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, 15 deletions

diff --git a/llvm/lib/CodeGen/BasicBlockSections.cpp b/llvm/lib/CodeGen/BasicBlockSections.cpp
index a3c3660..421c1d8 100644
--- a/llvm/lib/CodeGen/BasicBlockSections.cpp
+++ b/llvm/lib/CodeGen/BasicBlockSections.cpp
@@ -48,19 +48,11 @@
 // Basic Block Labels
 // ==================
 //
-// With -fbasic-block-sections=labels, or when a basic block is placed in a
-// unique section, it is labelled with a symbol.  This allows easy mapping of
-// virtual addresses from PMU profiles back to the corresponding basic blocks.
-// Since the number of basic blocks is large, the labeling bloats the symbol
-// table sizes and the string table sizes significantly. While the binary size
-// does increase, it does not affect performance as the symbol table is not
-// loaded in memory during run-time. The string table size bloat is kept very
-// minimal using a unary naming scheme that uses string suffix compression. The
-// basic blocks for function foo are named "a.BB.foo", "aa.BB.foo", ... This
-// turns out to be very good for string table sizes and the bloat in the string
-// table size for a very large binary is ~8 %.  The naming also allows using
-// the --symbol-ordering-file option in LLD to arbitrarily reorder the
-// sections.
+// With -fbasic-block-sections=labels, we emit the offsets of BB addresses of
+// every function into a .bb_addr_map section. Along with the function symbols,
+// this allows for mapping of virtual addresses in PMU profiles back to the
+// corresponding basic blocks. This logic is implemented in AsmPrinter. This
+// pass only assigns the BBSectionType of every function to ``labels``.
 //
 //===----------------------------------------------------------------------===//
 
@@ -304,7 +296,6 @@ bool BasicBlockSections::runOnMachineFunction(MachineFunction &MF) {
 
   if (BBSectionsType == BasicBlockSection::Labels) {
     MF.setBBSectionsType(BBSectionsType);
-    MF.createBBLabels();
     return true;
   }
 
@@ -314,7 +305,6 @@ bool BasicBlockSections::runOnMachineFunction(MachineFunction &MF) {
                                    FuncBBClusterInfo))
     return true;
   MF.setBBSectionsType(BBSectionsType);
-  MF.createBBLabels();
   assignSections(MF, FuncBBClusterInfo);
 
   // We make sure that the cluster including the entry basic block precedes all