On Fri, 24 Mar 2006, Eric Botcazou wrote:
> After the change it is simplified to (int)((unsigned int)j + (unsigned int)o).
> Now take j=0 and o=-1. There is signed-overflow undefinedness neither in the
> original expression nor in the original simplified expression while there is
> in the new simplified expression when UINT_MAX is cast to int.
Conversion of unsigned to int does not involve undefined behavior, it
involves implementation-defined behavior, which GCC defines in
implement-c.texi:
@item
@cite{The result of, or the signal raised by, converting an integer to a
signed integer type when the value cannot be represented in an object of
that type (C90 6.2.1.2, C99 6.3.1.3).}
For conversion to a type of width @math{N}, the value is reduced
modulo @math{2^N} to be within range of the type; no signal is raised.
This is of course a definition of C semantics rather than tree semantics,
but I believe our trees follow the C semantics here.
--
Joseph S. Myers http://www.srcf.ucam.org/~jsm28/gcc/
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