fix #37872, avoid cycles in codegen for ===

Author: JeffBezanson

src/codegen.cpp

@@ -2564,8 +2564,8 @@ static Value *emit_f_is(jl_codectx_t &ctx, const jl_cgval_t &arg1, const jl_cgva
         (jl_pointer_egal(rt1) || jl_pointer_egal(rt2)))
         return ctx.builder.CreateICmpEQ(boxed(ctx, arg1), boxed(ctx, arg2));
 
-    bool justbits1 = jl_is_concrete_immutable(rt1);
-    bool justbits2 = jl_is_concrete_immutable(rt2);
+    bool justbits1 = jl_is_concrete_immutable(rt1) && ((jl_datatype_t*)rt1)->isinlinealloc;
+    bool justbits2 = jl_is_concrete_immutable(rt2) && ((jl_datatype_t*)rt2)->isinlinealloc;
     if (justbits1 || justbits2) { // whether this type is unique'd by value
         return emit_nullcheck_guard2(ctx, nullcheck1, nullcheck2, [&] () -> Value* {
             jl_value_t *typ = justbits1 ? rt1 : rt2;

test/compiler/codegen.jl

@@ -495,3 +495,8 @@ let x = reinterpret(Has256Bits, [0xfcdac822cac89d82de4f9b3326da8294, 0x6ebac4d59
     @test reinterpret(UInt128, [h(x)]) == lshifted
     @test reinterpret(UInt128, [Base.shl_int(x, 0x8)]) == lshifted
 end
+
+# issue #37872
+let f(@nospecialize(x)) = x===Base.ImmutableDict(Int128=>:big)
+    @test !f(Dict(Int=>Int))
+end