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From: Lars Brinkhoff <lars@nocrew.org>
To: Warner Losh <imp@bsdimp.com>
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Subject: [TUHS] Re: C history question: why is signed integer overflow UB?
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Warner Losh writes:
> I suspect that it was the absence of a signed right shift. In my
> Decsystem-20 OS class, one of the differences between the compiler on
> the VAX and the compiler on the '20 was that -1 >> 1 was -1 on the VAX
> and 2^35-1 on the '20.

The DEC-20 (aka PDP-10) does have a signed right shift instruction.
Apparently the compiler didn't use it.