[Remarks] Emit a section containing remark diagnostics metadata

A section containing metadata on remark diagnostics will be emitted if
the flag (-mllvm) -remarks-section is present.

For now, the metadata is:

* a magic number for remarks: "REMARKS\0"
* the version number: a little-endian uint64_t
* the absolute file path to the serialized remark diagnostics: a
  null-terminated string.

Differential Revision: https://reviews.llvm.org/D59571

llvm-svn: 357043

llvm/docs/CodeGenerator.rst

@@ -1589,6 +1589,17 @@ and stack sizes (unsigned LEB128). The stack size values only include the space
 allocated in the function prologue. Functions with dynamic stack allocations are
 not included.
 
+Emitting remark diagnostics in the object file
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A section containing metadata on remark diagnostics will be emitted when
+-remarks-section is passed. The section contains:
+
+* a magic number: "REMARKS\0"
+* the version number: a little-endian uint64_t
+* the absolute file path to the serialized remark diagnostics: a
+  null-terminated string.
+
 VLIW Packetizer
 ---------------
 

llvm/docs/CommandGuide/llc.rst

@@ -140,6 +140,10 @@ End-user Options
  in the function prologue. Functions with dynamic stack allocations are not
  included.
 
+.. option:: -remarks-section
+
+ Emit the .remarks (ELF) / __remarks (MachO) section which contains metadata
+ about remark diagnostics.
 
 Tuning/Configuration Options
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

llvm/include/llvm/CodeGen/AsmPrinter.h

@@ -315,6 +315,8 @@ class AsmPrinter : public MachineFunctionPass {
 
   void emitStackSizeSection(const MachineFunction &MF);
 
+  void emitRemarksSection(Module &M);
+
   enum CFIMoveType { CFI_M_None, CFI_M_EH, CFI_M_Debug };
   CFIMoveType needsCFIMoves() const;
 

llvm/include/llvm/MC/MCObjectFileInfo.h

@@ -159,6 +159,9 @@ class MCObjectFileInfo {
   /// FaultMap section.
   MCSection *FaultMapSection;
 
+  /// Remarks section.
+  MCSection *RemarksSection;
+
   /// EH frame section.
   ///
   /// It is initialized on demand so it can be overwritten (with uniquing).
@@ -314,6 +317,7 @@ class MCObjectFileInfo {
 
   MCSection *getStackMapSection() const { return StackMapSection; }
   MCSection *getFaultMapSection() const { return FaultMapSection; }
+  MCSection *getRemarksSection() const { return RemarksSection; }
 
   MCSection *getStackSizesSection(const MCSection &TextSec) const;
 

llvm/include/llvm/Remarks/Remark.h

@@ -23,6 +23,9 @@
 namespace llvm {
 namespace remarks {
 
+constexpr uint64_t Version = 0;
+constexpr StringRef Magic("REMARKS", 7);
+
 /// The debug location used to track a remark back to the source file.
 struct RemarkLocation {
   /// Absolute path of the source file corresponding to this remark.

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -78,6 +78,7 @@
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
+#include "llvm/IR/RemarkStreamer.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Value.h"
 #include "llvm/MC/MCAsmInfo.h"
@@ -99,6 +100,7 @@
 #include "llvm/MC/MCValue.h"
 #include "llvm/MC/SectionKind.h"
 #include "llvm/Pass.h"
+#include "llvm/Remarks/Remark.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
@@ -141,6 +143,11 @@ static const char *const CodeViewLineTablesGroupDescription =
 
 STATISTIC(EmittedInsts, "Number of machine instrs printed");
 
+static cl::opt<bool> EnableRemarksSection(
+    "remarks-section",
+    cl::desc("Emit a section containing remark diagnostics metadata"),
+    cl::init(false));
+
 char AsmPrinter::ID = 0;
 
 using gcp_map_type = DenseMap<GCStrategy *, std::unique_ptr<GCMetadataPrinter>>;
@@ -1331,6 +1338,38 @@ void AsmPrinter::emitGlobalIndirectSymbol(Module &M,
   }
 }
 
