[StatepointLowering] Use Constant instead of TargetConstant for undef value (#81635)

Prevents isel errors when trying to lower gc relocate of undef value
(which turns into CopyToReg of TargetConstant). Such relocates may occur
after DCE (e.g. after GVN removes some dead blocks) if there are not
passes like instcombine scheduled after to clean them up.

Fixes #80294

---------

Co-authored-by: Matt Arsenault <[email protected]>

Author: danilaml

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

@@ -1287,7 +1287,7 @@ void SelectionDAGBuilder::visitGCRelocate(const GCRelocateInst &Relocate) {
   if (SD.isUndef() && SD.getValueType().getSizeInBits() <= 64) {
     // Lowering relocate(undef) as arbitrary constant. Current constant value
     // is chosen such that it's unlikely to be a valid pointer.
-    setValue(&Relocate, DAG.getTargetConstant(0xFEFEFEFE, SDLoc(SD), MVT::i64));
+    setValue(&Relocate, DAG.getConstant(0xFEFEFEFE, SDLoc(SD), MVT::i64));
     return;
   }
 

llvm/test/CodeGen/X86/statepoint-relocate-undef.ll

@@ -0,0 +1,32 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -verify-machineinstrs < %s | FileCheck %s
+
+target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+declare void @use(...)
+declare void @f()
+declare token @llvm.experimental.gc.statepoint.p0(i64, i32, ptr, i32, i32, ...)
+declare ptr addrspace(1) @llvm.experimental.gc.relocate.p1(token, i32, i32)
+
+;; Check that llvm doesn't crash if relocate with undef base/derived ptr survives until isel
+define void @test_gcrelocate_undef(ptr addrspace(1) %ptr) gc "statepoint-example" {
+; CHECK-LABEL: test_gcrelocate_undef:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    pushq %rax
+; CHECK-NEXT:    .cfi_def_cfa_offset 16
+; CHECK-NEXT:    movq %rdi, (%rsp)
+; CHECK-NEXT:    callq f@PLT
+; CHECK-NEXT:  .Ltmp0:
+; CHECK-NEXT:    movl $4278124286, %edi # imm = 0xFEFEFEFE
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    callq use@PLT
+; CHECK-NEXT:    popq %rax
+; CHECK-NEXT:    .cfi_def_cfa_offset 8
+; CHECK-NEXT:    retq
+  %tok = tail call token (i64, i32, ptr, i32, i32, ...)
+      @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(void ()) @f, i32 0, i32 0, i32 0, i32 0) ["gc-live" (ptr addrspace(1) %ptr, ptr addrspace(1) undef), "deopt" (ptr addrspace(1) %ptr, i32 undef)]
+  %a = call ptr addrspace(1) @llvm.experimental.gc.relocate.p1(token %tok, i32 0, i32 1)
+  call void (...) @use(ptr addrspace(1) %a)
+  ret void
+}