[Statepoint] Remove redundant clear of call target on register

[skatkov]

Summary

Patchable statepoint is lowered into sequence of nops, so zeroed call target
should not be on register. It is better to use getTargetConstant instead
of getConstant to select zero constant for call target.

[reames]

LGTM. Looking through the code, it really looks like the null callee value isn't even really being used by the lowering. It's an operand on the MachineInst and thus gets emitted, but in the nop-padding case it doesn't appear to make into the MC Stream. We could, arguably should, restructure to remove it. Your patch appears to be correct, and a step in the right direction, so go for it.

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

if (ISP.getNumPatchBytes() > 0) {
     // `null` instead.  Not lowering the call target lets statepoint clients get
     // away without providing a physical address for the symbolic call target at
     // link time.
 
     const auto &TLI = DAG.getTargetLoweringInfo();
     const auto &DL = DAG.getDataLayout();
 
     unsigned AS = ISP.getCalledValue()->getType()->getPointerAddressSpace();
-    ActualCallee = DAG.getConstant(0, getCurSDLoc(), TLI.getPointerTy(DL, AS));
+    ActualCallee =
+        DAG.getTargetConstant(0, getCurSDLoc(), TLI.getPointerTy(DL, AS));
   } else {
     ActualCallee = getValue(ISP.getCalledValue());
   }
 
   StatepointLoweringInfo SI(DAG);
   populateCallLoweringInfo(SI.CLI, ISP.getCall(),
                            ImmutableStatepoint::CallArgsBeginPos,
                            ISP.getNumCallArgs(), ActualCallee,

llvm/test/CodeGen/X86/statepoint-call-lowering.ll

entry:
   ret void
 }
 
 define i1 @test_i1_return_patchable() gc "statepoint-example" {
 ; CHECK-LABEL: test_i1_return_patchable:
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    pushq %rax
 ; CHECK-NEXT:    .cfi_def_cfa_offset 16
-; CHECK-NEXT:    xorl %eax, %eax
 ; CHECK-NEXT:    nopl (%rax)
 ; CHECK-NEXT:  .Ltmp7:
 ; CHECK-NEXT:    popq %rcx
 ; CHECK-NEXT:    .cfi_def_cfa_offset 8
 ; CHECK-NEXT:    retq
 ; A patchable variant of test_i1_return
 entry:
   %safepoint_token = tail call token (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 3, i1 ()*null, i32 0, i32 0, i32 0, i32 0)