[PATCH] Retain alignment requirements for load->selects modified by DAGCombine

[Owen Anderson]

Perhaps this optimization should be disabled if it would reduce alignment and the target does not support unaligned accesses?  See TargetLoweringBase::allowsMisalignedMemoryAccesses() for a way to determine this.

—Owen

On Jul 30, 2014, at 10:40 AM, Louis Gerbarg <lgg at apple.com> wrote:

> This patch fixes an issue where we can generate aligned loads for unaligned data. It is a latent bug that has been around for years, but we have started tripping it recently with the introduction of more aggressive vectorization, etc. On x86 this manifests as generating a movapd when we should have used a movupd, which then results in a segfault if the data does not happened to be aligned. The problem is in platform independent codegen, so this may manifest on other architectures as well. I suspect on PPC this may result in some subtle data corruption bugs since altivec loads will take an unaligned address without faulting and just ignore the low bits.
> 
> Tests included.
> 
> Louis
> 
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