llvm dev - Feb 2013 - [LLVMdev] mem2reg for non entry blocks?

Sorry if this has been discussed before, but I would appreciate any
pointers.

I am trying to understand why mem2reg only looks at allocas in entry
blocks, and not for any allocas in a function. One case where allocas could
be used to build local data structures like linked list (and so on make it
unsafe), and for that the existing conditions in IsAllocaPromotable (i.e.
the alloca pointer cannot escape or be stored or cast etc) should guard
against that, regardless of the position of the alloca. right? Is there a
reason for this?

Thanks
Vinod
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Hi Vinod,

On 23/02/13 02:20, Vinod Grover wrote:
> Sorry if this has been discussed before, but I would appreciate any
pointers.
> I am trying to understand why mem2reg only looks at allocas in entry
blocks, and
> not for any allocas in a function. One case where allocas could be used to
build
> local data structures like linked list (and so on make it unsafe), and for
that
> the existing conditions in IsAllocaPromotable (i.e. the alloca pointer
cannot
> escape or be stored or cast etc) should guard against that, regardless of
the
> position of the alloca. right? Is there a reason for this?
an alloca outside of the entry block might be inside a loop, in which case the
semantics are that it would allocate more stack space on every loop iteration.
I think some of the optimizers that run later try to move allocas into the entry
block if possible, but in general it is simpler to have the front-end just put
them there in the first place.

Ciao, Duncan.

On Tue, Feb 26, 2013 at 5:42 AM, Duncan Sands <baldrick at free.fr> wrote:
> Hi Vinod,
>
>
> On 23/02/13 02:20, Vinod Grover wrote:
>
>> Sorry if this has been discussed before, but I would appreciate any
>> pointers.
>> I am trying to understand why mem2reg only looks at allocas in entry
>> blocks, and
>> not for any allocas in a function. One case where allocas could be used
>> to build
>> local data structures like linked list (and so on make it unsafe), and
>> for that
>> the existing conditions in IsAllocaPromotable (i.e. the alloca pointer
>> cannot
>> escape or be stored or cast etc) should guard against that, regardless
of
>> the
>> position of the alloca. right? Is there a reason for this?
>>
>
> an alloca outside of the entry block might be inside a loop, in which case
> the
> semantics are that it would allocate more stack space on every loop
> iteration.
> I think some of the optimizers that run later try to move allocas into the
> entry
> block if possible, but in general it is simpler to have the front-end just
> put
> them there in the first place.
>
Mem2reg is already changing that semantic, though.  If I use an "alloca
i32" in the entry block, then I am saying I want 4 bytes of stack space,
but mem2reg may replace that with registers.

>
> Ciao, Duncan.
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-- 

Thanks,

Justin Holewinski
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