[BOLT] Offset LPStart to avoid unnecessary instructions (#116713)

For C++ exception handling, when we write a call site table, we must
avoid emitting 0-value offsets for landing pads unless the call site has
no landing pad. However, 0 can be a real offset from the start of the
FDE if the FDE corresponds to a function fragment that starts with a
landing pad. In such cases, we used to emit a trap instruction at the
start of the fragment to guarantee non-zero LP offset.

To avoid emitting unnecessary trap instructions, we can instead set
LPStart to an offset from the FDE. If we emit it as [FDEStart - 1], then
all real offsets from LPStart in FDE become non-negative.

Author: maksfb

bolt/lib/Core/BinaryEmitter.cpp

@@ -416,17 +416,6 @@ void BinaryEmitter::emitFunctionBody(BinaryFunction &BF, FunctionFragment &FF,
     BF.duplicateConstantIslands();
   }
 
-  if (!FF.empty() && FF.front()->isLandingPad()) {
-    assert(!FF.front()->isEntryPoint() &&
-           "Landing pad cannot be entry point of function");
-    // If the first block of the fragment is a landing pad, it's offset from the
-    // start of the area that the corresponding LSDA describes is zero. In this
-    // case, the call site entries in that LSDA have 0 as offset to the landing
-    // pad, which the runtime interprets as "no handler". To prevent this,
-    // insert some padding.
-    Streamer.emitBytes(BC.MIB->getTrapFillValue());
-  }
-
   // Track the first emitted instruction with debug info.
   bool FirstInstr = true;
   for (BinaryBasicBlock *const BB : FF) {
@@ -926,39 +915,54 @@ void BinaryEmitter::emitLSDA(BinaryFunction &BF, const FunctionFragment &FF) {
   // Emit the LSDA header.
 
   // If LPStart is omitted, then the start of the FDE is used as a base for
-  // landing pad displacements. Then if a cold fragment starts with
-  // a landing pad, this means that the first landing pad offset will be 0.
-  // As a result, the exception handling runtime will ignore this landing pad
-  // because zero offset denotes the absence of a landing pad.
-  // For this reason, when the binary has fixed starting address we emit LPStart
-  // as 0 and output the absolute value of the landing pad in the table.
+  // landing pad displacements. Then, if a cold fragment starts with a landing
+  // pad, this means that the first landing pad offset will be 0. However, C++
+  // runtime treats 0 as if there is no landing pad present, thus we *must* emit
+  // non-zero offsets for all valid LPs.
   //
-  // If the base address can change, we cannot use absolute addresses for
-  // landing pads (at least not without runtime relocations). Hence, we fall
-  // back to emitting landing pads relative to the FDE start.
-  // As we are emitting label differences, we have to guarantee both labels are
-  // defined in the same section and hence cannot place the landing pad into a
-  // cold fragment when the corresponding call site is in the hot fragment.
-  // Because of this issue and the previously described issue of possible
-  // zero-offset landing pad we have to place landing pads in the same section
-  // as the corresponding invokes for shared objects.
+  // As a solution, for fixed-address binaries we set LPStart to 0, and for
+  // position-independent binaries we set LP start to FDE start minus one byte
+  // for FDEs that start with a landing pad.
+  const bool NeedsLPAdjustment = !FF.empty() && FF.front()->isLandingPad();
   std::function<void(const MCSymbol *)> emitLandingPad;
   if (BC.HasFixedLoadAddress) {
     Streamer.emitIntValue(dwarf::DW_EH_PE_udata4, 1); // LPStart format
     Streamer.emitIntValue(0, 4);                      // LPStart
     emitLandingPad = [&](const MCSymbol *LPSymbol) {
-      if (!LPSymbol)
-        Streamer.emitIntValue(0, 4);
-      else
+      if (LPSymbol)
         Streamer.emitSymbolValue(LPSymbol, 4);
+      else
+        Streamer.emitIntValue(0, 4);
     };
   } else {
-    Streamer.emitIntValue(dwarf::DW_EH_PE_omit, 1); // LPStart format
+    if (NeedsLPAdjustment) {
+      // Use relative LPStart format and emit LPStart as [SymbolStart - 1].
+      Streamer.emitIntValue(dwarf::DW_EH_PE_pcrel | dwarf::DW_EH_PE_sdata4, 1);
+      MCSymbol *DotSymbol = BC.Ctx->createTempSymbol("LPBase");
+      Streamer.emitLabel(DotSymbol);
+
+      const MCExpr *LPStartExpr = MCBinaryExpr::createSub(
+          MCSymbolRefExpr::create(StartSymbol, *BC.Ctx),
+          MCSymbolRefExpr::create(DotSymbol, *BC.Ctx), *BC.Ctx);
+      LPStartExpr = MCBinaryExpr::createSub(
+          LPStartExpr, MCConstantExpr::create(1, *BC.Ctx), *BC.Ctx);
+      Streamer.emitValue(LPStartExpr, 4);
+    } else {
+      // DW_EH_PE_omit means FDE start (StartSymbol) will be used as LPStart.
+      Streamer.emitIntValue(dwarf::DW_EH_PE_omit, 1);
+    }
     emitLandingPad = [&](const MCSymbol *LPSymbol) {
-      if (!LPSymbol)
+      if (LPSymbol) {
+        const MCExpr *LPOffsetExpr = MCBinaryExpr::createSub(
+            MCSymbolRefExpr::create(LPSymbol, *BC.Ctx),
+            MCSymbolRefExpr::create(StartSymbol, *BC.Ctx), *BC.Ctx);
+        if (NeedsLPAdjustment)
+          LPOffsetExpr = MCBinaryExpr::createAdd(
+              LPOffsetExpr, MCConstantExpr::create(1, *BC.Ctx), *BC.Ctx);
+        Streamer.emitValue(LPOffsetExpr, 4);
+      } else {
         Streamer.emitIntValue(0, 4);
-      else
-        Streamer.emitAbsoluteSymbolDiff(LPSymbol, StartSymbol, 4);
+      }
     };
   }