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Date: Fri, 17 Jul 2015 22:23:34 +0200
From: Jens Gustedt <>
Subject: Re: Re: Left-shift of negative number

Hello Pascal,
you a probably right with your findings and so we should change the
code as I indicated. I think we just have to promote the one constant
and then all types are unsigned and of the right width.

But I should comment on this:

Am Freitag, den 17.07.2015, 19:55 +0000 schrieb Pascal Cuoq:
> I know that this is not well-known, so perhaps I should explain: ever since
> the late 1990s, C compilers have been taking advantage of undefined behavior
> in the C standard to introduce new optimizations.

No it is actually the other way around. The C standard voluntarily
leaves certain behavior undefined to have leeway for compiler and
library implementors to do what they (think they) have to do. So there
is a clear distinction what UB means for "user" code and for the

musl (and other C libraries I suppose) heavily rely on specific
properties of compilers, that is basically all what the writing of a C
library for a specific platform is about.

So the argument to show that this is a bug can not be "that is UB",
but must be, as you did, there is this-and-that compiler for which
this behavior is different.


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