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Re: locating unsigned type for non-standard precision


On Tue, Apr 24, 2012 at 3:50 PM, Peter Bigot <bigotp@acm.org> wrote:
> I've run into another issue supporting a 20-bit integer for which I'd
> appreciate a hint.? With this code:
>
> ? typedef long int __attribute__((__a20__)) int20_t;
> ? int20_t xi;
> ? int20_t addit () { xi += 0x54321L; }
>
> xi ends up in mode PSImode, which is a MODE_PARTIAL_INT with 20 bits of
> precision and 32-bit width.
>
> convert() notices that, because the constant in the add expression is
> SImode, there's an SImode add being truncated to a PSImode result, and
> pushes the truncation down into the operands.
>
> The problem is this ends up in convert_to_integer, which detects that the
> signed operation might overflow so calls unsigned_type_for() to get the
> unsigned variant.
>
> Unfortunately, this ends up in c_common_type_for_size(), which knows nothing
> about PSImode, and returns an unsigned type with 32 bits of precision when
> asked for one with 20 bits of precision.

That's expected - this function returns a type that is suitable for holding all
values, not a type that has necessarily matching precision.  If the caller
wants such type it needs to verify what the function returned.  Which seems
to me to be the correct fix for this (premature) optimization in
convert_to_integer.

Richard.

>? The expression is rewritten with
> the 32-bit constant integer recast to the 32-bit unsigned type (instead
> of the 20-bit one it might have used), and infinite recursion results.
>
> Is it proper for c_common_type_for_size() to know about partial int modes
> and return the best one available?? Is there a hook that would allow me to
> do this customized in my back-end?? Or is there another way to get
> unsigned_type_for() to return the "right" type for an unusual integer
> precision?
>
> Thanks.
>
> Peter


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