AMDGPU: Fix return of non-void-returning shaders

[nhaehnle]

Summary

Since "AMDGPU: Fix verifier errors in SILowerControlFlow", the logic that
ensures that a non-void-returning shader falls off the end of the last
basic block was effectively disabled, since SI_RETURN is now used.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96731

[arsenm]

LGTM

lib/Target/AMDGPU/SILowerControlFlow.cpp

for (I = MBB.begin(); I != MBB.end(); I = Next) {
             // block's instructions.
             NextBB = std::next(BI);
             BE = MF.end();
             Next = MBB.end();
           }
 
           break;
 
-        case AMDGPU::S_ENDPGM: {
-          if (MF.getInfo<SIMachineFunctionInfo>()->returnsVoid())
-            break;
+        case AMDGPU::SI_RETURN: {
+          assert(!MF.getInfo<SIMachineFunctionInfo>()->returnsVoid());
 
           // Graphics shaders returning non-void shouldn't contain S_ENDPGM,
           // because external bytecode will be appended at the end.
           if (BI != --MF.end() || I != MBB.getFirstTerminator()) {
-            // S_ENDPGM is not the last instruction. Add an empty block at
+            // SI_RETURN is not the last instruction. Add an empty block at
             // the end and jump there.
             if (!EmptyMBBAtEnd) {
               EmptyMBBAtEnd = MF.CreateMachineBasicBlock();
               MF.insert(MF.end(), EmptyMBBAtEnd);
             }
 
             MBB.addSuccessor(EmptyMBBAtEnd);
             BuildMI(*BI, I, MI.getDebugLoc(), TII->get(AMDGPU::S_BRANCH))
                     .addMBB(EmptyMBBAtEnd);
-          }
-
             I->eraseFromParent();
+          }
           break;
         }
       }
     }
   }
 
   if (NeedFlat && MFI->IsKernel) {
     // TODO: What to use with function calls?
     // We will need to Initialize the flat scratch register pair.
     if (NeedFlat)
       MFI->setHasFlatInstructions(true);
   }
 
   return true;
 }

test/CodeGen/AMDGPU/ret_jump.ll

 ; RUN: llc -march=amdgcn -mcpu=tahiti -verify-machineinstrs < %s | FileCheck -check-prefix=GCN %s
 ; RUN: llc -march=amdgcn -mcpu=tonga -verify-machineinstrs < %s | FileCheck -check-prefix=GCN %s
 
 ; This should end with an no-op sequence of exec mask manipulations
 ; Mask should be in original state after executed unreachable block
 
 ; GCN-LABEL: {{^}}main:
 ; GCN: s_cbranch_vccnz [[RET_BB:BB[0-9]+_[0-9]+]]
 
 ; GCN: s_and_saveexec_b64 [[SAVE_EXEC:s\[[0-9]+:[0-9]+\]]], vcc
 ; GCN-NEXT: s_xor_b64 [[XOR_EXEC:s\[[0-9]+:[0-9]+\]]], exec, [[SAVE_EXEC]]
 ; GCN-NEXT: ; mask branch [[UNREACHABLE_BB:BB[0-9]+_[0-9]+]]
 
 ; GCN: [[RET_BB]]:
-; GCN-NEXT: ; return
+; GCN-NEXT: s_branch [[FINAL_BB:BB[0-9]+_[0-9]+]]
 
 ; GCN-NEXT: [[UNREACHABLE_BB]]:
 ; GCN-NEXT: s_or_b64 exec, exec, [[XOR_EXEC]]
+; GCN-NEXT: [[FINAL_BB]]:
 ; GCN-NEXT: .Lfunc_end0
 define amdgpu_ps <{ i32, i32, i32, i32, i32, i32, i32, i32, i32, float, float, float, float, float, float, float, float, float, float, float, float, float, float }> @main([9 x <16 x i8>] addrspace(2)* byval, [17 x <16 x i8>] addrspace(2)* byval, [17 x <8 x i32>] addrspace(2)* byval, i32 addrspace(2)* byval, float inreg, i32 inreg, <2 x i32>, <2 x i32>, <2 x i32>, <3 x i32>, <2 x i32>, <2 x i32>, <2 x i32>, float, float, float, float, float, i32, i32, float, i32) #0 {
 main_body:
   %p83 = call float @llvm.SI.fs.interp(i32 1, i32 0, i32 %5, <2 x i32> %7)
   %p87 = fmul float undef, %p83
   %p88 = fadd float %p87, undef
   %p93 = fadd float %p88, undef
   %p97 = fmul float %p93, undef