sys_core_fold: Improve optimization of 'not'

Optimize away 'not' in sys_core_fold instead of in beam_block
and beam_dead, as we can do a better job in sys_core_fold.

I modified the test suite temporarily to never turn off Core Erlang
modifications and looked at the coverage. With the new optimizations
active in sys_core_fold, the code in beam_block and beam_dead did not
find a single 'not' that it could optimize.  That proves that the new
optimization is at least as good as the old one. Manually, I could
also verify that the new optimization would optimize some variations
of 'not' that the old one would not handle.

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/compiler/test/andor_SUITE.erl b/lib/compiler/test/andor_SUITE.erl
index 3199440d84..4d7f444c4f 100644
--- a/lib/compiler/test/andor_SUITE.erl
+++ b/lib/compiler/test/andor_SUITE.erl
@@ -370,6 +370,11 @@ combined(Config) when is_list(Config) ->
     ?line true = ?COMB(false, blurf, true),
     ?line true = ?COMB(true, true, blurf),
 
+    false = simple_comb(false, false),
+    false = simple_comb(false, true),
+    false = simple_comb(true, false),
+    true = simple_comb(true, true),
+
     ok.
 -undef(COMB).
 
@@ -396,6 +401,13 @@ comb(A, B, C) ->
 	  end,
     id(Res).
 
+simple_comb(A, B) ->
+    %% Use Res twice, to ensure that a careless optimization of 'not'
+    %% doesn't leave Res as a free variable.
+    Res = A andalso B,
+    _ = id(not Res),
+    Res.
+
 %% Test that a boolean expression in a case expression is properly
 %% optimized (in particular, that the error behaviour is correct).
 in_case(Config) when is_list(Config) ->