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, 0 insertions, 27 deletions

diff --git a/llvm/lib/CodeGen/MachineFunction.cpp b/llvm/lib/CodeGen/MachineFunction.cpp
index 0950d64..e4473fd 100644
--- a/llvm/lib/CodeGen/MachineFunction.cpp
+++ b/llvm/lib/CodeGen/MachineFunction.cpp
@@ -341,33 +341,6 @@ void MachineFunction::RenumberBlocks(MachineBasicBlock *MBB) {
   MBBNumbering.resize(BlockNo);
 }
 
-/// This is used with -fbasic-block-sections or -fbasicblock-labels option.
-/// A unary encoding of basic block labels is done to keep ".strtab" sizes
-/// small.
-void MachineFunction::createBBLabels() {
-  const TargetInstrInfo *TII = getSubtarget().getInstrInfo();
-  this->BBSectionsSymbolPrefix.resize(getNumBlockIDs(), 'a');
-  for (auto MBBI = begin(), E = end(); MBBI != E; ++MBBI) {
-    assert(
-        (MBBI->getNumber() >= 0 && MBBI->getNumber() < (int)getNumBlockIDs()) &&
-        "BasicBlock number was out of range!");
-    // 'a' - Normal block.
-    // 'r' - Return block.
-    // 'l' - Landing Pad.
-    // 'L' - Return and landing pad.
-    bool isEHPad = MBBI->isEHPad();
-    bool isRetBlock = MBBI->isReturnBlock() && !TII->isTailCall(MBBI->back());
-    char type = 'a';
-    if (isEHPad && isRetBlock)
-      type = 'L';
-    else if (isEHPad)
-      type = 'l';
-    else if (isRetBlock)
-      type = 'r';
-    BBSectionsSymbolPrefix[MBBI->getNumber()] = type;
-  }
-}
-
 /// This method iterates over the basic blocks and assigns their IsBeginSection
 /// and IsEndSection fields. This must be called after MBB layout is finalized
 /// and the SectionID's are assigned to MBBs.