From bb627b0a0c05e0bb04ae7f984b8e7fcd84906061 Mon Sep 17 00:00:00 2001
From: Amir Ayupov <aaupov@fb.com>
Date: Mon, 20 May 2024 16:55:11 -0700
Subject: [PATCH] [BOLT] Ignore special symbols as function aliases in
 updateELFSymbolTable

Exempt special symbols (hot text/data and _end symbol) from normal
handling. We only need to set their value and make them absolute.

If these symbols are handled as normal symbols and if they alias
functions we may create non-sensical symbols, e.g. __hot_start.cold.

Test Plan: updated hot-end-symbol.s

Reviewers: maksfb, rafaelauler, ayermolo, dcci

Reviewed By: dcci, maksfb

Pull Request: https://github.com/llvm/llvm-project/pull/92713
---
 bolt/lib/Rewrite/RewriteInstance.cpp   | 62 +++++++++++++++-----------
 bolt/test/runtime/X86/hot-end-symbol.s |  3 +-
 2 files changed, 37 insertions(+), 28 deletions(-)

diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index 85b39176754b64..6e1021a6df22c8 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -4808,6 +4808,40 @@ void RewriteInstance::updateELFSymbolTable(
     // Create a new symbol based on the existing symbol.
     ELFSymTy NewSymbol = Symbol;
 
+    // Handle special symbols based on their name.
+    Expected<StringRef> SymbolName = Symbol.getName(StringSection);
+    assert(SymbolName && "cannot get symbol name");
+
+    auto updateSymbolValue = [&](const StringRef Name,
+                                 std::optional<uint64_t> Value = std::nullopt) {
+      NewSymbol.st_value = Value ? *Value : getNewValueForSymbol(Name);
+      NewSymbol.st_shndx = ELF::SHN_ABS;
+      BC->outs() << "BOLT-INFO: setting " << Name << " to 0x"
+                 << Twine::utohexstr(NewSymbol.st_value) << '\n';
+    };
+
+    if (*SymbolName == "__hot_start" || *SymbolName == "__hot_end") {
+      if (opts::HotText) {
+        updateSymbolValue(*SymbolName);
+        ++NumHotTextSymsUpdated;
+      }
+      goto registerSymbol;
+    }
+
+    if (*SymbolName == "__hot_data_start" || *SymbolName == "__hot_data_end") {
+      if (opts::HotData) {
+        updateSymbolValue(*SymbolName);
+        ++NumHotDataSymsUpdated;
+      }
+      goto registerSymbol;
+    }
+
+    if (*SymbolName == "_end") {
+      if (NextAvailableAddress > Symbol.st_value)
+        updateSymbolValue(*SymbolName, NextAvailableAddress);
+      goto registerSymbol;
+    }
+
     if (Function) {
       // If the symbol matched a function that was not emitted, update the
       // corresponding section index but otherwise leave it unchanged.
@@ -4904,33 +4938,7 @@ void RewriteInstance::updateELFSymbolTable(
       }
     }
 
-    // Handle special symbols based on their name.
-    Expected<StringRef> SymbolName = Symbol.getName(StringSection);
-    assert(SymbolName && "cannot get symbol name");
-
-    auto updateSymbolValue = [&](const StringRef Name,
-                                 std::optional<uint64_t> Value = std::nullopt) {
-      NewSymbol.st_value = Value ? *Value : getNewValueForSymbol(Name);
-      NewSymbol.st_shndx = ELF::SHN_ABS;
-      BC->outs() << "BOLT-INFO: setting " << Name << " to 0x"
-                 << Twine::utohexstr(NewSymbol.st_value) << '\n';
-    };
-
-    if (opts::HotText &&
-        (*SymbolName == "__hot_start" || *SymbolName == "__hot_end")) {
-      updateSymbolValue(*SymbolName);
-      ++NumHotTextSymsUpdated;
-    }
-
-    if (opts::HotData && (*SymbolName == "__hot_data_start" ||
-                          *SymbolName == "__hot_data_end")) {
-      updateSymbolValue(*SymbolName);
-      ++NumHotDataSymsUpdated;
-    }
-
-    if (*SymbolName == "_end" && NextAvailableAddress > Symbol.st_value)
-      updateSymbolValue(*SymbolName, NextAvailableAddress);
-
+  registerSymbol:
     if (IsDynSym)
       Write((&Symbol - cantFail(Obj.symbols(&SymTabSection)).begin()) *
                 sizeof(ELFSymTy),
diff --git a/bolt/test/runtime/X86/hot-end-symbol.s b/bolt/test/runtime/X86/hot-end-symbol.s
index e6d83d77167acd..6ae771cead7568 100755
--- a/bolt/test/runtime/X86/hot-end-symbol.s
+++ b/bolt/test/runtime/X86/hot-end-symbol.s
@@ -12,6 +12,7 @@
 # RUN: %clang %cflags -no-pie %t.o -o %t.exe -Wl,-q
 
 # RUN: llvm-bolt %t.exe --relocs=1 --hot-text --reorder-functions=hfsort \
+# RUN:    --split-functions --split-strategy=all \
 # RUN:    --data %t.fdata -o %t.out | FileCheck %s
 
 # RUN: %t.out 1
@@ -30,12 +31,12 @@
 # CHECK-OUTPUT:       __hot_start
 # CHECK-OUTPUT-NEXT:  main
 # CHECK-OUTPUT-NEXT:  __hot_end
+# CHECK-OUTPUT-NOT:   __hot_start.cold
 
   .text
   .globl  main
   .type main, %function
   .globl  __hot_start
-  .type __hot_start, %object
   .p2align  4
 main:
 __hot_start: