Re: [Patch AArch64] Add cpu_defines.h for AArch64.

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

On 19/05/15 17:03, Ramana Radhakrishnan wrote:
> On Tue, May 19, 2015 at 4:54 PM,  <pinskia@gmail.com> wrote:
>>> On May 19, 2015, at 5:54 AM, Ramana Radhakrishnan <ramana.radhakrishnan@foss.arm.com> wrote:
>>> Like the ARM port, the AArch64 ports needs to set glibc_integral_traps to false as integer divide instructions do not trap.
>>>
>>> Bootstrapped and regression tested on aarch64-none-linux-gnu
>>>
>>> Ok to apply ?
>>
>> Not really questioning your patch but questioning libstdc++'s defaults.
>>  I wonder if this should be the default as most targets don't trap, only a few that does. And it should be safer default to say they don't trap too?
> 
> How about we  #error out if targets do *not* define some of these
> defaults in libstdc++  ?

__glibcxx_integral_traps seems to be used for the 'traps'
numeric_limits<T> member.

i think it can only be meaningful if LIA-1 is properly supported
(eg. div-by-zero is never optimized away) otherwise the standard
dictates UB and anything can happen. (Note that LIA-1 also requires
well-defined semantics for signed int overflow).

i don't see a way for conforming c++ code to use traps, or for
the library to guarantee either traps==true or traps==false on
any machine.