From e8f770a925db7ed4319944d738472afbc9c6a608 Mon Sep 17 00:00:00 2001
From: Frej Drejhammar <frej.drejhammar@gmail.com>
Date: Wed, 24 Jul 2024 14:48:28 +0200
Subject: [PATCH] compiler alias analysis: Fix bug in status of map extracts

As the destructive update pass does not have any support for patching
literal maps, as it can do for tuples to ensure that the term that
will be destructively updated is on the heap, we must conservatively
consider anything extracted from a literal map as aliased.

This is not a significant limitation as tracking the alias status of
individual associations of non-literal maps is currently considered
non feasible.
---
 lib/compiler/src/beam_ssa_alias.erl                  |  2 ++
 .../test/beam_ssa_check_SUITE_data/alias.erl         | 12 ++++++++++++
 2 files changed, 14 insertions(+)

diff --git a/lib/compiler/src/beam_ssa_alias.erl b/lib/compiler/src/beam_ssa_alias.erl
index 34353feb6fd5..de53ab24aea2 100644
--- a/lib/compiler/src/beam_ssa_alias.erl
+++ b/lib/compiler/src/beam_ssa_alias.erl
@@ -918,6 +918,8 @@ aa_derive_from(Dst, [Parent|Parents], State0) ->
     aa_derive_from(Dst, Parents, aa_derive_from(Dst, Parent, State0));
 aa_derive_from(_Dst, [], State0) ->
     State0;
+aa_derive_from(#b_var{}=Dst, #b_literal{val=Val}, State) when is_map(Val) ->
+    aa_set_aliased(Dst, State);
 aa_derive_from(#b_var{}, #b_literal{}, State) ->
     State;
 aa_derive_from(#b_var{}=Dst, #b_var{}=Parent, State) ->
diff --git a/lib/compiler/test/beam_ssa_check_SUITE_data/alias.erl b/lib/compiler/test/beam_ssa_check_SUITE_data/alias.erl
index 762fb243b23d..08cdf68a35c5 100644
--- a/lib/compiler/test/beam_ssa_check_SUITE_data/alias.erl
+++ b/lib/compiler/test/beam_ssa_check_SUITE_data/alias.erl
@@ -60,6 +60,7 @@
 	 not_transformable3/1,
 	 not_transformable4/1,
 	 not_transformable5/1,
+	 not_transformable6/0,
 
          bad_get_status_by_type/0,
          stacktrace0/0,
@@ -607,6 +608,17 @@ not_transformable5b([H|T], Acc) ->
 not_transformable5b([], Acc) ->
     Acc.
 
+%% Check that anything extracted from a map is aliased. This is
+%% required as otherwise the destructive update pass could try to
+%% update a literal.
+not_transformable6() ->
+%ssa% () when post_ssa_opt ->
+%ssa% E = get_map_element(...),
+%ssa% _ = bs_create_bin(append, _, E, ...) { aliased => [E], first_fragment_dies => true }.
+    M = #{a=> <<>>},
+    #{a:=X} = M,
+    <<X/binary, 17:8>>.
+
 %% Reproducer for a bug in beam_ssa_alias:aa_get_status_by_type/2
 %% where it would return the wrong alias/uniqe status for certain
 %% combinations of returned types.