codegen: cfunction for inferred union return (JuliaLang#35118)

Regression caused by slight changing to union-return calling convention
to handle pointer gc-tagging.

Fixes JuliaLang#26078

src/codegen.cpp

@@ -4320,6 +4320,7 @@ static Function* gen_cfun_wrapper(
  FunctionType *cft = returninfo.decl->getFunctionType();
  jlfunc_sret = (returninfo.cc == jl_returninfo_t::SRet);
 
+ // TODO: Can use use emit_call_specfun_other here?
  std::vector<Value*> args;
  Value *result;
  if (jlfunc_sret || returninfo.cc == jl_returninfo_t::Union) {
@@ -4391,14 +4392,22 @@ static Function* gen_cfun_wrapper(
  case jl_returninfo_t::SRet:
  retval = mark_julia_slot(result, astrt, NULL, tbaa_stack);
  break;
- case jl_returninfo_t::Union:
- retval = mark_julia_slot(ctx.builder.CreateExtractValue(call, 0),
+ case jl_returninfo_t::Union: {
+ Value *box = ctx.builder.CreateExtractValue(call, 0);
+ Value *tindex = ctx.builder.CreateExtractValue(call, 1);
+ Value *derived = ctx.builder.CreateSelect(
+ ctx.builder.CreateICmpEQ(
+ ctx.builder.CreateAnd(tindex, ConstantInt::get(T_int8, 0x80)),
+ ConstantInt::get(T_int8, 0)),
+ decay_derived(ctx.builder.CreateBitCast(result, T_pjlvalue)),
+ decay_derived(box));
+ retval = mark_julia_slot(derived,
  astrt,
- ctx.builder.CreateExtractValue(call, 1),
+ tindex,
  tbaa_stack);
- // note that the value may not be rooted here (on the return path)
- // XXX: should have a root if we need to emit a typeassert abort
+ retval.Vboxed = box;
  break;
+ }
  case jl_returninfo_t::Ghosts:
  retval = mark_julia_slot(NULL, astrt, call, tbaa_stack);
  break;

test/ccall.jl

@@ -994,6 +994,12 @@ foo13031(x,y,z) = z
 foo13031p = @cfunction(foo13031, Cint, (Ref{Tuple{}}, Ref{Tuple{}}, Cint))
 ccall(foo13031p, Cint, (Ref{Tuple{}},Ref{Tuple{}},Cint), (), (), 8)
 
+# issue 26078
+
+unstable26078(x) = x > 0 ? x : "foo"
+handle26078 = @cfunction(unstable26078, Int32, (Int32,))
+@test ccall(handle26078, Int32, (Int32,), 1) == 1
+
 # issue 17219
 function ccall_reassigned_ptr(ptr::Ptr{Cvoid})
  ptr = Libdl.dlsym(Libdl.dlopen(libccalltest), "test_echo_p")