Generate an .sframe section with a skeleton header

llvm/include/llvm/MC/MCObjectFileInfo.h

@@ -13,6 +13,7 @@
 #ifndef LLVM_MC_MCOBJECTFILEINFO_H
 #define LLVM_MC_MCOBJECTFILEINFO_H
 
+#include "llvm/BinaryFormat/SFrame.h"
 #include "llvm/BinaryFormat/Swift.h"
 #include "llvm/MC/MCSection.h"
 #include "llvm/Support/Compiler.h"
@@ -50,6 +51,9 @@ class LLVM_ABI MCObjectFileInfo {
   /// Compact unwind encoding indicating that we should emit only an EH frame.
   unsigned CompactUnwindDwarfEHFrameOnly = 0;
 
+  /// SFrame ABI architecture byte
+  std::optional<sframe::ABI> SFrameABIArch = {};
+
   /// Section directive for standard text.
   MCSection *TextSection = nullptr;
 
@@ -174,6 +178,9 @@ class LLVM_ABI MCObjectFileInfo {
   /// It is initialized on demand so it can be overwritten (with uniquing).
   MCSection *EHFrameSection = nullptr;
 
+  /// SFrame section.
+  MCSection *SFrameSection = nullptr;
+
   /// Section containing metadata on function stack sizes.
   MCSection *StackSizesSection = nullptr;
 
@@ -266,6 +273,7 @@ class LLVM_ABI MCObjectFileInfo {
     return CompactUnwindDwarfEHFrameOnly;
   }
 
+  std::optional<sframe::ABI> getSFrameABIArch() const { return SFrameABIArch; }
   virtual unsigned getTextSectionAlignment() const { return 4; }
   MCSection *getTextSection() const { return TextSection; }
   MCSection *getDataSection() const { return DataSection; }
@@ -445,6 +453,7 @@ class LLVM_ABI MCObjectFileInfo {
   MCSection *getTOCBaseSection() const { return TOCBaseSection; }
 
   MCSection *getEHFrameSection() const { return EHFrameSection; }
+  MCSection *getSFrameSection() const { return SFrameSection; }
 
   bool isPositionIndependent() const { return PositionIndependent; }
 

llvm/include/llvm/MC/MCSFrame.h

@@ -0,0 +1,36 @@
+//===- MCSFrame.h - Machine Code SFrame support -----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains the declaration of MCSFrameEmitter to support emitting
+// sframe unwinding info from .cfi_* directives. It relies on FDEs and CIEs
+// created for Dwarf frame info, but emits that info in a different format.
+//
+// See https://sourceware.org/binutils/docs-2.41/sframe-spec.html
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_MC_MCSFRAME_H
+#define LLVM_MC_MCSFRAME_H
+
+#include <cstdint>
+
+#include "llvm/ADT/SmallVector.h"
+
+namespace llvm {
+
+class MCObjectStreamer;
+
+class MCSFrameEmitter {
+public:
+  //
+  // Emits the sframe section.
+  //
+  static void Emit(MCObjectStreamer &streamer);
+};
+
+} // end namespace llvm
+#endif // LLVM_MC_MCSFRAME_H

llvm/lib/MC/CMakeLists.txt

@@ -45,6 +45,7 @@ add_llvm_component_library(LLVMMC
   MCSection.cpp
   MCSectionMachO.cpp
   MCStreamer.cpp
+  MCSFrame.cpp
   MCSPIRVStreamer.cpp
   MCSubtargetInfo.cpp
   MCSymbol.cpp

llvm/lib/MC/MCObjectFileInfo.cpp

@@ -10,6 +10,7 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/BinaryFormat/COFF.h"
 #include "llvm/BinaryFormat/ELF.h"
+#include "llvm/BinaryFormat/SFrame.h"
 #include "llvm/BinaryFormat/Wasm.h"
 #include "llvm/MC/MCAsmInfo.h"
 #include "llvm/MC/MCContext.h"
@@ -380,6 +381,19 @@ void MCObjectFileInfo::initELFMCObjectFileInfo(const Triple &T, bool Large) {
   unsigned EHSectionType = T.getArch() == Triple::x86_64
                                ? ELF::SHT_X86_64_UNWIND
                                : ELF::SHT_PROGBITS;
+  switch (T.getArch()) {
+  case Triple::x86_64:
+    SFrameABIArch = sframe::ABI::AMD64EndianLittle;
+    break;
+  case Triple::aarch64:
+    SFrameABIArch = sframe::ABI::AArch64EndianLittle;
+    break;
+  case Triple::aarch64_be:
+    SFrameABIArch = sframe::ABI::AArch64EndianBig;
+    break;
+  default:
+    break;
+  }
 
