[llvm-bugs] [Bug 31691] New: Reporting of predicted benefits or vectorisation

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31691

            Bug ID: 31691
           Summary: Reporting of predicted benefits or vectorisation
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: drraph at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

When running the Intel Compiler with -qopt-report=4, say, you get told the
expected performance gain from vectorisation, amongst other useful information.
For example,

 LOOP BEGIN at permanent-in-c.c(47,7)
      remark #25045: Fused Loops: ( 47 50 )

      remark #15388: vectorization support: reference v has aligned access   [
permanent-in-c.c(48,2) ]
      remark #15388: vectorization support: reference v has aligned access   [
permanent-in-c.c(48,2) ]
      remark #15389: vectorization support: reference M has unaligned access  
[ permanent-in-c.c(48,2) ]
      remark #15388: vectorization support: reference v has aligned access   [
permanent-in-c.c(51,8) ]
      remark #15381: vectorization support: unaligned access used inside loop
body
      remark #15305: vectorization support: vector length 4
      remark #15399: vectorization support: unroll factor set to 2
      remark #15309: vectorization support: normalized vectorization overhead
0.600
      remark #15301: FUSED LOOP WAS VECTORIZED
      remark #15442: entire loop may be executed in remainder
      remark #15448: unmasked aligned unit stride loads: 1 
      remark #15449: unmasked aligned unit stride stores: 1 
      remark #15450: unmasked unaligned unit stride loads: 1 
      remark #15475: --- begin vector loop cost summary ---
      remark #15476: scalar loop cost: 49 
      remark #15477: vector loop cost: 10.000 
      remark #15478: estimated potential speedup: 4.580 
      remark #15487: type converts: 3 
      remark #15488: --- end vector loop cost summary ---
      remark #25456: Number of Array Refs Scalar Replaced In Loop: 2
   LOOP END

gcc also gives you similar if perhaps less useful information.  

[...]
vect_model_reduction_cost: inside_cost = 6, prologue_cost = 2, epilogue_cost =
6 .
test2.c:50:9: note: ==> examining statement: j_73 = j_186 + 1;
test2.c:50:9: note: irrelevant.
test2.c:50:9: note: ==> examining statement: if (j_73 < n.2_201)
test2.c:50:9: note: irrelevant.
test2.c:50:9: note: === vect_update_slp_costs_according_to_vf ===
test2.c:50:9: note: cost model: epilogue peel iters set to vf/2 because loop
iterations are unknown .
test2.c:50:9: note: Cost model analysis: 
  Vector inside of loop cost: 50
  Vector prologue cost: 8
  Vector epilogue cost: 52
  Scalar iteration cost: 20
  Scalar outside cost: 4
  Vector outside cost: 60
  prologue iterations: 0
  epilogue iterations: 2
  Calculated minimum iters for profitability: 5
test2.c:50:9: note:   Runtime profitability threshold = 4
test2.c:50:9: note:   Static estimate profitability threshold = 4
test2.c:50:9: note: epilog loop required

[...]


It would be great if clang/llvm could provide similar information to the
user/coder.

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