[SystemZ] Recognize mnop-mcount in backend

[jonpa]

Summary

This patch is the backend part corresponding to https://reviews.llvm.org/D67763.

With -pg -mfentry -mnop-mcount, gcc emits 'brcl 0, 0'. Using EmitNop() gives a 'brcl 0, 6' instead - a jump to the next instruction. Is this acceptable?

Is report_fatal_error() the right way to exit with mcount + -mnop-mcount? It seems at least nicer than the stackdump given by llvm_unreachable().

[jonpa]

The backend check for fentry could perhaps be performed in MachineFunction::init() somehow, but doing it fairly early in SystemZDAGToDAG::runOnMachineFunction() seems reasonable, or (we don't have any pre-isel passes that are always run)?

It seems safe to do e.g. F.getFnAttribute("fentry-call").getValueAsString() without a preceding F.hasFnAttribute("fentry-call"), but I am not 100% sure...

[uweigand]

LGTM.

[jonpa]

Thanks for review - r372950.

[MaskRay]

The commit message should look like:

Reviewed By: uweigand
Differential Revision: https://reviews.llvm.org/D67765

Then the differential will be closed automatically when you push the commit to master.

Some people include unrelated things like Tags: #llvm and Subscribers: in their commit messages. I think they are not good practice. Reviewed By+ Differential Revision should be sufficient.

lib/Target/SystemZ/SystemZAsmPrinter.cpp

else {
     OutStreamer.EmitLabel(DotSym);
     return 6;
   }
 }
 
 void SystemZAsmPrinter::LowerFENTRY_CALL(const MachineInstr &MI,
                                          SystemZMCInstLower &Lower) {
   MCContext &Ctx = MF->getContext();
+  if (MF->getFunction().getFnAttribute("mnop-mcount")
+                       .getValueAsString() == "true") {
+    EmitNop(Ctx, *OutStreamer, 6, getSubtargetInfo());
+    return;
+  }
+
   MCSymbol *fentry = Ctx.getOrCreateSymbol("__fentry__");
   const MCSymbolRefExpr *Op =
       MCSymbolRefExpr::create(fentry, MCSymbolRefExpr::VK_PLT, Ctx);
   OutStreamer->EmitInstruction(MCInstBuilder(SystemZ::BRASL)
                        .addReg(SystemZ::R0D).addExpr(Op), getSubtargetInfo());
 }
 
 void SystemZAsmPrinter::LowerSTACKMAP(const MachineInstr &MI) {

lib/Target/SystemZ/SystemZISelDAGToDAG.cpp

class SystemZDAGToDAGISel : public SelectionDAGISel {
   // Try to expand a boolean SELECT_CCMASK using an IPM sequence.
   SDValue expandSelectBoolean(SDNode *Node);
 
 public:
   SystemZDAGToDAGISel(SystemZTargetMachine &TM, CodeGenOpt::Level OptLevel)
       : SelectionDAGISel(TM, OptLevel) {}
 
   bool runOnMachineFunction(MachineFunction &MF) override {
+    const Function &F = MF.getFunction();
+    if (F.getFnAttribute("mnop-mcount").getValueAsString() == "true" &&
+        F.getFnAttribute("fentry-call").getValueAsString() != "true")
+      report_fatal_error("mnop-mcount only supported with fentry-call");
+
     Subtarget = &MF.getSubtarget<SystemZSubtarget>();
     return SelectionDAGISel::runOnMachineFunction(MF);
   }
 
   // Override MachineFunctionPass.
   StringRef getPassName() const override {
     return "SystemZ DAG->DAG Pattern Instruction Selection";
   }

test/CodeGen/SystemZ/mnop-mcount-01.ll

+; RUN: llc %s -mtriple=s390x-linux-gnu -mcpu=z10 -o - -verify-machineinstrs \
+; RUN:   | FileCheck %s
+
+define void @test1() #0 {
+entry:
+  ret void
+
+; CHECK-LABEL: @test1
+; CHECK: brasl %r0, __fentry__
+; CHECK-NOT: brcl 0, .Ltmp0
+; CHECK: br %r14
+}
+
+define void @test2() #1 {
+entry:
+  ret void
+
+; CHECK-LABEL: @test2
+; CHECK-NOT: brasl %r0, __fentry__
+; CHECK: brcl 0, .Ltmp0
+; CHECK: br %r14
+}
+
+attributes #0 = { "fentry-call"="true" }
+attributes #1 = { "fentry-call"="true" "mnop-mcount"="true" }
+

test/CodeGen/SystemZ/mnop-mcount-02.ll

+; RUN: not llc %s -mtriple=s390x-linux-gnu -o - 2>&1 | FileCheck %s
+;
+; CHECK: LLVM ERROR: mnop-mcount only supported with fentry-call
+
+define void @test1() #0 {
+entry:
+  ret void
+}
+
+attributes #0 = { "instrument-function-entry-inlined"="mcount" "mnop-mcount"="true" }
+