[LLVMdev] unaligned AVX store gets split into two instructions

Zach Devito zdevito at stanford.edu
Wed Jul 10 02:12:31 PDT 2013

I've narrowed this down to a single kernel (kernel.ll), which does a
fixed-size matrix-matrix multiply:

# ~/llvm-32-final/bin/llc kernel.ll -o kernel32.s
# ~/llvm-33-final/bin/llc kernel.ll -o kernel33.s
# ~/llvm-32-final/bin/clang++ harness.cpp kernel32.s -o harness32
# ~/llvm-32-final/bin/clang++ harness.cpp kernel33.s -o harness33
# time ./harness32
real 0m0.584s
user 0m0.581s
sys 0m0.001s
# time ./harness33
real 0m0.730s
user 0m0.725s
sys 0m0.001s

If you look at kernel33.s, it has a register spill/reload in the inner
loop. This doesn't appear in the llvm 3.2 version and disappears from the
3.3 version if you remove the "align 8"s from kernel.ll which are making it
unaligned.  Do the two-instruction unaligned loads increase register
pressure? Or is something else going on?


On Tue, Jul 9, 2013 at 11:33 PM, Zach Devito <zdevito at stanford.edu> wrote:

> 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,
> Zach
> 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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