Re: A potential bug in lra-constraints.c for special_memory_constraint?

[Qing Zhao <qing dot zhao at oracle dot com>]

> On Jul 11, 2017, at 2:00 PM, Eric Botcazou <ebotcazou@adacore.com> wrote:
> 
>> the problem I had is:
>> 
>> 1. we added a new special_memory_constraint for misaligned memory access, 
>> one important requirement for this new special_memory_constraint is, the
>> address of the memory access is misaligned.
> 
> OK, it's the other way around from the usage of special_memory_constraint.
> In other words, I'm not sure that you really need to use it in this case,
> a standard memory_constraint could be sufficient.

A standard memory_constraint has the same handling on spilled code as special_memory_constraint:

2250                     case CT_MEMORY:
2251                       if (MEM_P (op)
2252                           && satisfies_memory_constraint_p (op, cn))
2253                         win = true;
2254                       else if (spilled_pseudo_p (op))
2255                         win = true;

as a result, if the new misaligned memory access was defined as a standard memory_constraint, will have the same issue.


> 
>> 2. per the current code in lra-constraints.c:
>> 
>> 2286                     case CT_SPECIAL_MEMORY:
>> 2287                       if (MEM_P (op)
>> 2288                           && satisfies_memory_constraint_p (op, cn))
>> 2289                         win = true;
>> 2290                       else if (spilled_pseudo_p (op))
>> 2291                         win = true;
>> 2292                       break;
>> 
>> if the op is a pseudo_p that can be spilled, then it's treated as a
>> PERFECT MATCH.
>> 
>> the issue only can be exposed by the following kind of RTL:
>> 
>> (insn 34 13 14 2 (set (reg:DI 122)
>>       (reg:DI 129)) misalign-3.c:12 125 {*movdi_insn_sp64}
>>    (nil))
>> 
>> i.e.
>>        (1). REG2 move to REG1
>> and. (2). REG2 is a virtual reg (> the max hard regno, on Sparc, its 103),
>> therefore, must be spilled to stack.
>> 
>> the current interpretation of special memory treat such REG2 as a perfect
>> match to special memory,  and then spill it.
>> however, such spilled memory RTL is NOT match the MISALIGN requirement,
>> (i.e, the address of the memory access for the spilled RTL is not
>> misaligned)
> 
> Yes, spilling will automatically meet alignment requirements, that's why it's 
> allowed for special_memory_constraint.

You mean, even for the mis-alignment requirement, the spilled memory access will met the mis-alignment?

> 
> Why do you absolutely need to have a misaligned address?
we need to generate misaligned load/store insns ONLY for misaligned memory access, therefore need a new 
constraints for misaligned address.

As I checked the GCC source code, the special_memory_constraints only were defined in i386 and sparc target, not
used quite often.

Qing

>  Can't you just avert 
> your eyes and pretend that the address is misaligned?  This will be suboptimal 
> but presumably work.  To be honest, I'm not even sure that you really need an 
> additional constraint, but I haven't investigated the subject seriously.
> 
> -- 
> Eric Botcazou