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From: lippin@skippy.berkeley.edu (The Apathist)
Newsgroups: comp.sys.mac.programmer
Subject: Re: THINK C Array Design Flaw
Message-ID: <24093@agate.BERKELEY.EDU>
Date: 6 May 89 02:20:19 GMT
References: <664@loligo.cc.fsu.edu>
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Recently pepke@gw.scri.fsu.edu (Eric Pepke) wrote:
[...]
]In one of my programs I need to use an array of 32*32*32 short integers,
]which is 64K long.  The segment loader problem doesn't bother me.  I am
]perfectly happy allocating the memory, passing a pointer into a function,
]and declaring it as I want it in the function.  So, I tried something like
]
]AddHist(theHist, r, g, b)
]short theHist[32][32][32];
]
]which caused a compiler error, in spite of the fact that the function does
]not have to define the array at all.

Looks like they blew that one.  I'm surprised it hasn't come up
before.

]short Access7(array, a, b)
]short array[7][7];
]short a, b;
]{
]    register short result;
]    result = array[a][b];
]}
[more code deleted]
]The techniques in both functions generalize to higher order and larger
]arrays.  ACCESS4 does an arithmetic shift on a longword, so it can be
]expected to work when the array is larger than 32K.  ACCESS7 only does a
]single word multiply, so it can be expected to fail on large arrays.  It is
]also a *signed* multiply, which makes little sense.

The code generated for Access7 looks correct to me.  The key point is
that the MULS.W instruction produces a long result.  Since the code
needs to multiply a, a *signed* word, by 14, MULS.W is the most
straightforward way to get the right result.  Problems with 32K will
only arise if a>=32K (which it can't be) or if the row size>=32K, in
which case I presume (or at least hope) different code is generated.

					--Tom Lippincott
					  lippin@math.berkeley.edu

		"Foo, you are nothing but a charlatan!"
					--Adventure