries where
all fragments of global parent functions are put under bolt-pseudo.o
file symbol:

• before: iterating over symbols under bolt-pseudo.o only to fail
  to find a parent,
• after: bail out immediately and use a global parent by name.

Test Plan: NFC, updated register-fragments-bolt-symbols.s

[llvmbot]

@llvm/pr-subscribers-llvm-binary-utilities

@llvm/pr-subscribers-bolt

Author: Amir Ayupov (aaupov)

Changes

There should be no functional change, only faster fragment matching for
large BOLTed binaries where all fragments of global parent functions
are put under bolt-pseudo.o file symbol.

Test Plan: NFC

---

Full diff: https://github.com/llvm/llvm-project/pull/90429.diff

1 Files Affected:

• (modified) bolt/lib/Rewrite/RewriteInstance.cpp (+5)

diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index 8eb2e5a9d9120a..d92775e16b7cf1 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -1511,6 +1511,11 @@ void RewriteInstance::registerFragments() {
 
     uint64_t ParentAddress{0};
 
+    // Check if containing FILE symbol is BOLT emitted synthetic symbol marking
+    // local fragments of global parents.
+    if (cantFail(FSI[-1].getName()) == getBOLTFileSymbolName())
+      goto registerParent;
+
     // BOLT split fragment symbols are emitted just before the main function
     // symbol.
     for (ELFSymbolRef NextSymbol = Symbol; NextSymbol < StopSymbol;

[rafaelauler]

lgtm