   // Solaris requires different flags for .eh_frame to seemingly every other
   // platform.
@@ -537,6 +551,9 @@ void MCObjectFileInfo::initELFMCObjectFileInfo(const Triple &T, bool Large) {
   EHFrameSection =
       Ctx->getELFSection(".eh_frame", EHSectionType, EHSectionFlags);
 
+  SFrameSection =
+      Ctx->getELFSection(".sframe", ELF::SHT_GNU_SFRAME, ELF::SHF_ALLOC);
+
   StackSizesSection = Ctx->getELFSection(".stack_sizes", ELF::SHT_PROGBITS, 0);
 
   PseudoProbeSection = Ctx->getELFSection(".pseudo_probe", DebugSecType, 0);
@@ -1062,6 +1079,7 @@ void MCObjectFileInfo::initMCObjectFileInfo(MCContext &MCCtx, bool PIC,
   CompactUnwindDwarfEHFrameOnly = 0;
 
   EHFrameSection = nullptr;             // Created on demand.
+  SFrameSection = nullptr;              // Created on demand.
   CompactUnwindSection = nullptr;       // Used only by selected targets.
   DwarfAccelNamesSection = nullptr;     // Used only by selected targets.
   DwarfAccelObjCSection = nullptr;      // Used only by selected targets.

llvm/lib/MC/MCObjectStreamer.cpp

@@ -17,6 +17,7 @@
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCObjectFileInfo.h"
 #include "llvm/MC/MCObjectWriter.h"
+#include "llvm/MC/MCSFrame.h"
 #include "llvm/MC/MCSection.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -30,7 +31,7 @@ MCObjectStreamer::MCObjectStreamer(MCContext &Context,
     : MCStreamer(Context),
       Assembler(std::make_unique<MCAssembler>(
           Context, std::move(TAB), std::move(Emitter), std::move(OW))),
-      EmitEHFrame(true), EmitDebugFrame(false) {
+      EmitEHFrame(true), EmitDebugFrame(false), EmitSFrame(false) {
   assert(Assembler->getBackendPtr() && Assembler->getEmitterPtr());
   IsObj = true;
   setAllowAutoPadding(Assembler->getBackend().allowAutoPadding());
@@ -185,6 +186,10 @@ void MCObjectStreamer::emitFrames(MCAsmBackend *MAB) {
 
   if (EmitDebugFrame)
     MCDwarfFrameEmitter::Emit(*this, MAB, false);
+
+  if (EmitSFrame || (getContext().getTargetOptions() &&
+                     getContext().getTargetOptions()->EmitSFrameUnwind))
+    MCSFrameEmitter::Emit(*this);
 }
 
