Let -basic-block-sections=labels emit basicblock metadata in a new .bb_addr_map section, instead of emitting special unary-encoded symbols.

This patch introduces the new .bb_addr_map section feature which allows us to emit the bits needed for mapping binary profiles to basic blocks into a separate section.
The format of the emitted data is represented as follows. It includes a header for every function:

|  Address of the function                      |  -> 8 bytes (pointer size)
|  Number of basic blocks in this function (>0) |  -> ULEB128

The header is followed by a BB record for every basic block. These records are ordered in the same order as MachineBasicBlocks are placed in the function. Each BB Info is structured as follows:

|  Offset of the basic block relative to function begin |  -> ULEB128
|  Binary size of the basic block                       |  -> ULEB128
|  BB metadata                                          |  -> ULEB128  [ MBB.isReturn() OR MBB.hasTailCall() << 1  OR  MBB.isEHPad() << 2 ]

The new feature will replace the existing "BB labels" functionality with -basic-block-sections=labels.
The .bb_addr_map section scrubs the specially-encoded BB symbols from the binary and makes it friendly to profilers and debuggers.
Furthermore, the new feature reduces the binary size overhead from 70% bloat to only 12%.

For more information and results please refer to the RFC: https://lists.llvm.org/pipermail/llvm-dev/2020-July/143512.html

Reviewed By: MaskRay, snehasish

Differential Revision: https://reviews.llvm.org/D85408

1 files changed, 5 insertions, 21 deletions

diff --git a/llvm/lib/CodeGen/MachineBasicBlock.cpp b/llvm/lib/CodeGen/MachineBasicBlock.cpp
index ebdd17f..b260af7 100644
--- a/llvm/lib/CodeGen/MachineBasicBlock.cpp
+++ b/llvm/lib/CodeGen/MachineBasicBlock.cpp
@@ -60,28 +60,11 @@ MCSymbol *MachineBasicBlock::getSymbol() const {
   if (!CachedMCSymbol) {
     const MachineFunction *MF = getParent();
     MCContext &Ctx = MF->getContext();
-    auto Prefix = Ctx.getAsmInfo()->getPrivateLabelPrefix();
 
-    assert(getNumber() >= 0 && "cannot get label for unreachable MBB");
-
-    // We emit a non-temporary symbol for every basic block if we have BBLabels
-    // or -- with basic block sections -- when a basic block begins a section.
-    // With basic block symbols, we use a unary encoding which can
-    // compress the symbol names significantly. For basic block sections where
-    // this block is the first in a cluster, we use a non-temp descriptive name.
-    // Otherwise we fall back to use temp label.
-    if (MF->hasBBLabels()) {
-      auto Iter = MF->getBBSectionsSymbolPrefix().begin();
-      if (getNumber() < 0 ||
-          getNumber() >= (int)MF->getBBSectionsSymbolPrefix().size())
-        report_fatal_error("Unreachable MBB: " + Twine(getNumber()));
-      // The basic blocks for function foo are named a.BB.foo, aa.BB.foo, and
-      // so on.
-      std::string Prefix(Iter + 1, Iter + getNumber() + 1);
-      std::reverse(Prefix.begin(), Prefix.end());
-      CachedMCSymbol =
-          Ctx.getOrCreateSymbol(Twine(Prefix) + ".BB." + Twine(MF->getName()));
-    } else if (MF->hasBBSections() && isBeginSection()) {
+    // We emit a non-temporary symbol -- with a descriptive name -- if it begins
+    // a section (with basic block sections). Otherwise we fall back to use temp
+    // label.
+    if (MF->hasBBSections() && isBeginSection()) {
       SmallString<5> Suffix;
       if (SectionID == MBBSectionID::ColdSectionID) {
         Suffix += ".cold";
@@ -92,6 +75,7 @@ MCSymbol *MachineBasicBlock::getSymbol() const {
       }
       CachedMCSymbol = Ctx.getOrCreateSymbol(MF->getName() + Suffix);
     } else {
+      const StringRef Prefix = Ctx.getAsmInfo()->getPrivateLabelPrefix();
       CachedMCSymbol = Ctx.getOrCreateSymbol(Twine(Prefix) + "BB" +
                                              Twine(MF->getFunctionNumber()) +
                                              "_" + Twine(getNumber()));