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

[Mark Seaborn]

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.

Cheers,
Mark
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