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

[Manman Ren]

Ping

On Dec 18, 2012, at 4:13 PM, Manman Ren <mren at apple.com> wrote:

> 
> Stack Alignment: throw error if we can't satisfy the minimal alignment
> requirement when creating stack objects in MachineFrameInfo.
> 
> Add CreateStackObjectWithMinAlign to throw error when the minimal alignment
> can't be achieved and to clamp the alignment when the preferred alignment
> can't be achieved. Same is true for CreateVariableSizedObject.
> Will not emit error in CreateSpillStackObject or CreateStackObject.
> 
> As long as callers of CreateStackObject do not assume the object will be
> aligned at the requested alignment, we should not have miscompile since
> later optimizations looking at the object's alignment will have the correct
> information.
> 
> 
> <emit_error.patch>
> 
> Thanks,
> Manman
> 
> On Dec 18, 2012, at 12:47 PM, Bob Wilson wrote:
> 
>> 
>> 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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