[m-rev.] diff: MLDS back-end: optimize away trivial switches

[Fergus Henderson]

Estimated hours taken: 0.5
Branches: main

compiler/ml_simplify_switch.m:
	Optimize away trivial switches (those with only one case,
	and where the default is unreachable).

Workspace: /home/earth/fjh/ws-earth2/mercury
Index: compiler/ml_simplify_switch.m
===================================================================
RCS file: /home/mercury1/repository/mercury/compiler/ml_simplify_switch.m,v
retrieving revision 1.2
diff -u -d -r1.2 ml_simplify_switch.m
--- compiler/ml_simplify_switch.m	20 Jan 2001 15:42:47 -0000	1.2
+++ compiler/ml_simplify_switch.m	16 Aug 2001 15:35:29 -0000
@@ -93,6 +93,18 @@
 		{ Statement = ml_switch_to_if_else_chain(Cases, Default, Rval,
 			MLDS_Context) }
 	;
+	%
+	% Optimize away trivial switches
+	% (these can occur e.g. with --tags none, where the
+	% primary tag test always has only one reachable case)
+	%
+		{ Stmt0 = switch(_Type, _Rval, _Range, Cases, Default) },
+		{ Cases = [SingleCase] },
+		{ Default = default_is_unreachable }
+	->
+		{ SingleCase = _MatchCondition - CaseStatement },
+		{ Statement = CaseStatement }
+	;
 		{ Stmt = Stmt0 },
 		{ Statement = mlds__statement(Stmt, MLDS_Context) }
 	).

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  | "... it seems to me that 15 years of
The University of Melbourne         | email is plenty for one lifetime."
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- Prof. Donald E. Knuth
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