ELF: Add --preferred-function-alignment option.

lld/ELF/Arch/X86_64.cpp

@@ -354,6 +354,9 @@ void X86_64::relaxCFIJumpTables() const {
             sec->file, sec->name, sec->type, sec->flags, sec->entsize,
             sec->entsize,
             sec->contentMaybeDecompress().slice(begin, end - begin));
+        // Ensure that --preferred-function-alignment does not mess with the
+        // placement of this section.
+        slice->retainAlignment = true;
         for (const Relocation &r : ArrayRef<Relocation>(rbegin, rend)) {
           slice->relocations.push_back(
               Relocation{r.expr, r.type, r.offset - begin, r.addend, r.sym});
@@ -421,6 +424,9 @@ void X86_64::relaxCFIJumpTables() const {
             // table. First add a slice for the unmodified jump table entries
             // before this one.
             addSectionSlice(begin, cur, rbegin, rcur);
+            // Ensure that --preferred-function-alignment does not mess with the
+            // placement of this section.
+            target->retainAlignment = true;
             // Add the target to our replacement list, and set the target's
             // replacement list to the empty list. This removes it from its
             // original position and adds it here, as well as causing
@@ -441,6 +447,7 @@ void X86_64::relaxCFIJumpTables() const {
       // jump table where it is and keep the last entry.
       if (lastSec) {
         addSectionSlice(begin, cur, rbegin, rcur);
+        lastSec->retainAlignment = true;
         replacements.push_back(lastSec);
         sectionReplacements[sec] = {};
         sectionReplacements[lastSec] = replacements;

lld/ELF/Config.h

@@ -358,6 +358,7 @@ struct Config {
   bool optRemarksWithHotness;
   bool picThunk;
   bool pie;
+  std::optional<uint64_t> preferredFunctionAlignment;
   bool printGcSections;
   bool printIcfSections;
   bool printMemoryUsage;

lld/ELF/Driver.cpp

@@ -1508,6 +1508,9 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
   if (auto *arg = args.getLastArg(OPT_package_metadata))
     parsePackageMetadata(ctx, *arg);
   ctx.arg.pie = args.hasFlag(OPT_pie, OPT_no_pie, false);
+  if (args.hasArg(OPT_preferred_function_alignment))
+    ctx.arg.preferredFunctionAlignment =
+        args::getInteger(args, OPT_preferred_function_alignment, 0);
   ctx.arg.printIcfSections =
       args.hasFlag(OPT_print_icf_sections, OPT_no_print_icf_sections, false);
   ctx.arg.printGcSections =

lld/ELF/InputSection.cpp

@@ -58,7 +58,7 @@ InputSectionBase::InputSectionBase(InputFile *file, StringRef name,
                                    Kind sectionKind)
     : SectionBase(sectionKind, file, name, type, flags, link, info, addralign,
                   entsize),
-      bss(0), decodedCrel(0), keepUnique(0), nopFiller(0),
+      bss(0), decodedCrel(0), keepUnique(0), nopFiller(0), retainAlignment(0),
       content_(data.data()), size(data.size()) {
   // In order to reduce memory allocation, we assume that mergeable
   // sections are smaller than 4 GiB, which is not an unreasonable

lld/ELF/InputSection.h

@@ -183,6 +183,11 @@ class InputSectionBase : public SectionBase {
   LLVM_PREFERRED_TYPE(bool)
   uint8_t nopFiller : 1;
 
+  // If true, --preferred-function-alignment has no effect on this section.
+  // Set by the CFI jump table relaxation pass.
+  LLVM_PREFERRED_TYPE(bool)
+  uint8_t retainAlignment : 1;
+
   mutable bool compressed = false;
 
   // Input sections are part of an output section. Special sections

lld/ELF/Options.td

@@ -406,6 +406,9 @@ def pop_state: F<"pop-state">,
 def push_state: F<"push-state">,
   HelpText<"Save the current state of --as-needed, -static and --whole-archive">;
 
+def preferred_function_alignment: JJ<"preferred-function-alignment=">,
+  HelpText<"Align functions to the given alignment if possible">;
+
 def print_map: F<"print-map">,
   HelpText<"Print a link map to the standard output">;
 

lld/ELF/Writer.cpp

@@ -1499,6 +1499,20 @@ static void randomizeSectionPadding(Ctx &ctx) {
   }
 }
 
+static void applyPreferredFunctionAlignment(Ctx &ctx) {
+  if (!ctx.arg.preferredFunctionAlignment)
+    return;
+  SmallVector<InputSection *, 0> storage;
+  for (OutputSection *osec : ctx.outputSections) {
+    if (!(osec->flags & SHF_EXECINSTR))
+      continue;
+    for (InputSection *sec : getInputSections(*osec, storage))
+      if (!isa<SyntheticSection>(sec) && !sec->retainAlignment)
+        sec->addralign = std::max<uint32_t>(
+            sec->addralign, *ctx.arg.preferredFunctionAlignment);
+  }
+}
+
 // We need to generate and finalize the content that depends on the address of
 // InputSections. As the generation of the content may also alter InputSection
 // addresses we must converge to a fixed point. We do that here. See the comment
@@ -1530,6 +1544,9 @@ template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() {
 
   ctx.target->relaxCFIJumpTables();
 
+  if (ctx.arg.preferredFunctionAlignment)
+    applyPreferredFunctionAlignment(ctx);
+
   uint32_t pass = 0, assignPasses = 0;
   for (;;) {
     bool changed = ctx.target->needsThunks

lld/docs/ld.lld.1

@@ -520,6 +520,13 @@ Currently the same as yes.
 Don't use.
 .El
 
+.It Fl -preferred-function-alignment Ns = Ns Ar align
+Specify the preferred function alignment. This flag increases the
+alignment of all user-provided executable (SHF_EXECINSTR) sections to the
+specified value unless retaining the original alignment is beneficial
+for performance reasons. For example, when using LLVM's control flow
+integrity feature, functions may retain their original alignment if
+necessary in order to move them into the jump table.
 .It Fl -print-gc-sections
 List removed unused sections.
 .It Fl -print-icf-sections

lld/test/ELF/preferred-function-alignment.s

@@ -0,0 +1,15 @@
+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+// RUN: ld.lld --preferred-function-alignment=32 -o %t %t.o
+// RUN: llvm-nm -n %t | FileCheck %s
+
+// CHECK: 0000000000201120 t f1
+.section .text.f1,"ax",@progbits
+f1:
+ret
+
+// CHECK: 0000000000201140 t f2
+.section .text.f2,"ax",@progbits
+f2:
+ret
+

lld/test/ELF/x86_64-relax-jump-tables.s

@@ -2,6 +2,8 @@
 // RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
 // RUN: ld.lld %t.o -shared -o %t
 // RUN: llvm-objdump -d --show-all-symbols %t | FileCheck %s
+// RUN: ld.lld %t.o -shared -o %t32 --preferred-function-alignment=32
+// RUN: llvm-objdump -d --show-all-symbols %t32 | FileCheck %s
 
 // Mostly positive cases, except for f2.
 .section .text.jt1,"ax",@llvm_cfi_jump_table,8