[llvm] r174336 - [Stack Alignment] emit warning instead of a hard error

[Jim Grosbach]

Thanks for clarifying.

With that in mind, I'm not personally opposed to demoting this to a warning anyway, but I will grant that it does somewhat weaken the case for it.

Eric, what do you think?

-Jim

On Feb 5, 2013, at 10:43 AM, Manman Ren <mren at apple.com> wrote:

> 
> Sorry for the confusion :]
> We are not performing the frontend analysis to check for whether the low bits are assumed to be zeros.
> I was stating that we should emit a hard error if the low bits are assumed to be zeros in the source code, and a warning if the low bits are not assumed to be zeros.
> But since we don't currently perform the frontend analysis, to make sure existing codes that compile with previous version can still build, we emit a warning.
> 
> Hope that clears things out.
> 
> Manman
> 
> On Feb 5, 2013, at 10:36 AM, Jim Grosbach <grosbach at apple.com> wrote:
> 
>> Hi Manman,
>> 
>>> Per discussion in rdar://13127907, we should emit a hard error only if
>>> people write code where the requested alignment is larger than achievable
>>> and assumes the low bits are zeros. A warning should be good enough when
>>> we are not sure if the source code assumes the low bits are zeros.
>> 
>> I'm a bit confused. This implies that we're doing the frontend analysis to check for that condition and issue a hard error for it. The below is saying that's not actually the case. Can you elaborate a bit on what exactly is happening?
>> 
>> -Jim
>> 
>> On Feb 5, 2013, at 10:18 AM, Manman Ren <mren at apple.com> wrote:
>> 
>>> 
>>> We currently do not analyze the source code to check the usage of the low bits.
>>> 
>>> In the backend, the alignment is already clamped to the correct value, so the backend optimizations will not treat those low bits as zero.
>>> 
>>> -Manman
>>> 
>>> On Feb 4, 2013, at 6:08 PM, Eric Christopher <echristo at gmail.com> wrote:
>>> 
>>>> Also I could be missing it but I couldn't spot the code that checks for the usage of the bits in not aligning/warning/erroring? Quick pointer?
>>>> 
>>>> Thanks!
>>>> 
>>>> -eric
>>>> 
>>>> 
>>>> On Mon, Feb 4, 2013 at 5:41 PM, Eric Christopher <echristo at gmail.com> wrote:
>>>> 
>>>> 
>>>> 
>>>> On Mon, Feb 4, 2013 at 5:35 PM, Manman Ren <mren at apple.com> wrote:
>>>> 
>>>> Yes, there are related discussions in r169197 and "[PATCH] Stack Alignment: clamp the alignment of stack objects in MachineFrameInfo".
>>>> 
>>>> But people can use a 32-byte alignment attribute on a machine which only supports 16-byte stack alignment.
>>>> If the source code further assumes the low bits are zeros, they will get wrong result.
>>>> But if not, a hard error is too much and it will make existing code which can compile with earlier version failed to build with this patch.
>>>> 
>>>> And to use the other side of the argument that won last time :)
>>>> 
>>>> But this means that if people aren't looking at the warning or hard erroring on warnings then we're going to emit bad code instead of making it an error.
>>>> 
>>>> -eric
>>>> 
>>>> 
>>> 
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>> 
> 

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