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Re: Register allocation trouble

On 07/24/2017 11:37 AM, Georg-Johann Lay wrote:
> Andrew Stubbs schrieb:
>> On 24/07/17 14:58, Georg-Johann Lay wrote:
>>> Dunno if that works in all situation.  For example, when the register
>>> allocator is facing high register pressure and decides to spill the
>>> target register, it uses the constraints of the matched insn.
>> That would be a memory to memory move, and therefore not valid in any
>> mov insn on many architectures. Are you sure that's a real thing?
> Hard to tell, it depends on many gory details...
> If the additional insn is a mov insn that only catches specific cases
> by its predicates, then you have definitely a problem (more than 1 mov
> insn per mode).
Correct.  Sadly everything will seem OK here, then one day you'll have
an abort or wrong code generation in reload.  Been there, done that.

> If it has an explicit mem: then the problems are completely different.
> For example, it will also match volatile MEMs even for cases when
> volatile_ok is false.
Though presumably you could extract the MEM out of the insn so that you
could check the volatile flag.  Though that's fairly gross.

> In general, it's hard to find test cases which show that something
> breaks.


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