Encode address offsets of basic blocks relative to the end of the pre…

…vious basic blocks.

Conceptually, the new encoding emits the offsets and sizes as label differences between each two consecutive basic block begin and end label. When decoding, the offsets must be aggregated along with basic block sizes to calculate the final relative-to-function offsets of basic blocks.

This encoding uses smaller values compared to the existing one (offsets relative to function symbol).
Smaller values tend to occupy fewer bytes in ULEB128 encoding. As a result, we get about 25% reduction
in the size of the bb-address-map section (reduction from about 9MB to 7MB).

Reviewed By: tmsriram, jhenderson

Differential Revision: https://reviews.llvm.org/D106421

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -1152,16 +1152,19 @@ void AsmPrinter::emitBBAddrMapSection(const MachineFunction &MF) {
   OutStreamer->emitSymbolValue(FunctionSymbol, getPointerSize());
   // Emit the total number of basic blocks in this function.
   OutStreamer->emitULEB128IntValue(MF.size());
+  const MCSymbol *PrevMBBEndSymbol = FunctionSymbol;
   // Emit BB Information for each basic block in the funciton.
   for (const MachineBasicBlock &MBB : MF) {
     const MCSymbol *MBBSymbol =
         MBB.isEntryBlock() ? FunctionSymbol : MBB.getSymbol();
-    // Emit the basic block offset.
-    emitLabelDifferenceAsULEB128(MBBSymbol, FunctionSymbol);
+    // Emit the basic block offset relative to the end of the previous block.
+    // This is zero unless the block is padded due to alignment.
+    emitLabelDifferenceAsULEB128(MBBSymbol, PrevMBBEndSymbol);
     // Emit the basic block size. When BBs have alignments, their size cannot
     // always be computed from their offsets.
     emitLabelDifferenceAsULEB128(MBB.getEndSymbol(), MBBSymbol);
     OutStreamer->emitULEB128IntValue(getBBAddrMapMetadata(MBB));
+    PrevMBBEndSymbol = MBB.getEndSymbol();
   }
   OutStreamer->PopSection();
 }

llvm/lib/CodeGen/BasicBlockSections.cpp

@@ -60,7 +60,7 @@
 // Basic Block Labels
 // ==================
 //
-// With -fbasic-block-sections=labels, we emit the offsets of BB addresses of
+// With -fbasic-block-sections=labels, we encode the offsets of BB addresses of
 // every function into the .llvm_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

llvm/test/CodeGen/X86/basic-block-sections-labels.ll

@@ -51,12 +51,12 @@ declare i32 @__gxx_personality_v0(...)
 ; CHECK-NEXT:	.uleb128 .Lfunc_begin0-.Lfunc_begin0
 ; CHECK-NEXT:	.uleb128 .LBB_END0_0-.Lfunc_begin0
 ; CHECK-NEXT:	.byte	8
-; CHECK-NEXT:	.uleb128 .LBB0_1-.Lfunc_begin0
+; CHECK-NEXT:	.uleb128 .LBB0_1-.LBB_END0_0
 ; CHECK-NEXT:	.uleb128 .LBB_END0_1-.LBB0_1
 ; CHECK-NEXT:	.byte	8
-; CHECK-NEXT:	.uleb128 .LBB0_2-.Lfunc_begin0
+; CHECK-NEXT:	.uleb128 .LBB0_2-.LBB_END0_1
 ; CHECK-NEXT:	.uleb128 .LBB_END0_2-.LBB0_2
 ; CHECK-NEXT:	.byte	1
-; CHECK-NEXT:	.uleb128 .LBB0_3-.Lfunc_begin0
+; CHECK-NEXT:	.uleb128 .LBB0_3-.LBB_END0_2
 ; CHECK-NEXT:	.uleb128 .LBB_END0_3-.LBB0_3
 ; CHECK-NEXT:	.byte	5

llvm/test/tools/llvm-readobj/ELF/bb-addr-map.test

@@ -30,7 +30,7 @@
 # LLVM-NEXT:         CanFallThrough: No
 # LLVM-NEXT:       }
 # LLVM-NEXT:       {
-# LLVM-NEXT:         Offset: 0x3
+# LLVM-NEXT:         Offset: 0x4
 # LLVM-NEXT:         Size: 0x4
 # LLVM-NEXT:         HasReturn: Yes
 # LLVM-NEXT:         HasTailCall: No

llvm/tools/llvm-readobj/ELFDumper.cpp

@@ -6962,10 +6962,13 @@ template <class ELFT> void LLVMELFDumper<ELFT>::printBBAddrMaps() {
       W.printString("Name", FuncName);
 
       ListScope L(W, "BB entries");
+      uint32_t FunctionRelativeAddress = 0;
       for (const BBAddrMap::BBEntry &BBE : AM.BBEntries) {
         DictScope L(W);
-        W.printHex("Offset", BBE.Offset);
+        FunctionRelativeAddress += BBE.Offset;
+        W.printHex("Offset", FunctionRelativeAddress);
         W.printHex("Size", BBE.Size);
+        FunctionRelativeAddress += BBE.Size;
         W.printBoolean("HasReturn", BBE.HasReturn);
         W.printBoolean("HasTailCall", BBE.HasTailCall);
         W.printBoolean("IsEHPad", BBE.IsEHPad);