llvm dev - Feb 2011 - [LLVMdev] broken alignment in stack(caused by bug in SelectionDAGBuilder) causes invalid schedules with r125471 and newer

The following problems happens with architectures, where stack alignment is
smaller than the biggest preferred alignment for any data type


SP pointer may point anywhere with alignment of stack alignment (4 in our case)

SelectionDAGBuilder however calls CreateStackObject with preferred alignment is
given data type(8 in our problemaric case. The ABI alignment for this data type
is only 4)

This means, that the MachineFrameInfo thinks that the stack object is aligned by
8 bytes, even though in reality if may also be aligned only by 4 bytes, of the
SP points to address which is aligned by 4, but not 8.


r125471 introduced optimization which can optimize add into xor in case object
is aligned so that the operation only touches the lowest bits, which were zero
on the aligned pointer.

But when the MachineFrameInfo contains invalid align information, this
optimization breaks and it incorrectly optimizes add by 4 into xor by 4, when
the object is (in reality) aligned by 4, not by 8(which MFI thinks about
it's alignment)



Two solutions come into my mind:

1)  changiing the SelectionDAGBuilder to use std::min(StackAlignment,
PreferredAlignment) instead of PreferredAlignment. This might require asserting
(StackAlignment <= ABI alignment).

2) changing the SelectionAGBuilder to use ABI alignment instead of the preferred
alignment for the stack objects.


The affected code lines are around 4820, and 5694 of SelectionDAGBuilder.cpp

On 14 Feb 2011, at 20:10, Heikki Kultala wrote:
> The following problems happens with architectures, where stack alignment is
smaller than the biggest preferred alignment for any data type
> 
> 
> SP pointer may point anywhere with alignment of stack alignment (4 in our
case)
> 
> SelectionDAGBuilder however calls CreateStackObject with preferred
alignment is given data type(8 in our problemaric case. The ABI alignment for
this data type is only 4)
> 
> This means, that the MachineFrameInfo thinks that the stack object is
aligned by 8 bytes, even though in reality if may also be aligned only by 4
bytes, of the SP points to address which is aligned by 4, but not 8.
> 
> 
> r125471 introduced optimization which can optimize add into xor in case
object is aligned so that the operation only touches the lowest bits, which were
zero on the aligned pointer.
> 
> But when the MachineFrameInfo contains invalid align information, this
optimization breaks and it incorrectly optimizes add by 4 into xor by 4, when
the object is (in reality) aligned by 4, not by 8(which MFI thinks about
it's alignment)
> 
> 
> 
> Two solutions come into my mind:
> 
> 1)  changiing the SelectionDAGBuilder to use std::min(StackAlignment,
PreferredAlignment) instead of PreferredAlignment. This might require asserting
(StackAlignment <= ABI alignment).
> 
> 2) changing the SelectionAGBuilder to use ABI alignment instead of the
preferred alignment for the stack objects.
> 
> 
> The affected code lines are around 4820, and 5694 of
SelectionDAGBuilder.cpp

Hmm, it seems that CreateStackObject is called also in many other places with
preferred alignment.
So maybe it would be better to instead change the part which checks the
alignment (InferPtrAlignment?)
, so that for objects in stack , the alignment would be correctly calculated
taking the stack alignment into consideration