Updated Extensions.rst.

Author: rlavaee

llvm/docs/Extensions.rst

@@ -405,31 +405,72 @@ This section is emitted with ``-basic-block-address-map`` and will contain
 a BB address map table for every function.
 
 The ``SHT_LLVM_BB_ADDR_MAP`` type provides backward compatibility to allow
-reading older versions of the BB address map generated by older compilers. Each
-function entry starts with a version byte which specifies the encoding version
-to use. The following versioning schemes are currently supported.
+reading older versions of the BB address map generated by older compilers (up to
+two years old). Each function entry starts with a version byte which specifies
+the encoding version to use. This follows by a feature byte which specifies the
+features specific to this particular entry. The function base address is stored
+as a full address. Other addresses in the entry (block begin and end addresses
+and callsite addresses) are stored in a running-offset fashion, as offset
+relative to the prior address.
 
-Version 1 (newest): basic block address offsets are computed relative to the end
-of previous blocks.
+The following versioning schemes are currently supported (newer versions support
+features of the older versions).
+
+Version 3 (newest): Capable of encoding callsite offsets. Enabled by the 6th bit
+of the feature byte.
 
 Example:
 
 .. code-block:: gas
 
   .section  ".llvm_bb_addr_map","",@llvm_bb_addr_map
-  .byte     1                             # version number
-  .byte     0                             # feature byte (reserved for future use)
+  .byte     3                             # version number
+  .byte     32                            # feature byte
   .quad     .Lfunc_begin0                 # address of the function
   .byte     2                             # number of basic blocks
   # BB record for BB_0
-   .uleb128  .Lfunc_beign0-.Lfunc_begin0  # BB_0 offset relative to function entry (always zero)
+   .byte     0                            # BB_0 ID
+   .uleb128  .Lfunc_begin0-.Lfunc_begin0  # BB_0 offset relative to function entry (always zero)
+   .byte     0                            # number of callsites in this block
    .uleb128  .LBB_END0_0-.Lfunc_begin0    # BB_0 size
    .byte     x                            # BB_0 metadata
   # BB record for BB_1
+   .byte     1                            # BB_1 ID
    .uleb128  .LBB0_1-.LBB_END0_0          # BB_1 offset relative to the end of last block (BB_0).
-   .uleb128  .LBB_END0_1-.LBB0_1          # BB_1 size
+   .byte     2                            # number of callsites in this block
+   .uleb128  .LBB0_1_CS0-.LBB0_1          # offset of callsite relative to the previous offset (.LBB0_1)
+   .uleb128  .LBB0_1_CS1-.LBB0_1_CS0      # offset of callsite relative to the previous offset (.LBB0_1_CS0)
+   .uleb128  .LBB_END0_1-.LBB0_1_CS1      # BB_1 size offset (Offset of the block end relative to the previous offset).
    .byte     y                            # BB_1 metadata
 
+Version 2: Capable of encoding split functions. Enabled by the 4th bit of the
+feature byte. The base address of each split range is stored as a full address.
+The first range corresponds to the function entry.
+
+Example:
+
+.. code-block:: gas
+  .section  ".llvm_bb_addr_map","",@llvm_bb_addr_map
+  .byte     2                             # version number
+  .byte     8                             # feature byte
+  .byte     2                             # number of basic block ranges
+  # 1st BB range (corresponding to the function entry)
+   .quad     .Lfunc_begin0                 # base address
+   .byte     1                             # number of basic blocks in this range
+    # BB record for BB_0
+    .byte     0                            # BB_0 ID
+    .uleb128  .Lfunc_begin0-.Lfunc_begin0  # BB_0 offset relative to function entry (always zero)
+    .uleb128  .LBB_END0_0-.Lfunc_begin0    # BB_0 size
+    .byte     x                            # BB_0 metadata
+  # 2nd BB range
+   .quad     func.part.1
+   .byte     1                             # number of basic blocks in this range
+    # BB record for BB_1
+    .byte     1                            # BB_1 ID
+    .uleb128  func.part.1-func.part.1      # BB_1 offset relative to the range begin (always zero)
+    .uleb128  .LBB_END0_1-func.part.1      # BB_1 size
+    .byte     1                            # BB_1 metadata
+
 PGO Analysis Map
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