[PATCH] Use ARM-style representation for C++ method pointers under PNaCl

[Mark Seaborn]

On 19 June 2013 22:43, Mark Seaborn <mseaborn at chromium.org> wrote:

> On 19 June 2013 15:20, John McCall <rjmccall at apple.com> wrote:
>
>> On Jun 19, 2013, at 3:17 PM, Mark Seaborn <mseaborn at chromium.org> wrote:
>>
>> On 19 June 2013 13:17, John McCall <rjmccall at apple.com> wrote:
>>
>>> On Jun 19, 2013, at 1:05 PM, Mark Seaborn <mseaborn at chromium.org> wrote:
>>>
>>> On 19 June 2013 13:01, Mark Seaborn <mseaborn at chromium.org> wrote:
>>>
>>>> Use ARM-style representation for C++ method pointers under PNaCl
>>>>
>>>> Before this change, Clang uses the x86 representation for C++ method
>>>> pointers when generating code for PNaCl.  However, the resulting code
>>>> will assume that function pointers are 0 mod 2.  This assumption is
>>>> not safe for PNaCl, where function pointers could have any value
>>>> (especially in future sandboxing models).
>>>>
>>>> So, switch to using the ARM representation for PNaCl code, which makes
>>>> no assumptions about the alignment of function pointers.
>>>>
>>>> See: https://code.google.com/p/nativeclient/issues/detail?id=3450
>>>>
>>>
>>> Oops, I meant to send this to cfe-commits rather than llvm-commits.
>>>
>>>
>>> I do not think you should just unconditionally opt in to random ARM
>>> behavior.  In particular, ARM uses 32-bit guard variables because that's
>>> the size of a pointer on ARM;  PNaCl needs to be able to efficiently
>>> support 64-bit platforms as well.
>>>
>>
>> The code does always use 64-bit guard variables on 64-bit systems.  It
>> does this:
>>
>>     // Guard variables are 64 bits in the generic ABI and size width on
>> ARM
>>     // (i.e. 32-bit on AArch32, 64-bit on AArch64).
>>     guardTy = (IsARM ? CGF.SizeTy : CGF.Int64Ty);
>>
>> Having said that, PNaCl is 32-bit-only:  PNaCl programs assume a 32-bit
>> address space.  We don't support 64-bit pointers in PNaCl.  In Clang,
>> targeting PNaCl is identified by "le32" being in the triple, and I assume
>> there's no way to get 64-bit pointers with "le32". :-)
>>
>>
>> Interesting, okay.
>>
>> I still do not want PNaCl to claim to be ARM.  Abstract the code so that
>> you can opt into the specific behaviors you want without pretending to
>> be ARM.
>>
>
> OK, I have changed the patch to split IsARM into two separate fields.  I
> called the fields UseARMMethodPtrABI and UseARMGuardVarABI out of a lack of
> imagination.
>
> I've changed the patch to use the non-ARM guard variable ABI for PNaCl.
> Having looked at the code more carefully, I see it's inlining a different
> guard variable check on ARM, which we don't necessarily want to use for
> PNaCl; it's not just a different guard var size.
>

I've updated the patch to also add a test for C++ guard variable usage
under PNaCl.  Is this OK to commit?

Cheers,
Mark
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