[MC] Aligned bundling: remove special handling for RelaxAll

When both aligned bundling and RelaxAll are enabled, bundle padding is
directly written into fragments (https://reviews.llvm.org/D8072).
(The original motivation was memory usage, which has been achieved from
different angles with recent assembler improvement).

The code presents challenges with the work to replace fragment
representation (e.g. #94950 #95077). This patch removes the special
handling. RelaxAll still works but the behavior seems slightly different
as revealed by 2 changed tests. However, most `-mc-relax-all` tests are
unchanged.

RelaxAll used to be the default for clang -O0. This mode has significant
code size drawbacks and newer Clang doesn't use it (#90013).

---

flushPendingLabels: The FOffset parameter can be removed: pending labels
will be assigned to the incoming fragment at offset 0.

Pull Request: https://github.com/llvm/llvm-project/pull/95188

llvm/include/llvm/MC/MCELFStreamer.h

@@ -39,7 +39,6 @@ public:
   /// state management
   void reset() override {
     SeenIdent = false;
-    BundleGroups.clear();
     MCObjectStreamer::reset();
   }
 
@@ -142,14 +141,7 @@ private:
   void finalizeCGProfileEntry(const MCSymbolRefExpr *&S, uint64_t Offset);
   void finalizeCGProfile();
 
-  /// Merge the content of the fragment \p EF into the fragment \p DF.
-  void mergeFragment(MCDataFragment *, MCDataFragment *);
-
   bool SeenIdent = false;
-
-  /// BundleGroups - The stack of fragments holding the bundle-locked
-  /// instructions.
-  SmallVector<MCDataFragment *, 4> BundleGroups;
 };
 
 MCELFStreamer *createARMELFStreamer(MCContext &Context,

llvm/include/llvm/MC/MCSection.h

@@ -227,8 +227,7 @@ public:
   void addPendingLabel(MCSymbol* label, unsigned Subsection = 0);
 
   /// Associate all pending labels in a subsection with a fragment.
-  void flushPendingLabels(MCFragment *F, uint64_t FOffset = 0,
+  void flushPendingLabels(MCFragment *F, unsigned Subsection);
-			  unsigned Subsection = 0);
 
   /// Associate all pending labels with empty data fragments. One fragment
   /// will be created for each subsection as necessary.

llvm/include/llvm/MC/MCWasmStreamer.h

@@ -73,9 +73,6 @@ private:
 
   void fixSymbolsInTLSFixups(const MCExpr *expr);
 
-  /// Merge the content of the fragment \p EF into the fragment \p DF.
-  void mergeFragment(MCDataFragment *, MCDataFragment *);
-
   bool SeenIdent;
 };
 

llvm/lib/MC/MCAssembler.cpp

@@ -420,12 +420,6 @@ void MCAsmLayout::layoutBundle(MCFragment *F) {
   // The fragment's offset will point to after the padding, and its computed
   // size won't include the padding.
   //
-  // When the -mc-relax-all flag is used, we optimize bundling by writting the
-  // padding directly into fragments when the instructions are emitted inside
-  // the streamer. When the fragment is larger than the bundle size, we need to
-  // ensure that it's bundle aligned. This means that if we end up with
-  // multiple fragments, we must emit bundle padding between fragments.
-  //
   // ".align N" is an example of a directive that introduces multiple
   // fragments. We could add a special case to handle ".align N" by emitting
   // within-fragment padding (which would produce less padding when N is less
@@ -436,7 +430,7 @@ void MCAsmLayout::layoutBundle(MCFragment *F) {
   MCEncodedFragment *EF = cast<MCEncodedFragment>(F);
   uint64_t FSize = Assembler.computeFragmentSize(*this, *EF);
 
-  if (!Assembler.getRelaxAll() && FSize > Assembler.getBundleAlignSize())
+  if (FSize > Assembler.getBundleAlignSize())
     report_fatal_error("Fragment can't be larger than a bundle size");
 
   uint64_t RequiredBundlePadding =

llvm/lib/MC/MCELFStreamer.cpp

@@ -50,44 +50,6 @@ bool MCELFStreamer::isBundleLocked() const {
   return getCurrentSectionOnly()->isBundleLocked();
 }
 
