From fb24b4d46a0a8278031f42c1cba6c030eb6c6010 Mon Sep 17 00:00:00 2001
From: Maksim Panchenko <maks@fb.com>
Date: Sat, 28 Jun 2025 13:53:56 -0700
Subject: [PATCH] [BOLT] Push code to higher addresses under options (#146180)

When --hot-functions-at-end is used in combination with --use-old-text,
allocate code at the highest possible addresses withing old .text.

This feature is mostly useful for HHVM, where it is beneficial to have
hot static code placed as close as possible to jitted code.
---
 bolt/lib/Rewrite/RewriteInstance.cpp | 31 +++++++++++++++++++++++++---
 bolt/test/code-at-high-address.c     | 24 +++++++++++++++++++++
 2 files changed, 52 insertions(+), 3 deletions(-)
 create mode 100644 bolt/test/code-at-high-address.c

diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index 934768c244b3..462f8a582d73 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -3918,15 +3918,40 @@ void RewriteInstance::mapCodeSections(BOLTLinker::SectionMapper MapSection) {
       return Address;
     };
 
+    // Try to allocate sections before the \p Address and return an address for
+    // the allocation of the first section, or 0 if [0, Address) range is not
+    // big enough to fit all sections.
+    auto allocateBefore = [&](uint64_t Address) -> uint64_t {
+      for (BinarySection *Section : llvm::reverse(CodeSections)) {
+        if (Section->getOutputSize() > Address)
+          return 0;
+        Address -= Section->getOutputSize();
+        Address = alignDown(Address, Section->getAlignment());
+        Section->setOutputAddress(Address);
+      }
+      return Address;
+    };
+
     // Check if we can fit code in the original .text
     bool AllocationDone = false;
     if (opts::UseOldText) {
-      const uint64_t CodeSize =
-          allocateAt(BC->OldTextSectionAddress) - BC->OldTextSectionAddress;
+      uint64_t StartAddress;
+      uint64_t EndAddress;
+      if (opts::HotFunctionsAtEnd) {
+        EndAddress = BC->OldTextSectionAddress + BC->OldTextSectionSize;
+        StartAddress = allocateBefore(EndAddress);
+      } else {
+        StartAddress = BC->OldTextSectionAddress;
+        EndAddress = allocateAt(BC->OldTextSectionAddress);
+      }
 
+      const uint64_t CodeSize = EndAddress - StartAddress;
       if (CodeSize <= BC->OldTextSectionSize) {
         BC->outs() << "BOLT-INFO: using original .text for new code with 0x"
-                   << Twine::utohexstr(opts::AlignText) << " alignment\n";
+                   << Twine::utohexstr(opts::AlignText) << " alignment";
+        if (StartAddress != BC->OldTextSectionAddress)
+          BC->outs() << " at 0x" << Twine::utohexstr(StartAddress);
+        BC->outs() << '\n';
         AllocationDone = true;
       } else {
         BC->errs()
diff --git a/bolt/test/code-at-high-address.c b/bolt/test/code-at-high-address.c
new file mode 100644
index 000000000000..fa33c1eb342d
--- /dev/null
+++ b/bolt/test/code-at-high-address.c
@@ -0,0 +1,24 @@
+// Check that llvm-bolt pushes code to higher addresses under
+// --hot-functions-at-end when rewriting code in-place.
+
+// REQUIRES: system-linux
+
+// RUN: %clang %cflags -O0 %s -o %t -no-pie -Wl,-q -falign-functions=64 \
+// RUN:   -nostartfiles -nostdlib -ffreestanding
+// RUN: llvm-bolt %t -o %t.bolt --use-old-text --align-functions=1 \
+// RUN:   --no-huge-pages --align-text=1 --use-gnu-stack --hot-functions-at-end \
+// RUN:   | FileCheck %s --check-prefix=CHECK-BOLT
+// RUN: llvm-readelf --sections %t.bolt | FileCheck %s
+
+// CHECK-BOLT: using original .text for new code with 0x1 alignment at {{.*}}
+
+// As .text is pushed higher, preceding .bolt.org.text should have non-zero
+// size.
+// CHECK: .bolt.org.text PROGBITS
+// CHECK-NOT: {{ 000000 }}
+// CHECK-SAME: AX
+// CHECK-NEXT: .text PROGBITS
+
+int foo() { return 0; }
+
+int main() { return foo(); }