[BOLT] Use heuristic for matching split local functions (#90424)

Use known order of BOLT split function symbols: fragment symbols
immediately precede the parent fragment symbol.

Depends On: #89648

Test Plan: Added register-fragments-bolt-symbols.s

Author: aaupov

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -1513,6 +1513,23 @@ void RewriteInstance::registerFragments() {
       StopSymbol = *FSI;
 
     uint64_t ParentAddress{0};
+
+    // BOLT split fragment symbols are emitted just before the main function
+    // symbol.
+    for (ELFSymbolRef NextSymbol = Symbol; NextSymbol < StopSymbol;
+         NextSymbol.moveNext()) {
+      StringRef Name = cantFail(NextSymbol.getName());
+      if (Name == ParentName) {
+        ParentAddress = cantFail(NextSymbol.getValue());
+        goto registerParent;
+      }
+      if (Name.starts_with(ParentName))
+        // With multi-way splitting, there are multiple fragments with different
+        // suffixes. Parent follows the last fragment.
+        continue;
+      break;
+    }
+
     // Iterate over local file symbols and check symbol names to match parent.
     for (ELFSymbolRef Symbol(FSI[-1]); Symbol < StopSymbol; Symbol.moveNext()) {
       if (cantFail(Symbol.getName()) == ParentName) {
@@ -1521,6 +1538,7 @@ void RewriteInstance::registerFragments() {
       }
     }
 
+registerParent:
     // No local parent is found, use global parent function.
     if (!ParentAddress)
       if (BinaryData *ParentBD = BC->getBinaryDataByName(ParentName))

bolt/test/X86/register-fragments-bolt-symbols.s

@@ -0,0 +1,32 @@
+# Test the heuristics for matching BOLT-added split functions.
+
+# RUN: llvm-mc --filetype=obj --triple x86_64-unknown-unknown %S/cdsplit-symbol-names.s -o %t.main.o
+# RUN: llvm-mc --filetype=obj --triple x86_64-unknown-unknown %s -o %t.chain.o
+# RUN: link_fdata %S/cdsplit-symbol-names.s %t.main.o %t.fdata
+# RUN: sed -i 's|chain|chain/2|g' %t.fdata
+# RUN: llvm-strip --strip-unneeded %t.main.o
+# RUN: llvm-objcopy --localize-symbol=chain %t.main.o
+# RUN: %clang %cflags %t.chain.o %t.main.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.bolt --split-functions --split-strategy=randomN \
+# RUN:   --reorder-blocks=ext-tsp --enable-bat --bolt-seed=7 --data=%t.fdata
+# RUN: llvm-objdump --syms %t.bolt | FileCheck %s --check-prefix=CHECK-SYMS
+
+# RUN: link_fdata %s %t.bolt %t.preagg PREAGG
+# PREAGG: B X:0 #chain.cold.0# 1 0
+# RUN: perf2bolt %t.bolt -p %t.preagg --pa -o %t.bat.fdata -w %t.bat.yaml -v=1 \
+# RUN:   | FileCheck %s --check-prefix=CHECK-REGISTER
+
+# CHECK-SYMS: l df *ABS*          [[#]] chain.s
+# CHECK-SYMS: l  F .bolt.org.text [[#]] chain
+# CHECK-SYMS: l  F .text.cold     [[#]] chain.cold.0
+# CHECK-SYMS: l  F .text          [[#]] chain
+# CHECK-SYMS: l df *ABS*          [[#]] bolt-pseudo.o
+
+# CHECK-REGISTER: BOLT-INFO: marking chain.cold.0/1(*2) as a fragment of chain/2(*2)
+
+.file "chain.s"
+        .text
+        .type   chain, @function
+chain:
+        ret
+        .size   chain, .-chain