[llvm-commits] [PATCH] Stack Alignment: clamp the alignment of stack objects in MachineFrameInfo

[Bob Wilson]

On Dec 18, 2012, at 11:27 AM, Manman Ren <mren at apple.com> wrote:

> 
> There are some discussions in thread
> Re: [llvm-commits] [llvm] r169197 - in /llvm/trunk: include/llvm/CodeGen/MachineFrameInfo.h include/llvm/Target/TargetFrameLowering.h lib/CodeGen/MachineFunction.cpp test/CodeGen/ARM/alloc-no-stack-realign.ll

As you can tell, I'm way behind on patch reviews…  Sorry that I missed the other discussion.

To Chris's comment about LLVMContext::emitError: I'm embarrassed to admit that I'd forgotten that was available for general error messages.  I knew it was there for inline assembly, but didn't make the connection when I saw InstCombine writing a warning directly to errs().  I'll fix that and check for other places that may also need to be updated.

> 
> 
>>> When creating stack object, we use Align
>>> Align = std::max((unsigned)TLI.getDataLayout()->getPrefTypeAlignment(Ty),
>>>                  AI->getAlignment());
>>> The preferred alignment may be too big for the target, and it makes sense to give a warning instead of a fatal error.
>> 
>> If we can't realign the stack, we should print an error if
>> AI->getAlignment() is too large and simply ignore the return of
>> getPrefTypeAlignment if it's too large.
> 
> This means going through the call sites of these functions CreateStackObject … and checking the alignment there instead of
> wrapping it up in these functions.
> 
> Another option is to pass in an extra argument for the minimal alignment that must be satisfied.
> 
> //////////////////////////////////////////////
> --> I will pass an extra argument for the minimal alignment and will use emitError if the minimal alignment can't be satisfied and
> will ignore the case where getPrefTypeAlignment is too large.
> --> If that does not sound right, please let me know,
> 
> Thanks,
> Manman
> 
> On Dec 18, 2012, at 11:15 AM, Chris Lattner <clattner at apple.com> wrote:
> 
>> 
>> On Dec 18, 2012, at 11:06 AM, Bob Wilson <bob.wilson at apple.com> wrote:
>> 
>>>>> 
>>>>> Hi Manman,
>>>>> 
>>>>> Could you move the functions from MachineFrameInfo.h first and then add the clamping code? Also, the debug output you emit should be within 'DEBUG()' macro. Like this:
>>>> Hi Bill,
>>>> Sure, I will submit it as two patches and guard with DEBUG.
>>>>> 
>>>>> 	DEBUG(dbgs() << "Warning: requested alignment " << Align
>>>>> 	         << " exceeds the stack alignment " << StackAlign
>>>>> 	         << " when stack realignment is off\n");
>>>>> 
>>>>> Or better yet make it an 'assert' if you don't expect this situation to ever occur:
>>>> This can actually happen if the user specified a large alignment or the data type has a large alignment.
>>> 
>>> In that case, using DEBUG is absolutely the wrong thing.  The warning won't be produced in a compiler built without debugging.  Is there any way you could detect this in the front-end and produce a proper diagnostic?  (I haven't thought about that and haven't looked at the code, so forgive me if that's a stupid idea.)  Otherwise, you just have to print out the warning to "errs()".  In the long term, we need to figure out how to let the optimizer and code-gen produce real diagnostics (tracked at Apple in <rdar://problem/12867368>).
>> 
>> Backend errors like this should be reported with LLVMContext::emitError.
> 
>> 
>> -Chris
> 

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