Constant folding in Inform, i.e. the process of evaluating constant
arithmetic expression at compile-time, appears to be severely broken. For
instance, the following program: will print 13105 rather than -2. The problem does not apply just to
division -- pretty much any constant folding will fail once you involve
negative numbers. The problem, I believe, is that while folding constants Inform will also
translate the result into 16-bit Z-machine values. So what happens above is
that -10 (the operator "unary minus" applied to the constant 10) is folded
to the Z-machine representation of -10 (the constant). But when you pad
these 16 bits with zeroes to make a 32-bit value, you don't get -10, but
65526. Divide by 5 and you get, approximately, 13105. I see two possible ways of fixing this. Either the values have to be
converted back into native format before doing the constant folding, or the
constant folding has to be rewritten to work with Z-machine values. A quick
and dirty way of doing the former would be to add two lines to expressp.c,
around line 825, like this: However, I have no idea if this is sufficient, or if it breaks anything else
in the process. It wouldn't surprise me the least if it did.

About Patches
**Issue C62102**

**Constant folding is broken**

*Submitted by*: Torbjörn Andersson
*Appeared in*: Compiler 6.20 or before
*Fixed in*: Compiler 6.30
*Problem*
[ Main; print_ret -10 / 5; ];

*Solution*
if ((o1.marker == 0) && (o2.marker == 0)
&& ((o1.type == SHORT_CONSTANT_OT)||(o1.type == LONG_CONSTANT_OT))
&& ((o2.type == SHORT_CONSTANT_OT)||(o2.type == LONG_CONSTANT_OT)))
{
--> if (o1.value > 0x7fff) o1.value -= 0x10000;
--> if (o2.value > 0x7fff) o2.value -= 0x10000;
switch(t.value)
{
case PLUS_OP: x = o1.value + o2.value; goto FoldConstantC;

*
Last updated 17 April 2013.
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