[mercury-users] Why isn't deforestation working here?

[Fergus Henderson]

On 16-Oct-1999, Ralph Becket <rbeck at microsoft.com> wrote:
> The basic format of the code is something like this:
> 
> 	io__read_byte(Result, IO_in, IO_out),
> 	(	Result = ok(Byte), ...
> 	;	Result = eof, ...
> 	;	Result = error(ErrCode), ...
> 	)
> 
> and I was under the impression that compiling with
> 
> 	MCFLAGS = -O 6 --intermodule-optimization
> 
> should deforest (vb?) this code.  Any help would be greatly appreciated!

The basic problem is that the compiler decides that io__read_byte
is too complicated to be worth trying intermodule inlining.
That looks like a bad decision; probably we need to tweak the
compiler's inlining heuristics.  But in the mean time, the following
patch will probably give you some improvement.
This patch just adds some `pragma inline' declarations.

Index: io.m
===================================================================
RCS file: /home/staff/zs/imp/mercury/library/io.m,v
retrieving revision 1.185
diff -u -u -r1.185 io.m
--- io.m	1999/10/03 10:40:50	1.185
+++ io.m	1999/10/17 11:53:29
@@ -1149,6 +1149,9 @@
 
 % input predicates
 
+:- pragma inline(io__read_char/3).
+:- pragma inline(io__read_char/4).
+
 io__read_char(Result) -->
 	io__input_stream(Stream),
 	io__read_char(Stream, Result).
@@ -1168,6 +1171,9 @@
 		{ Result = error(io_error(Msg)) }
 	).
 
+:- pragma inline(io__read_byte/3).
+:- pragma inline(io__read_byte/4).
+
 io__read_byte(Result) -->
 	io__binary_input_stream(Stream),
 	io__read_byte(Stream, Result).
@@ -2800,6 +2806,8 @@
 ").
 
 /* input predicates */
+
+:- pragma inline(io__read_char_code/4).
 
 :- pragma c_code(io__read_char_code(File::in, CharCode::out, IO0::di, IO::uo),
 		will_not_call_mercury, "

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.
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