-void MCELFStreamer::mergeFragment(MCDataFragment *DF,
-                                  MCDataFragment *EF) {
-  MCAssembler &Assembler = getAssembler();
-
-  if (Assembler.isBundlingEnabled() && Assembler.getRelaxAll()) {
-    uint64_t FSize = EF->getContents().size();
-
-    if (FSize > Assembler.getBundleAlignSize())
-      report_fatal_error("Fragment can't be larger than a bundle size");
-
-    uint64_t RequiredBundlePadding = computeBundlePadding(
-        Assembler, EF, DF->getContents().size(), FSize);
-
-    if (RequiredBundlePadding > UINT8_MAX)
-      report_fatal_error("Padding cannot exceed 255 bytes");
-
-    if (RequiredBundlePadding > 0) {
-      SmallString<256> Code;
-      raw_svector_ostream VecOS(Code);
-      EF->setBundlePadding(static_cast<uint8_t>(RequiredBundlePadding));
-      Assembler.writeFragmentPadding(VecOS, *EF, FSize);
-
-      DF->getContents().append(Code.begin(), Code.end());
-    }
-  }
-
-  flushPendingLabels(DF, DF->getContents().size());
-
-  for (unsigned i = 0, e = EF->getFixups().size(); i != e; ++i) {
-    EF->getFixups()[i].setOffset(EF->getFixups()[i].getOffset() +
-                                 DF->getContents().size());
-    DF->getFixups().push_back(EF->getFixups()[i]);
-  }
-  if (DF->getSubtargetInfo() == nullptr && EF->getSubtargetInfo())
-    DF->setHasInstructions(*EF->getSubtargetInfo());
-  DF->getContents().append(EF->getContents().begin(), EF->getContents().end());
-}
-
 void MCELFStreamer::initSections(bool NoExecStack, const MCSubtargetInfo &STI) {
   MCContext &Ctx = getContext();
   switchSection(Ctx.getObjectFileInfo()->getTextSection());
@@ -575,24 +537,12 @@ void MCELFStreamer::emitInstToData(const MCInst &Inst,
 
   if (Assembler.isBundlingEnabled()) {
     MCSection &Sec = *getCurrentSectionOnly();
-    if (Assembler.getRelaxAll() && isBundleLocked()) {
+    if (isBundleLocked() && !Sec.isBundleGroupBeforeFirstInst()) {
-      // If the -mc-relax-all flag is used and we are bundle-locked, we re-use
-      // the current bundle group.
-      DF = BundleGroups.back();
-      CheckBundleSubtargets(DF->getSubtargetInfo(), &STI);
-    }
-    else if (Assembler.getRelaxAll() && !isBundleLocked())
-      // When not in a bundle-locked group and the -mc-relax-all flag is used,
-      // we create a new temporary fragment which will be later merged into
-      // the current fragment.
-      DF = getContext().allocFragment<MCDataFragment>();
-    else if (isBundleLocked() && !Sec.isBundleGroupBeforeFirstInst()) {
       // If we are bundle-locked, we re-use the current fragment.
       // The bundle-locking directive ensures this is a new data fragment.
       DF = cast<MCDataFragment>(getCurrentFragment());
       CheckBundleSubtargets(DF->getSubtargetInfo(), &STI);
-    }
+    } else if (!isBundleLocked() && Fixups.size() == 0) {
-    else if (!isBundleLocked() && Fixups.size() == 0) {
       // Optimize memory usage by emitting the instruction to a
       // MCCompactEncodedInstFragment when not in a bundle-locked group and
       // there are no fixups registered.
@@ -632,13 +582,6 @@ void MCELFStreamer::emitInstToData(const MCInst &Inst,
                              getAssembler().getBackend().RelaxFixupKind)
     DF->setLinkerRelaxable();
   DF->getContents().append(Code.begin(), Code.end());
-
-  if (Assembler.isBundlingEnabled() && Assembler.getRelaxAll()) {
-    if (!isBundleLocked()) {
-      mergeFragment(getOrCreateDataFragment(&STI), DF);
-      delete DF;
-    }
-  }
 }
 
 void MCELFStreamer::emitBundleAlignMode(Align Alignment) {
@@ -660,12 +603,6 @@ void MCELFStreamer::emitBundleLock(bool AlignToEnd) {
   if (!isBundleLocked())
     Sec.setBundleGroupBeforeFirstInst(true);
 
