[PPC][BOLT] Implement convertJmpToTailCall for PowerPC.

BOLT relies on treating tail calls as calls to build correct CFGs and preserve interprocedural analysis. On PowerPC, the compiler often emits plain branches (b, ba, bctr) instead of explicit calls when lowering tail calls. (TCO Tail Call Optimisation).

This patch overrides convertJmpToTailCall in PPCMCPlusBuilder so that uncoditional branches are converted into their link-bit variants (BL, BLA, BCTRL). This way, BOLT can consistently recognise them as calls/tail calls during dissasembly and optimisation, including when skipping PLT (Procedure Linkage Table) stubs.

Author: kostasalv

bolt/lib/Target/PowerPC/PPCMCPlusBuilder.cpp

@@ -98,8 +98,23 @@ const MCSymbol *PPCMCPlusBuilder::getTargetSymbol(const MCInst &Inst,
 }
 
 bool PPCMCPlusBuilder::convertJmpToTailCall(MCInst &Inst) {
-  (void)Inst;
-  return false;
+  switch (Inst.getOpcode()) {
+  // Uncoditional direct branch -> add link bit
+  case PPC::B: // relative
+    Inst.setOpcode(PPC::BL);
+    return true;
+  case PPC::BA: // absolute
+    Inst.setOpcode(PPC::BLA);
+    return true;
+
+  // Indirect branch via CTR -> add link bit
+  case PPC::BCTR:
+    Inst.setOpcode(PPC::BCTRL);
+    return true;
+  // Contitional branches
+  default:
+    return false;
+  }
 }
 
 bool PPCMCPlusBuilder::isTailCall(const MCInst &I) const {