From 53bd4974a1012669fb7db09815fabac01b586c12 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Wed, 21 Dec 2011 06:44:28 +0100
Subject: Eliminate the match_fail primop in v3_kernel and later passes
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In the v3_life pass, it is assumed that a 'match_fail' primop
only occur at the top-level and at the end of a function.
But this code:

do_split_cases(A) ->
    case A of
        x ->
	    Z = dummy1;
        _ ->
	    Z = dummy2,
	    a=b
    end,
    Z.

will be optimized by sys_core_fold to the following code:

'split_cases'/1 =
    fun (_cor0) ->
	let <_cor7,Z> =
	    case _cor0 of
	      <'x'> when 'true' ->
		   < 'dummy1','dummy1' >
	      <_cor6> when 'true' ->
                  %% Here follows a 'match_fail' primop inside
		  %% multiple return values:
		  < primop 'match_fail'({'badmatch','b'}),'dummy2' >
	    end
	in
	    Z

moving the 'match_fail' primop into a "values" construction.

In the future, we would like to get rid of the v3_life pass (it is
there for historical reasons), so in the mean-time we prefer to not
add more code to it by generalizing the handling of 'match_fail'.

Since the 'match_fail' primop can be simulated by erlang:error/{1,2},
the simplest solution is to translate 'match_fail' to a call to
erlang:error/{1,2} in v3_kernel and remove the handling of
'match_fail' in v3_life and v3_codegen.

It is tempting to get rid of 'match_fail' also in the Core Erlang
format, but there are two issues:

- Removing the support for 'match_fail' completely may break tools
  that generate Core Erlang code. We should not do that in a minor
  release.

- There is no easy way to generate a 'function_clause' exception
  that will remain correct if it will be inlined into another
  function. (Calling "erlang:error(function_clause, Args)" is
  fine only if it is not inlined into another function.) A good
  solution probably involves introducing new instructions, which
  is better done in a major release.

Noticed-by: Håkan Matsson
Minimized-test-case-by: Erik Søe Sørensen
---
 lib/compiler/test/compilation_SUITE.erl | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

(limited to 'lib/compiler/test')

diff --git a/lib/compiler/test/compilation_SUITE.erl b/lib/compiler/test/compilation_SUITE.erl
index 1343fbd1c9..1f17664b54 100644
--- a/lib/compiler/test/compilation_SUITE.erl
+++ b/lib/compiler/test/compilation_SUITE.erl
@@ -44,7 +44,7 @@ all() ->
      trycatch_4, opt_crash, otp_5404, otp_5436, otp_5481,
      otp_5553, otp_5632, otp_5714, otp_5872, otp_6121,
      otp_6121a, otp_6121b, otp_7202, otp_7345, on_load,
-     string_table,otp_8949_a,otp_8949_a].
+     string_table,otp_8949_a,otp_8949_a,split_cases].
 
 groups() -> 
     [{vsn, [], [vsn_1, vsn_2, vsn_3]}].
@@ -657,5 +657,19 @@ otp_8949_b(A, B) ->
 	    id(Var)
     end.
     
+split_cases(_) ->
+    dummy1 = do_split_cases(x),
+    {'EXIT',{{badmatch,b},_}} = (catch do_split_cases(y)),
+    ok.
+
+do_split_cases(A) ->
+    case A of
+        x ->
+	    Z = dummy1;
+        _ ->
+	    Z = dummy2,
+	    a=b
+    end,
+    Z.
 
 id(I) -> I.
-- 
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