[PATCH] Fix write-back value propagation for pre-indexed addressing modes

[Hal Finkel]

----- Original Message -----
> From: "Silviu Baranga" <Silviu.Baranga at arm.com>
> To: llvm-commits at cs.uiuc.edu
> Sent: Wednesday, April 24, 2013 11:02:01 AM
> Subject: [PATCH] Fix write-back value propagation for pre-indexed addressing	modes
> 
> Hi,
> 
> The attached patch fixes a bug in DAG combiner that causes a wrong
> usage
> of the value write-back value obtained from pre-indexed addressing
> modes.
> 
> The bug manifests itself when the base pointer and the offset are
> swapped
> in a load/store store instruction with a pre decrement addressing
> mode.
> 
> If a user needs to subtract the base from a constant value, the
> current
> code re-writes the expression as a SUB instruction, subtracting the
> write-back
> value from an adjusted constant. This is wrong, since the correct
> output
> is to add an adjusted constant to the write-back value.
> 
> Since the code is apparently impossible to test, the patch does not
> contain a
> regression test. Because of this I've re-written the code that
> handles the
> propagation of the write-back value so that it is easy to follow and
> unlikely
> to have fundamental bugs in it.

Silviu,

Thanks for working on this. This is PR15581, and Dmitry Antipov has proposed a simple fix (which seems to be covered by your code as well). I'll ask you the same question that I asked him:

Is is safe to add something like this?
assert(AM == ISD::PRE_DEC && OV <= 0 && "Offset for PRE_DEC must be negative");

My misunderstanding of how the offsets for PRE_DEC addressing are interpreted is, from what I now understand from Dmitry, the cause of the current underlying problem.

Regardless, I agree that your code is cleaner :)

 -Hal

> 
> Please review!
> 
> Thanks,
> Silviu
> 
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