[BOLT] Push code to higher addresses under options

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()

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(); }