[llvm-bugs] [Bug 36180] New: [x86] scalar FP code runs ~15% slower on Haswell when compiled with -mavx

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https://bugs.llvm.org/show_bug.cgi?id=36180

            Bug ID: 36180
           Summary: [x86] scalar FP code runs ~15% slower on Haswell when
                    compiled with -mavx
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 19783
  --> https://bugs.llvm.org/attachment.cgi?id=19783&action=edit
himeno.c source file

I have a Haswell perf mystery that I can't explain. The himeno program (see
attachment) is an FP and memory benchmark that plows through large
multi-dimensional arrays doing 32-bit fadd/fsub/fmul.

To eliminate potentially questionable transforms and variation from the
vectorizers, build it as scalar-ops only like this:

$ ./clang -O2 himeno.c -fno-vectorize -fno-slp-vectorize  -o himeno_novec_sse
$ ./clang -O2 himeno.c -fno-vectorize -fno-slp-vectorize -mavx -o
himeno_novec_avx

And I'm testing on a 4.0GHz Haswell iMac running macOS 10.13.3:

$ ./himeno_novec_sse
mimax = 257 mjmax = 129 mkmax = 129
imax = 256 jmax = 128 kmax =128
cpu : 13.244777 sec.
Loop executed for 500 times
Gosa : 9.897132e-04 
MFLOPS measured : 5175.818966
Score based on MMX Pentium 200MHz : 160.391043

$ ./himeno_novec_avx
mimax = 257 mjmax = 129 mkmax = 129
imax = 256 jmax = 128 kmax =128
cpu : 15.533612 sec.
Loop executed for 500 times
Gosa : 9.897132e-04 
MFLOPS measured : 4413.176279
Score based on MMX Pentium 200MHz : 136.757864

There's an unfortunate amount of noise (~5%) in the perf on this system with
this benchmark, but these results are reproducible. I'm consistently seeing
~15% better perf with the non-AVX build.

If we look at the inner loop asm, they are virtually identical in terms of
operations. The SSE code just has a few extra instructions needed to copy
values because of the destructive ops, but the loads, stores, and math are the
same.

A IACA analysis of these loops says they should have virtually the same
throughput on HSW:

Block Throughput: 20.89 Cycles       Throughput Bottleneck: Backend
Loop Count:  22
Port Binding In Cycles Per Iteration:
--------------------------------------------------------------------------------------------------
|  Port  |   0   -  DV   |   1   |   2   -  D    |   3   -  D    |   4   |   5 
 |   6   |   7   |
--------------------------------------------------------------------------------------------------
| Cycles | 13.0     0.0  | 21.0  | 12.0    12.0  | 12.0    11.0  |  1.0  |  2.0
 |  2.0  |  0.0  |

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