[LLVMdev] unaligned AVX store gets split into two instructions

Zach Devito zdevito at stanford.edu
Tue Jul 9 23:33:38 PDT 2013

Thanks for all the the info! I'm still in the process of narrowing down the
performance difference in my code. I'm no longer convinced its related to
only the unaligned loads/stores alone since extracting this part of the
kernel makes the performance difference disappear.  I will try to narrow
down what is going on and if it seems related LLVM, I will post an example.
Thanks again,


On Tue, Jul 9, 2013 at 10:15 PM, Nadav Rotem <nrotem at apple.com> wrote:

> Hi,
> Yes. On Sandybridge 256-bit loads/stores are double pumped.  This means
> that they go in one after the other in two cycles.  On Haswell the memory
> ports are wide enough to allow a 256bit memory operation in one cycle.  So,
> on Sandybridge we split unaligned memory operations into two 128bit parts
> to allow them to execute in two separate ports. This is also what GCC and
> ICC do.
> It is very possible that the decision to split the wide vectors causes a
> regression.  If the memory ports are busy it is better to double-pump them
> and save the cost of the insert/extract subvector.  Unfortunately, during
> ISel we don’t have a good way to estimate port pressure. In any case, it is
> a good idea to revise the heuristics that I put in and to see if it matches
> the Sandybridge optimization guide. If I remember correctly the
> optimization guide does not have too much information on this, but Elena
> looked over it and said that it made sense.
> BTW, you can validate that this is the problem using the IACA tool. It
> performs static analysis on your binary and tells you where the critical
> path is.
> http://software.intel.com/en-us/articles/intel-architecture-code-analyzer
> Thanks,
> Nadav
> On Jul 9, 2013, at 10:01 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Tue, Jul 9, 2013 at 9:01 PM, Zach Devito <zdevito at gmail.com> wrote:
> I'm seeing a difference in how LLVM 3.3 and 3.2 emit unaligned vector loads
> on AVX.
> 3.3 is splitting up an unaligned vector load but in 3.2, it was emitted as
> a
> single instruction (details below).
> In a matrix-matrix inner-kernel, I see a ~25% decrease in performance,
> which
> seems to be due to this.
> Any ideas why this changed? Thanks!
> This was intentional; apparently doing it with two instructions is
> supposed to be faster.  See r172868/r172894.
> Adding Nadav in case he has anything more to say.
> -Eli
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