 void MCObjectStreamer::visitUsedSymbol(const MCSymbol &Sym) {

llvm/lib/MC/MCSFrame.cpp

@@ -0,0 +1,98 @@
+//===- lib/MC/MCSFrame.cpp - MCSFrame implementation ----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/MC/MCSFrame.h"
+#include "llvm/BinaryFormat/SFrame.h"
+#include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCObjectFileInfo.h"
+#include "llvm/MC/MCObjectStreamer.h"
+#include "llvm/MC/MCSection.h"
+#include "llvm/MC/MCSymbol.h"
+#include "llvm/Support/EndianStream.h"
+
+using namespace llvm;
+using namespace sframe;
+
+namespace {
+
+// Emitting these field-by-field, instead of constructing the actual structures
+// lets Streamer do target endian-fixups for free.
+
+class SFrameEmitterImpl {
+  MCObjectStreamer &Streamer;
+  ABI SFrameABI;
+  MCSymbol *FDESubSectionStart;
+  MCSymbol *FRESubSectionStart;
+  MCSymbol *FRESubSectionEnd;
+
+public:
+  SFrameEmitterImpl(MCObjectStreamer &Streamer) : Streamer(Streamer) {
+    assert(Streamer.getContext()
+               .getObjectFileInfo()
+               ->getSFrameABIArch()
+               .has_value());
+    SFrameABI = *Streamer.getContext().getObjectFileInfo()->getSFrameABIArch();
+    FDESubSectionStart = Streamer.getContext().createTempSymbol();
+    FRESubSectionStart = Streamer.getContext().createTempSymbol();
+    FRESubSectionEnd = Streamer.getContext().createTempSymbol();
+  }
+
+  void EmitPreamble() {
+    Streamer.emitInt16(Magic);
+    Streamer.emitInt8(static_cast<uint8_t>(Version::V2));
+    Streamer.emitInt8(0);
+  }
+
+  void EmitHeader() {
+    EmitPreamble();
+    // sfh_abi_arch
+    Streamer.emitInt8(static_cast<uint8_t>(SFrameABI));
+    // sfh_cfa_fixed_fp_offset
+    Streamer.emitInt8(0);
+    // sfh_cfa_fixed_ra_offset
+    Streamer.emitInt8(0);
+    // sfh_auxhdr_len
+    Streamer.emitInt8(0);
+    // shf_num_fdes
+    Streamer.emitInt32(0);
+    // shf_num_fres
+    Streamer.emitInt32(0);
+    // shf_fre_len
+    Streamer.emitAbsoluteSymbolDiff(FRESubSectionEnd, FRESubSectionStart,
+                                    sizeof(int32_t));
+    // shf_fdeoff. With no sfh_auxhdr, these immediately follow this header.
+    Streamer.emitInt32(0);
+    // shf_freoff
+    Streamer.emitAbsoluteSymbolDiff(FRESubSectionStart, FDESubSectionStart,
+                                    sizeof(uint32_t));
+  }
+
+  void EmitFDEs() { Streamer.emitLabel(FDESubSectionStart); }
+
+  void EmitFREs() {
+    Streamer.emitLabel(FRESubSectionStart);
+    Streamer.emitLabel(FRESubSectionEnd);
+  }
+};
+
+} // end anonymous namespace
+
+void MCSFrameEmitter::Emit(MCObjectStreamer &Streamer) {
+  MCContext &Context = Streamer.getContext();
+  SFrameEmitterImpl Emitter(Streamer);
+
+  MCSection *Section = Context.getObjectFileInfo()->getSFrameSection();
+  // Not strictly necessary, but gas always aligns to 8, so match that.
+  Section->ensureMinAlignment(Align(8));
+  Streamer.switchSection(Section);
+  MCSymbol *SectionStart = Context.createTempSymbol();
+  Streamer.emitLabel(SectionStart);
+  Emitter.EmitHeader();
+  Emitter.EmitFDEs();
+  Emitter.EmitFREs();
+}

llvm/test/MC/ELF/cfi-sframe.s

@@ -0,0 +1,26 @@
+// TODO: Add other architectures as they gain sframe support
+// REQUIRES: x86-registered-target
+// RUN: llvm-mc --assemble --filetype=obj --gsframe -triple x86_64 %s -o %t.o
+// RUN: llvm-readelf --sframe %t.o | FileCheck %s
+
+.cfi_sections .sframe
+
+f1:
+        .cfi_startproc
+        nop
+        .cfi_endproc
+
+// CHECK: SFrame section '.sframe' {
+// CHECK-NEXT:  Header {
+// CHECK-NEXT:    Magic: 0xDEE2
+// CHECK-NEXT:    Version: V2 (0x2)
+// CHECK-NEXT:    Flags [ (0x0)
+// CHECK:    ABI: AMD64EndianLittle (0x3)
+// CHECK-NEXT:    CFA fixed FP offset (unused): 0
+// CHECK-NEXT:    CFA fixed RA offset: 0
+// CHECK-NEXT:    Auxiliary header length: 0
+// CHECK-NEXT:    Num FDEs: 0
+// CHECK-NEXT:    Num FREs: 0
+// CHECK-NEXT:    FRE subsection length: 0
+// CHECK-NEXT:    FDE subsection offset: 0
+// CHECK-NEXT:    FRE subsection offset: 0