Basic Block Sections support in LLVM.

This is the second patch in a series of patches to enable basic block
sections support.

This patch adds support for:

* Creating direct jumps at the end of basic blocks that have fall
through instructions.
* New pass, bbsections-prepare, that analyzes placement of basic blocks
in sections.
* Actual placing of a basic block in a unique section with special
handling of exception handling blocks.
* Supports placing a subset of basic blocks in a unique section.
* Support for MIR serialization and deserialization with basic block
sections.

Parent patch : D68063
Differential Revision: https://reviews.llvm.org/D73674

1 files changed, 65 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/MachineBasicBlock.cpp b/llvm/lib/CodeGen/MachineBasicBlock.cpp
index c2f459e..b273da8 100644
--- a/llvm/lib/CodeGen/MachineBasicBlock.cpp
+++ b/llvm/lib/CodeGen/MachineBasicBlock.cpp
@@ -61,12 +61,31 @@ MCSymbol *MachineBasicBlock::getSymbol() const {
     const MachineFunction *MF = getParent();
     MCContext &Ctx = MF->getContext();
     auto Prefix = Ctx.getAsmInfo()->getPrivateLabelPrefix();
+
+    bool BasicBlockSymbols = MF->hasBBSections() || MF->hasBBLabels();
+    auto Delimiter = BasicBlockSymbols ? "." : "_";
     assert(getNumber() >= 0 && "cannot get label for unreachable MBB");
-    CachedMCSymbol = Ctx.getOrCreateSymbol(Twine(Prefix) + "BB" +
-                                           Twine(MF->getFunctionNumber()) +
-                                           "_" + Twine(getNumber()));
-  }
 
+    // With Basic Block Sections, we emit a symbol for every basic block. To
+    // keep the size of strtab small, we choose a unary encoding which can
+    // compress the symbol names significantly.  The basic blocks for function
+    // foo are named a.BB.foo, aa.BB.foo, and so on.
+    if (BasicBlockSymbols) {
+      auto Iter = MF->getBBSectionsSymbolPrefix().begin();
+      if (getNumber() < 0 ||
+          getNumber() >= (int)MF->getBBSectionsSymbolPrefix().size())
+        report_fatal_error("Unreachable MBB: " + Twine(getNumber()));
+      std::string Prefix(Iter + 1, Iter + getNumber() + 1);
+      std::reverse(Prefix.begin(), Prefix.end());
+      CachedMCSymbol =
+          Ctx.getOrCreateSymbol(Prefix + Twine(Delimiter) + "BB" +
+                                Twine(Delimiter) + Twine(MF->getName()));
+    } else {
+      CachedMCSymbol = Ctx.getOrCreateSymbol(
+          Twine(Prefix) + "BB" + Twine(MF->getFunctionNumber()) +
+          Twine(Delimiter) + Twine(getNumber()));
+    }
+  }
   return CachedMCSymbol;
 }
 
@@ -529,6 +548,48 @@ void MachineBasicBlock::moveAfter(MachineBasicBlock *NewBefore) {
   getParent()->splice(++NewBefore->getIterator(), getIterator());
 }
 
+// Returns true if this basic block and the Other are in the same section.
+bool MachineBasicBlock::sameSection(const MachineBasicBlock *Other) const {
+  if (this == Other)
+    return true;
+
+  if (this->getSectionType() != Other->getSectionType())
+    return false;
+
+  // If either is in a unique section, return false.
+  if (this->getSectionType() == llvm::MachineBasicBlockSection::MBBS_Unique ||
+      Other->getSectionType() == llvm::MachineBasicBlockSection::MBBS_Unique)
+    return false;
+
+  return true;
+}
+
+const MachineBasicBlock *MachineBasicBlock::getSectionEndMBB() const {
+  if (this->isEndSection())
+    return this;
+  auto I = std::next(this->getIterator());
+  const MachineFunction *MF = getParent();
+  while (I != MF->end()) {
+    const MachineBasicBlock &MBB = *I;
+    if (MBB.isEndSection())
+      return &MBB;
+    I = std::next(I);
+  }
+  llvm_unreachable("No End Basic Block for this section.");
+}
+
+// Returns true if this block begins any section.
+bool MachineBasicBlock::isBeginSection() const {
+  return (SectionType == MBBS_Entry || SectionType == MBBS_Unique ||
+          getParent()->isSectionStartMBB(getNumber()));
+}
+
+// Returns true if this block begins any section.
+bool MachineBasicBlock::isEndSection() const {
+  return (SectionType == MBBS_Entry || SectionType == MBBS_Unique ||
+          getParent()->isSectionEndMBB(getNumber()));
+}
+
 void MachineBasicBlock::updateTerminator() {
   const TargetInstrInfo *TII = getParent()->getSubtarget().getInstrInfo();
   // A block with no successors has no concerns with fall-through edges.