[fpc-devel] Review of AVR patch for bug 31925

[Florian Klämpfl]

Am 22.09.2017 um 17:28 schrieb Christo:
> On Fri, 2017-09-22 at 07:10 +0200, Christo wrote:
>> On Wed, 2017-09-20 at 12:36 +0200, Karoly Balogh (Charlie/SGR) wrote:
>>>
> 
> A complication I've noted is that enabling overflow checking doesn't
> call the fpc_mul_byte_overflow function (as the naming convention
> suggested), rather the generated code calls fpc_mul_byte and then check
> the carry flag to determine whether an overflow occurred. Since the
> function in generic.inc is plain pascal code there is no guarantee that
> the carry flag will be set if an overflow should have occurred.  I
> assume that at a higher level the code generator assumes that a
> hardware MUL instruction _will_ be used, leading to incorrect overflow
> detection logic if a software helper is used.
> 
> A similar situation exist for the current 16 bit software MUL case with
> overflow checking.
> 
> Any pointers on where to start looking to try and adapt the overflow
> checking code generation?

AVR has no working overflow checking yet. Basically, tcg.a_op_reg_reg_reg_checkoverflow and
tcg.a_op_conbst_reg_reg_checkoverflow need to be overridden and they have to set ovloc properly
which is checked later on by g_overflowCheck_loc,