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

[Manman Ren]

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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