[SHT_LLVM_BB_ADDR_MAP] Add an option to skip emitting bb entries (#114447)

Sometimes we want to use a `PgoAnalysisMap` feature that doesn't require
the BB entries info, e.g. only the `FuncEntryCount`, but the BB entries
is emitted by default, so I'm adding an option to skip the info for this
case to save the binary size(can save ~90% size of the section). For
implementation, it extends a new field(`OmitBBEntries`) in
`BBAddrMap::Features` for this and it's controlled by a switch
`--basic-block-address-map-skip-bb-entries`.

Note that this naturally supports backwards compatibility as the field
is zero for the old version, matches the decoding in the new version
llvm.

1 files changed, 23 insertions, 21 deletions

diff --git a/llvm/lib/Object/ELF.cpp b/llvm/lib/Object/ELF.cpp
index 545a672..b6d0699 100644
--- a/llvm/lib/Object/ELF.cpp
+++ b/llvm/lib/Object/ELF.cpp
@@ -851,29 +851,31 @@ decodeBBAddrMapImpl(const ELFFile<ELFT> &EF,
         NumBlocksInBBRange = readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr);
       }
       std::vector<BBAddrMap::BBEntry> BBEntries;
-      for (uint32_t BlockIndex = 0; !MetadataDecodeErr && !ULEBSizeErr && Cur &&
-                                    (BlockIndex < NumBlocksInBBRange);
-           ++BlockIndex) {
-        uint32_t ID = Version >= 2
-                          ? readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr)
-                          : BlockIndex;
-        uint32_t Offset = readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr);
-        uint32_t Size = readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr);
-        uint32_t MD = readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr);
-        if (Version >= 1) {
-          // Offset is calculated relative to the end of the previous BB.
-          Offset += PrevBBEndOffset;
-          PrevBBEndOffset = Offset + Size;
-        }
-        Expected<BBAddrMap::BBEntry::Metadata> MetadataOrErr =
-            BBAddrMap::BBEntry::Metadata::decode(MD);
-        if (!MetadataOrErr) {
-          MetadataDecodeErr = MetadataOrErr.takeError();
-          break;
+      if (!FeatEnable.OmitBBEntries) {
+        for (uint32_t BlockIndex = 0; !MetadataDecodeErr && !ULEBSizeErr &&
+                                      Cur && (BlockIndex < NumBlocksInBBRange);
+             ++BlockIndex) {
+          uint32_t ID = Version >= 2
+                            ? readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr)
+                            : BlockIndex;
+          uint32_t Offset = readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr);
+          uint32_t Size = readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr);
+          uint32_t MD = readULEB128As<uint32_t>(Data, Cur, ULEBSizeErr);
+          if (Version >= 1) {
+            // Offset is calculated relative to the end of the previous BB.
+            Offset += PrevBBEndOffset;
+            PrevBBEndOffset = Offset + Size;
+          }
+          Expected<BBAddrMap::BBEntry::Metadata> MetadataOrErr =
+              BBAddrMap::BBEntry::Metadata::decode(MD);
+          if (!MetadataOrErr) {
+            MetadataDecodeErr = MetadataOrErr.takeError();
+            break;
+          }
+          BBEntries.push_back({ID, Offset, Size, *MetadataOrErr});
         }
-        BBEntries.push_back({ID, Offset, Size, *MetadataOrErr});
+        TotalNumBlocks += BBEntries.size();
       }
-      TotalNumBlocks += BBEntries.size();
       BBRangeEntries.push_back({RangeBaseAddress, std::move(BBEntries)});
     }
     FunctionEntries.push_back({std::move(BBRangeEntries)});