(j3.2006) BEQ, BNE?

[Toon Moene]

On 01/30/2013 02:33 PM, Bill Long wrote:

> I assume this is a theoretical question since I don't know of any
> relevant hardware implementation that distinguishes between +0 and -0
> for integers.  If such hardware ever appears on the horizon, we  could
> add in BEQ and BNE.  If I recall, the goal was to add the actually
> useful cases while trying to limit the ever-expanding list of intrinsics.

I get it you never worked on Univac 1100 systems ?  1111...1111 and 
0000....0000 both being treated as zero (especially if only *some* of 
the instructions do that) leads to .... hmmm, interesting problems.

The following might serve as an example.

Some thirty years ago I *did* do support for Fortran and Fortran related 
applications (DISSPLA, SPSS) on Univac 1100 systems at the Dutch Postal 
Services (then the largest employer in the Netherlands, with 100,000 
employees).

In the bug report system of the vendor the following appeared (paraphrased):

"I have a large subroutine, and upon (optimized) compilation, only a 
tiny object file remains, which doesn't do anything useful."

Analysis:

"The Fortran compiler, upon optimizing your routine, first made a list 
of all basic blocks, and then determined which of them were unreachable, 
giving them a 0 bit in an bit-array of double (72-bit) words. 
Unfortunately, on checking whether basic blocks were reachable, the 
compiler, as an optimization, checked a full double word for 0 to deal 
with 72 blocks at once."

Of course, at that time (late 70s, early 80s) the hype of the day in 
systems programming was to use a high-level (Pascal-like) language to 
write programs like compilers in.  It was the compiler for *that* 
language that made the mistake of optimizing a bit-array check for zero 
as a series of jump-on-double-word-equal-zero instructions, which 
happened to be just as satisfied with a double-word filled with 72 one 
bits for zero.

-- 
Toon Moene - e-mail: toon at moene.org - phone: +31 346 214290
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