Handle union of immutables in codegen of GC.@preserve (#39520)

(cherry picked from commit fb0287f)

Author: maleadt

src/codegen.cpp

@@ -4542,7 +4542,7 @@ static jl_cgval_t emit_expr(jl_codectx_t &ctx, jl_value_t *expr, ssize_t ssaval)
             if (ai.isboxed) {
                 vals.push_back(ai.Vboxed);
             }
-            else if (!jl_is_pointerfree(ai.typ)) {
+            else if (jl_is_concrete_immutable(ai.typ) && !jl_is_pointerfree(ai.typ)) {
                 Type *at = julia_type_to_llvm(ctx, ai.typ);
                 vals.push_back(emit_unbox(ctx, at, ai, ai.typ));
             }

test/compiler/codegen.jl

@@ -522,3 +522,32 @@ function f39232(a)
     return z
 end
 @test f39232((+, -)) == Any[+, -]
+
+@testset "GC.@preserve" begin
+    # main use case
+    function f1(cond)
+        val = [1]
+        GC.@preserve val begin end
+    end
+    @test occursin("llvm.julia.gc_preserve_begin", get_llvm(f1, Tuple{Bool}, true, false, false))
+
+    # stack allocated objects (JuliaLang/julia#34241)
+    function f3(cond)
+        val = ([1],)
+        GC.@preserve val begin end
+    end
+    @test occursin("llvm.julia.gc_preserve_begin", get_llvm(f3, Tuple{Bool}, true, false, false))
+
+    # unions of immutables (JuliaLang/julia#39501)
+    function f2(cond)
+        val = cond ? 1 : 1f0
+        GC.@preserve val begin end
+    end
+    @test !occursin("llvm.julia.gc_preserve_begin", get_llvm(f2, Tuple{Bool}, true, false, false))
+    # make sure the fix for the above doesn't regress #34241
+    function f4(cond)
+        val = cond ? ([1],) : ([1f0],)
+        GC.@preserve val begin end
+    end
+    @test occursin("llvm.julia.gc_preserve_begin", get_llvm(f4, Tuple{Bool}, true, false, false))
+end