From 57e2ac9da97747b574180c4846998cb0b1411bd7 Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Mon, 5 Oct 2020 21:26:21 -0400
Subject: [PATCH] fix #37872, avoid cycles in codegen for `===`

---
 src/codegen.cpp          | 12 ++++++++++--
 test/compiler/codegen.jl |  5 +++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/src/codegen.cpp b/src/codegen.cpp
index be9aaf075858f..1fc71a923f1a2 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -2500,8 +2500,16 @@ static Value *emit_bits_compare(jl_codectx_t &ctx, jl_cgval_t arg1, jl_cgval_t a
                 Value *nullcheck2 = nullptr;
                 auto fld1 = emit_getfield_knownidx(ctx, arg1, i, sty, &nullcheck1);
                 auto fld2 = emit_getfield_knownidx(ctx, arg2, i, sty, &nullcheck2);
-                answer = ctx.builder.CreateAnd(answer, emit_f_is(ctx, fld1, fld2,
-                                                                 nullcheck1, nullcheck2));
+                Value *fld_answer;
+                if (jl_field_isptr(sty, i) && jl_is_concrete_immutable(fldty)) {
+                    // concrete immutables that are !isinlinealloc might be reference cycles
+                    // issue #37872
+                    fld_answer = emit_box_compare(ctx, fld1, fld2, nullcheck1, nullcheck2);
+                }
+                else {
+                    fld_answer = emit_f_is(ctx, fld1, fld2, nullcheck1, nullcheck2);
+                }
+                answer = ctx.builder.CreateAnd(answer, fld_answer);
             }
             return answer;
         }
diff --git a/test/compiler/codegen.jl b/test/compiler/codegen.jl
index 5f131f3e80e48..2ccffc40b2f16 100644
--- a/test/compiler/codegen.jl
+++ b/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