+void AsmPrinter::emitRemarksSection(Module &M) {
+  RemarkStreamer *RS = M.getContext().getRemarkStreamer();
+  if (!RS)
+    return;
+
+  // Switch to the right section: .remarks/__remarks.
+  MCSection *RemarksSection =
+      OutContext.getObjectFileInfo()->getRemarksSection();
+  OutStreamer->SwitchSection(RemarksSection);
+
+  // Emit the magic number.
+  OutStreamer->EmitBytes(remarks::Magic);
+  // Explicitly emit a '\0'.
+  OutStreamer->EmitIntValue(/*Value=*/0, /*Size=*/1);
+
+  // Emit the version number: little-endian uint64_t.
+  // The version number is located at the offset 0x0 in the section.
+  std::array<char, 8> Version;
+  support::endian::write64le(Version.data(), remarks::Version);
+  OutStreamer->EmitBinaryData(StringRef(Version.data(), Version.size()));
+
+  // Emit the null-terminated absolute path to the remark file.
+  // The path is located at the offset 0x4 in the section.
+  StringRef FilenameRef = RS->getFilename();
+  SmallString<128> Filename = FilenameRef;
+  sys::fs::make_absolute(Filename);
+  assert(!Filename.empty() && "The filename can't be empty.");
+  OutStreamer->EmitBytes(Filename);
+  // Explicitly emit a '\0'.
+  OutStreamer->EmitIntValue(/*Value=*/0, /*Size=*/1);
+}
+
 bool AsmPrinter::doFinalization(Module &M) {
   // Set the MachineFunction to nullptr so that we can catch attempted
   // accesses to MF specific features at the module level and so that
@@ -1362,6 +1401,12 @@ bool AsmPrinter::doFinalization(Module &M) {
     EmitVisibility(Name, V, false);
   }
 
+  // Emit the remarks section contents.
+  // FIXME: Figure out when is the safest time to emit this section. It should
+  // not come after debug info.
+  if (EnableRemarksSection)
+    emitRemarksSection(M);
+
   const TargetLoweringObjectFile &TLOF = getObjFileLowering();
 
   TLOF.emitModuleMetadata(*OutStreamer, M);

llvm/lib/MC/MCObjectFileInfo.cpp

@@ -290,6 +290,9 @@ void MCObjectFileInfo::initMachOMCObjectFileInfo(const Triple &T) {
   FaultMapSection = Ctx->getMachOSection("__LLVM_FAULTMAPS", "__llvm_faultmaps",
                                          0, SectionKind::getMetadata());
 
+  RemarksSection = Ctx->getMachOSection(
+      "__LLVM", "__remarks", MachO::S_ATTR_DEBUG, SectionKind::getMetadata());
+
   TLSExtraDataSection = TLSTLVSection;
 }
 
@@ -475,6 +478,9 @@ void MCObjectFileInfo::initELFMCObjectFileInfo(const Triple &T, bool Large) {
       Ctx->getELFSection(".eh_frame", EHSectionType, EHSectionFlags);
 
   StackSizesSection = Ctx->getELFSection(".stack_sizes", ELF::SHT_PROGBITS, 0);
+
+  RemarksSection =
+      Ctx->getELFSection(".remarks", ELF::SHT_PROGBITS, ELF::SHF_EXCLUDE);
 }
 
 void MCObjectFileInfo::initCOFFMCObjectFileInfo(const Triple &T) {

llvm/test/CodeGen/X86/remarks-section.ll

@@ -0,0 +1,33 @@
+; RUN: llc < %s -mtriple=x86_64-linux -remarks-section -pass-remarks-output=%/t.yaml | FileCheck -DPATH=%/t.yaml %s
+; RUN: llc < %s -mtriple=x86_64-darwin -remarks-section -pass-remarks-output=%/t.yaml | FileCheck --check-prefix=CHECK-DARWIN -DPATH=%/t.yaml %s
+
+; CHECK-LABEL: func1:
+
+; CHECK: .section .remarks,"e",@progbits
+; The magic number:
+; CHECK-NEXT: .ascii "REMARKS"
+; Null-terminator:
+; CHECK-NEXT: .byte 0
+; The version:
+; CHECK-NEXT: .byte 0x00, 0x00, 0x00, 0x00
+; CHECK-NEXT: .byte 0x00, 0x00, 0x00, 0x00
+; The remark file path:
+; CHECK-NEXT: .ascii "[[PATH]]"
+; Null-terminator:
+; CHECK-NEXT: .byte 0
+
+; CHECK-DARWIN: .section __LLVM,__remarks,regular,debug
+; The magic number:
+; CHECK-DARWIN-NEXT: .ascii "REMARKS"
+; Null-terminator:
+; CHECK-DARWIN-NEXT: .byte 0
+; The version:
+; CHECK-DARWIN-NEXT: .byte 0x00, 0x00, 0x00, 0x00
+; CHECK-DARWIN-NEXT: .byte 0x00, 0x00, 0x00, 0x00
+; The remark file path:
+; CHECK-DARWIN-NEXT: .ascii "[[PATH]]"
+; Null-terminator:
+; CHECK-DARWIN-NEXT: .byte 0
+define void @func1() {
+  ret void
+}