[AsmPrinter] Print label if MBB's address is taken

[void]

Summary

Make sure that we emit a label for the MBB if its address is taken.

[nickdesaulniers]

lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

438

Make MBB a non-const MachineBasicBlock&, then you can drop the ugly const_cast and then also the & then -> and just use ..

for (MachineBasicBlock& MBB : *MF) {
  ...
  MBB.setLabelMustBeEmitted();

[void]

lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

438

The MF is coming from MI which itself is const. There's probably going to be a const_cast somewhere along the line. :-(

[nickdesaulniers]

Sounds like this may address: https://bugs.llvm.org/show_bug.cgi?id=42309#c1?

lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

438

Ok, is the &+-> still really necessary?

const_cast<MachineBasicBlock>(MBB).setLabelMustBeEmitted();

(I still feel like const_cast is a code smell; if we modify something, then our function signature should really not accept a const param. const parameters are meant to be an agreement between caller and callee that callee doesn't modify the param).

[void]

Cast to reference.

[void]

lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

438

I can cast it to a reference.

I don't like const_casts either. We would need to calculate that it should be set long before getting here.

lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

case '$': {
           if (Modifier[0] == 'l') { // Labels are target independent.
             if (MI->getOperand(OpNo).isBlockAddress()) {
               const BlockAddress *BA = MI->getOperand(OpNo).getBlockAddress();
               const BasicBlock *BB = BA->getBasicBlock();
               const MachineFunction *MF = MI->getParent()->getParent();
               for (const MachineBasicBlock &MBB : *MF)
                 if (BB == MBB.getBasicBlock()) {
                   MBB.getSymbol()->print(OS, AP->MAI);
+                  const_cast<MachineBasicBlock&>(MBB).setLabelMustBeEmitted();
                   break;
                 }
             } else if (MI->getOperand(OpNo).isMBB()) {
               const MCSymbol *Sym = MI->getOperand(OpNo).getMBB()->getSymbol();
               Sym->print(OS, AP->MAI);
             } else {
               Error = true;
             }

test/CodeGen/X86/callbr-asm-label-addr.ll

+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+; CHECK-LABEL: test1:
+; CHECK:       .quad .LBB0_2
+; CHECK-NEXT:  .quad .LBB0_1
+; CHECK-LABEL: .LBB0_1:
+; CHECK-LABEL: .LBB0_2:
+
+define i32 @test1(i32 %x) {
+entry:
+  callbr void asm sideeffect ".quad ${0:l}\0A\09.quad ${1:l}", "i,X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@test1, %baz), i8* blockaddress(@test1, %bar))
+          to label %asm.fallthrough [label %bar]
+
+asm.fallthrough:
+  br label %bar
+
+bar:
+  %call = tail call i32 @foo(i32 %x) #2
+  br label %baz
+
+baz:
+  %call1 = tail call i32 @mux(i32 %call) #2
+  ret i32 %call1
+}
+
+declare dso_local i32 @foo(i32) local_unnamed_addr #1
+
+declare dso_local i32 @mux(i32) local_unnamed_addr #1

test/CodeGen/X86/callbr-asm.ll

 ; CHECK-NEXT:    xorl %eax, %eax
 ; CHECK-NEXT:    jmp .LBB0_2
 ; CHECK-NEXT:    #NO_APP
 ; CHECK-NEXT:  .LBB0_1: # %normal
 ; CHECK-NEXT:    xorl %eax, %eax
 ; CHECK-NEXT:    retl
 ; CHECK-NEXT:  .Ltmp0: # Block address taken
 ; CHECK-NEXT:  .LBB0_2: # %fail
+; CHECK-NEXT:  # Label of block must be emitted
 ; CHECK-NEXT:    movl $1, %eax
 ; CHECK-NEXT:    retl
 entry:
   %0 = add i32 %a, 4
   callbr void asm "xorl $0, $0; jmp ${1:l}", "r,X,~{dirflag},~{fpsr},~{flags}"(i32 %0, i8* blockaddress(@test1, %fail)) to label %normal [label %fail]
 
 normal:
   ret i32 0