-  if (getAssembler().getRelaxAll() && !isBundleLocked()) {
-    // TODO: drop the lock state and set directly in the fragment
-    MCDataFragment *DF = getContext().allocFragment<MCDataFragment>();
-    BundleGroups.push_back(DF);
-  }
-
   Sec.setBundleLockState(AlignToEnd ? MCSection::BundleLockedAlignToEnd
                                     : MCSection::BundleLocked);
 }
@@ -680,27 +617,7 @@ void MCELFStreamer::emitBundleUnlock() {
   else if (Sec.isBundleGroupBeforeFirstInst())
     report_fatal_error("Empty bundle-locked group is forbidden");
 
-  // When the -mc-relax-all flag is used, we emit instructions to fragments
+  Sec.setBundleLockState(MCSection::NotBundleLocked);
-  // stored on a stack. When the bundle unlock is emitted, we pop a fragment
-  // from the stack a merge it to the one below.
-  if (getAssembler().getRelaxAll()) {
-    assert(!BundleGroups.empty() && "There are no bundle groups");
-    MCDataFragment *DF = BundleGroups.back();
-
-    // FIXME: Use BundleGroups to track the lock state instead.
-    Sec.setBundleLockState(MCSection::NotBundleLocked);
-
-    // FIXME: Use more separate fragments for nested groups.
-    if (!isBundleLocked()) {
-      mergeFragment(getOrCreateDataFragment(DF->getSubtargetInfo()), DF);
-      BundleGroups.pop_back();
-      delete DF;
-    }
-
-    if (Sec.getBundleLockState() != MCSection::BundleLockedAlignToEnd)
-      getOrCreateDataFragment()->setAlignToBundleEnd(false);
-  } else
-    Sec.setBundleLockState(MCSection::NotBundleLocked);
 }
 
 void MCELFStreamer::finishImpl() {

llvm/lib/MC/MCObjectStreamer.cpp

@@ -81,7 +81,7 @@ void MCObjectStreamer::flushPendingLabels(MCFragment *F, uint64_t FOffset) {
   }
 
   // Associate the labels with F.
-  CurSection->flushPendingLabels(F, FOffset, CurSubsectionIdx);
+  CurSection->flushPendingLabels(F, CurSubsectionIdx);
 }
 
 void MCObjectStreamer::flushPendingLabels() {
@@ -215,7 +215,7 @@ static bool canReuseDataFragment(const MCDataFragment &F,
   // When bundling is enabled, we don't want to add data to a fragment that
   // already has instructions (see MCELFStreamer::emitInstToData for details)
   if (Assembler.isBundlingEnabled())
-    return Assembler.getRelaxAll();
+    return false;
   // If the subtarget is changed mid fragment we start a new fragment to record
   // the new STI.
   return !STI || F.getSubtargetInfo() == STI;
@@ -292,8 +292,7 @@ void MCObjectStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
   // Otherwise queue the label and set its fragment pointer when we emit the
   // next fragment.
   auto *F = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
-  if (F && !(getAssembler().isBundlingEnabled() &&
+  if (F) {
-             getAssembler().getRelaxAll())) {
     Symbol->setFragment(F);
     Symbol->setOffset(F->getContents().size());
   } else {
@@ -465,9 +464,6 @@ void MCObjectStreamer::emitInstructionImpl(const MCInst &Inst,
 
 void MCObjectStreamer::emitInstToFragment(const MCInst &Inst,
                                           const MCSubtargetInfo &STI) {
-  if (getAssembler().getRelaxAll() && getAssembler().isBundlingEnabled())
-    llvm_unreachable("All instructions should have already been relaxed");
-
   // Always create a new, separate fragment here, because its size can change
   // during relaxation.
   MCRelaxableFragment *IF =

llvm/lib/MC/MCSection.cpp

@@ -82,15 +82,14 @@ void MCSection::addPendingLabel(MCSymbol *label, unsigned Subsection) {
   PendingLabels.push_back(PendingLabel(label, Subsection));
 }
 
-void MCSection::flushPendingLabels(MCFragment *F, uint64_t FOffset,
+void MCSection::flushPendingLabels(MCFragment *F, unsigned Subsection) {
-				   unsigned Subsection) {
   // Set the fragment and fragment offset for all pending symbols in the
   // specified Subsection, and remove those symbols from the pending list.
   for (auto It = PendingLabels.begin(); It != PendingLabels.end(); ++It) {
     PendingLabel& Label = *It;
     if (Label.Subsection == Subsection) {
       Label.Sym->setFragment(F);
-      Label.Sym->setOffset(FOffset);
+      assert(Label.Sym->getOffset() == 0);
       PendingLabels.erase(It--);
     }
   }
@@ -105,7 +104,7 @@ void MCSection::flushPendingLabels() {
     MCFragment *F = new MCDataFragment();
     addFragment(*F);
     F->setParent(this);
-    flushPendingLabels(F, 0, Label.Subsection);
+    flushPendingLabels(F, Label.Subsection);
   }
 }
 

llvm/lib/MC/MCWasmStreamer.cpp

@@ -39,19 +39,6 @@ using namespace llvm;
 
 MCWasmStreamer::~MCWasmStreamer() = default; // anchor.
 
