[Systemz/z/OS] Centralize emitting the call type information

[Kai]

Summary

With XPLINK, a no-op with information about the call type is emitted
after each call instruction. Centralizing it has the advantage that it is
easy to document all cases, and it makes it easier to extend it later
(e.g. dynamic stack allocation, 32 bit mode).
Also add a test checking the call types emitted so far.

[uweigand]

See inline comment. Otherwise this LGTM.

llvm/lib/Target/SystemZ/SystemZAsmPrinter.cpp

127

Minor nit: this function could use a comment explaining that it is deliberately emitting a NOP instruction that encodes information into the register used, in accordance with the XPLINK ABI.

llvm/lib/Target/SystemZ/SystemZAsmPrinter.h

private:
   StackMaps SM;
 
   SystemZTargetStreamer *getTargetStreamer() {
     MCTargetStreamer *TS = OutStreamer->getTargetStreamer();
     assert(TS && "do not have a target streamer");
     return static_cast<SystemZTargetStreamer *>(TS);
   }
 
+  /// Call type information for XPLINK.
+  enum class CallType {
+    BASR76 = 0,   // b'x000' == BASR  r7,r6
+    BRAS7 = 1,    // b'x001' == BRAS  r7,ep
+    RESVD_2 = 2,  // b'x010'
+    BRASL7 = 3,   // b'x011' == BRASL r7,ep
+    RESVD_4 = 4,  // b'x100'
+    RESVD_5 = 5,  // b'x101'
+    BALR1415 = 6, // b'x110' == BALR  r14,r15
+    BASR33 = 7,   // b'x111' == BASR  r3,r3
+  };
+
 public:
   SystemZAsmPrinter(TargetMachine &TM, std::unique_ptr<MCStreamer> Streamer)
       : AsmPrinter(TM, std::move(Streamer)), SM(*this) {}
 
   // Override AsmPrinter.
   StringRef getPassName() const override { return "SystemZ Assembly Printer"; }
   void emitInstruction(const MachineInstr *MI) override;
   void emitMachineConstantPoolValue(MachineConstantPoolValue *MCPV) override;
   void emitEndOfAsmFile(Module &M) override;
   bool PrintAsmOperand(const MachineInstr *MI, unsigned OpNo,
                        const char *ExtraCode, raw_ostream &OS) override;
   bool PrintAsmMemoryOperand(const MachineInstr *MI, unsigned OpNo,
                              const char *ExtraCode, raw_ostream &OS) override;
 
   bool doInitialization(Module &M) override {
     SM.reset();
     return AsmPrinter::doInitialization(M);
   }
   void emitFunctionEntryLabel() override;
 
 private:
+  void emitCallInformation(CallType CT);
   void LowerFENTRY_CALL(const MachineInstr &MI, SystemZMCInstLower &MCIL);
   void LowerSTACKMAP(const MachineInstr &MI);
   void LowerPATCHPOINT(const MachineInstr &MI, SystemZMCInstLower &Lower);
 };
 } // end namespace llvm
 
 #endif

llvm/lib/Target/SystemZ/SystemZAsmPrinter.cpp

static MCInst lowerSubvectorStore(const MachineInstr *MI, unsigned Opcode) {
   return MCInstBuilder(Opcode)
     .addReg(SystemZMC::getRegAsVR128(MI->getOperand(0).getReg()))
     .addReg(MI->getOperand(1).getReg())
     .addImm(MI->getOperand(2).getImm())
     .addReg(MI->getOperand(3).getReg())
     .addImm(0);
 }
 
+// The XPLINK ABI requires that a no-op encoding the call type is emitted after
+// each call to a subroutine. This information can be used by the called
+// function to determine its entry point, e.g. for generating a backtrace. The
+// call type is encoded as a register number in the bcr instruction. See
+// enumeration CallType for the possible values.
+void SystemZAsmPrinter::emitCallInformation(CallType CT) {
+  EmitToStreamer(*OutStreamer,
+                 MCInstBuilder(SystemZ::BCRAsm)
+                     .addImm(0)
+                     .addReg(SystemZMC::GR64Regs[static_cast<unsigned>(CT)]));
+}
+
 void SystemZAsmPrinter::emitInstruction(const MachineInstr *MI) {
   SystemZMCInstLower Lower(MF->getContext(), *this);
   MCInst LoweredMI;
   switch (MI->getOpcode()) {
   case SystemZ::Return:
     LoweredMI = MCInstBuilder(SystemZ::BR)
       .addReg(SystemZ::R14D);
     break;
case SystemZ::CLGIBReturn:
     break;
 
   case SystemZ::CallBRASL_XPLINK64:
     EmitToStreamer(*OutStreamer,
                    MCInstBuilder(SystemZ::BRASL)
                        .addReg(SystemZ::R7D)
                        .addExpr(Lower.getExpr(MI->getOperand(0),
                                               MCSymbolRefExpr::VK_PLT)));
-    EmitToStreamer(
-        *OutStreamer,
-        MCInstBuilder(SystemZ::BCRAsm).addImm(0).addReg(SystemZ::R3D));
+    emitCallInformation(CallType::BRASL7);
     return;
 
   case SystemZ::CallBASR_XPLINK64:
     EmitToStreamer(*OutStreamer, MCInstBuilder(SystemZ::BASR)
                                      .addReg(SystemZ::R7D)
                                      .addReg(MI->getOperand(0).getReg()));
-    EmitToStreamer(
-        *OutStreamer,
-        MCInstBuilder(SystemZ::BCRAsm).addImm(0).addReg(SystemZ::R0D));
+    emitCallInformation(CallType::BASR76);
     return;
 
   case SystemZ::CallBRASL:
     LoweredMI = MCInstBuilder(SystemZ::BRASL)
       .addReg(SystemZ::R14D)
       .addExpr(Lower.getExpr(MI->getOperand(0), MCSymbolRefExpr::VK_PLT));
     break;
 

llvm/test/CodeGen/SystemZ/call-zos-02.ll

+; RUN: llc --mtriple=s390x-ibm-zos --show-mc-encoding < %s | FileCheck %s
+
+define internal signext i32 @caller() {
+entry:
+  ret i32 0
+}
+
+
+define hidden signext i32 @caller2() {
+entry:
+; CHECK-LABEL:   caller2:
+; CHECK:         brasl 7, caller@PLT   * encoding: [0xc0,0x75,A,A,A,A]
+; CHECK-NEXT:    * fixup A - offset: 2, value: caller@PLT+2, kind: FK_390_PC32DBL
+; CHECK-NEXT:    bcr     0, 3          * encoding: [0x07,0x03]
+  %call = call signext i32 @caller()
+  ret i32 %call
+}