[LLVMdev] unaligned AVX store gets split into two instructions

Nadav Rotem nrotem at apple.com
Tue Jul 9 22:15:02 PDT 2013


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


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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