-void MCWasmStreamer::mergeFragment(MCDataFragment *DF, MCDataFragment *EF) {
-  flushPendingLabels(DF, DF->getContents().size());
-
-  for (unsigned I = 0, E = EF->getFixups().size(); I != E; ++I) {
-    EF->getFixups()[I].setOffset(EF->getFixups()[I].getOffset() +
-                                 DF->getContents().size());
-    DF->getFixups().push_back(EF->getFixups()[I]);
-  }
-  if (DF->getSubtargetInfo() == nullptr && EF->getSubtargetInfo())
-    DF->setHasInstructions(*EF->getSubtargetInfo());
-  DF->getContents().append(EF->getContents().begin(), EF->getContents().end());
-}
-
 void MCWasmStreamer::emitLabel(MCSymbol *S, SMLoc Loc) {
   auto *Symbol = cast<MCSymbolWasm>(S);
   MCObjectStreamer::emitLabel(Symbol, Loc);

llvm/test/MC/X86/AlignedBundling/misaligned-bundle-group.s

@@ -3,7 +3,7 @@
 # RUN:   | FileCheck -check-prefix=CHECK -check-prefix=CHECK-OPT %s
 # RUN: llvm-mc -filetype=obj -triple i686-pc-linux-gnu -mcpu=pentiumpro -mc-relax-all %s -o - \
 # RUN:   | llvm-objdump -d --no-show-raw-insn - \
-# RUN:   | FileCheck -check-prefix=CHECK -check-prefix=CHECK-RELAX %s
+# RUN:   | FileCheck --check-prefixes=CHECK,CHECK-OPT %s
 
         .text
 foo:
@@ -13,11 +13,7 @@ foo:
         .bundle_lock align_to_end
 # CHECK:            1:  nopw %cs:(%eax,%eax)
 # CHECK:            10: nopw %cs:(%eax,%eax)
-# CHECK-RELAX:      1a: nop
-# CHECK-RELAX:      20: nopw %cs:(%eax,%eax)
-# CHECK-RELAX:      2a: nopw %cs:(%eax,%eax)
 # CHECK-OPT:        1b: calll 0x1c
-# CHECK-RELAX:      3b: calll 0x3c
         calll   bar # 5 bytes
         .bundle_unlock
         ret         # 1 byte

llvm/test/MC/X86/AlignedBundling/misaligned-bundle.s

@@ -3,7 +3,7 @@
 # RUN:   | FileCheck -check-prefix=CHECK -check-prefix=CHECK-OPT %s
 # RUN: llvm-mc -filetype=obj -triple i686-pc-linux-gnu -mcpu=pentiumpro -mc-relax-all %s -o - \
 # RUN:   | llvm-objdump --no-print-imm-hex -d --no-show-raw-insn - \
-# RUN:   | FileCheck -check-prefix=CHECK -check-prefix=CHECK-RELAX %s
+# RUN:   | FileCheck --check-prefixes=CHECK,CHECK-OPT %s
 
         .text
 foo:
@@ -11,17 +11,12 @@ foo:
         push    %ebp          # 1 byte
         .align  16
 # CHECK:            1:  nopw %cs:(%eax,%eax)
-# CHECK-RELAX:      10: nopw %cs:(%eax,%eax)
-# CHECK-RELAX:      1a: nop
 # CHECK-OPT:        10: movl $1, (%esp)
-# CHECK-RELAX:      20: movl $1, (%esp)
         movl $0x1, (%esp)     # 7 bytes
         movl $0x1, (%esp)     # 7 bytes
 # CHECK-OPT:        1e: nop
         movl $0x2, 0x1(%esp)  # 8 bytes
         movl $0x2, 0x1(%esp)  # 8 bytes
-# CHECK-RELAX:      3e: nop
-# CHECK-RELAX:      40: movl $2, 1(%esp)
         movl $0x2, 0x1(%esp)  # 8 bytes
         movl $0x2, (%esp)     # 7 bytes
 # CHECK-OPT:        3f: nop