Re: [patch][version 6] add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

[Qing Zhao <qing.zhao@oracle.com>]

> On Aug 11, 2021, at 11:15 AM, Richard Sandiford <richard.sandiford@arm.com> wrote:
> 
> Qing Zhao <qing.zhao@oracle.com> writes:
>>> On Aug 11, 2021, at 4:02 AM, Richard Sandiford <richard.sandiford@arm.com> wrote:
>>>> I came up with the following solution:
>>>> 
>>>> Define the IFN_DEFERRED_INIT function as:
>>>> 
>>>>  LHS = DEFERRED_INIT (SIZE of the DECL, INIT_TYPE, IS_VLA);
>>>> 
>>>>  if IS_VLA is false, the LHS is the DECL itself,
>>>>  if IS_VLA is true, the LHS is the pointer to this DECL that created by
>>>>  gimplify_vla_decl.
>>>> 
>>>> 
>>>> The benefit of this solution are:
>>>> 
>>>> 1. Resolved the invalid IR issue;
>>>> 2. The call stmt carries the address of the VLA natually;
>>>> 
>>>> The issue with this solution is:
>>>> 
>>>> For VLA and non-VLA, the LHS will be different, 
>>>> 
>>>> Do you see any other potential issues with this solution?
>>> 
>>> The idea behind the DECL version of the .DEFERRED_INIT semantics was
>>> that .DEFERRED_INIT just returns a SIZE-byte value that the caller
>>> then assigns to a SIZE-byte lhs (with the caller choosing the lhs).
>>> .DEFEREED_INIT itself doesn't read or write memory and so can be const,
>>> which in turn allows alias analysis to be more precise.
>> Yes. That’s right.
>> 
>>> 
>>> If we want to handle the VLA case using pointers instead then I think
>>> that needs to be a different IFN.
>>> 
>>> If we did handle the VLA case using pointers (not expressing an opinion
>>> on that), then it would be the caller's job to allocate the VLA and work
>>> out the address of the VLA;
>> 
>> the current routine “gimplify_vla_decl” has done this already:
>> 
>> It created a temporary variable for the address of the VLA, and created a call to “alloca” to allocate the VLA.
> 
> Right, that's what I mean.  It's this alloca that allocates the VLA
> and determines its address.  This address is therefore logically an
> input rather than an output to the following zero/pattern initialisation.
> 
> In C you wouldn't write:
> 
>  addr = alloca(size);
>  addr = initialise(size);
> 
> to allocate and initialise a size-byte buffer, because initialise()
> would need to know the address of the memory it's supposed to initialise.
> The same is true for this gimple code.

This really make good sense to me.  :-)

> 
>> My -ftrivial-auto-var-init work just try to use the “address variable of the VLA” in the new .DEFERRED_INIT call to carry it to RTL expansion phase.
>> 
>> 
>>> this isn't something that .DEFERRED_INIT
>>> would work out on the caller's behalf.  The address of the VLA should
>>> therefore be an argument to the new IFN, rather than something that
>>> the IFN returns.
>> 
>> Then what’s the LHS of this call? Currently the major issue is the LHS is invalid gimple.
> 
> For this (different, address-taking, VLA-only) IFN, there would be no lhs.
> The IFN would be similar to a memset.

I see. 
> 
> Like I say, this is all hypothetical, based on “if we did handle the VLA
> case using pointers”.  As discussed, it would make alias analysis less
> precise.  I was just answering the question about whether there were
> potential issues.

Okay,  understood.

I will not handle the VLA case using pointers at this time. 

Per discussion with Richard Biener in the other emails, I might go the other route to special handle the 

_1 = .DEFERRED_INIT (4, 2, 0);
  alt_reloc = _1;

To see whether that can resolve the issues.

Let me know your opinion.

Thanks a lot.

Qing

> 
> Thanks,
> Richard