[LLVMbugs] [Bug 18773] New: LICM sinking a single unused load out of a loop prevents DSE in some other function!?!

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18773

            Bug ID: 18773
           Summary: LICM sinking a single unused load out of a loop
                    prevents DSE in some other function!?!
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: chandlerc at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 12031
  --> http://llvm.org/bugs/attachment.cgi?id=12031&action=edit
reduced test case

This is crazy weird. The essential problem is that LICM got slightly more
powerful in r200067 and started sinking loads out of the loop body in a corner
case. When it does so, we mysteriously stop being able to DSE stores in *a
separate function*!

The code for this is heavily reduced from Adobe-C++/loop_unroll.cpp. To
reproduce the weird behavior, take an "opt" binary from trunk and "old opt"
from r200066, and compare:

% opt < loop_unroll.reduced.ll -std-link-opts | llc -O3 -o loop_unroll.new.s
% clang++ -lm -o loop_unroll.new loop_unroll.new.s

vs.

% old_opt < loop_unroll.reduced.ll -std-link-opts | llc -O3 -o
loop_unroll.old.s
% clang++ -lm -o loop_unroll.old loop_unroll.old.s


When I benchmark these on my sandybridge machine I get:

% perf stat -r5 ./loop_unroll.new && perf stat -r5 ./loop_unroll.old

 Performance counter stats for './loop_unroll.new' (5 runs):

       1376.720033 task-clock                #    0.998 CPUs utilized          
 ( +-  0.10% )
                 1 context-switches          #    0.001 K/sec                  
 ( +- 31.62% )
                 0 cpu-migrations            #    0.000 K/sec                  
 ( +- 61.24% )
               152 page-faults               #    0.111 K/sec                  
 ( +-  0.16% )
     5,208,493,602 cycles                    #    3.783 GHz                    
 ( +-  0.02% )
     2,657,152,524 stalled-cycles-frontend   #   51.02% frontend cycles idle   
 ( +-  0.04% )
       279,081,881 stalled-cycles-backend    #    5.36% backend  cycles idle   
 ( +-  0.34% )
     8,515,758,351 instructions              #    1.63  insns per cycle        
                                             #    0.31  stalled cycles per insn
 ( +-  0.00% )
       356,723,894 branches                  #  259.111 M/sec                  
 ( +-  0.00% )
           201,373 branch-misses             #    0.06% of all branches        
 ( +-  0.01% )

       1.378844222 seconds time elapsed                                        
 ( +-  0.10% )


 Performance counter stats for './loop_unroll.old' (5 runs):

        877.683533 task-clock                #    0.998 CPUs utilized          
 ( +-  0.09% )
                 1 context-switches          #    0.001 K/sec                  
 ( +- 40.82% )
                 0 cpu-migrations            #    0.000 K/sec                  
 ( +-100.00% )
               152 page-faults               #    0.174 K/sec                  
 ( +-  0.16% )
     3,320,502,190 cycles                    #    3.783 GHz                    
 ( +-  0.00% )
        11,331,992 stalled-cycles-frontend   #    0.34% frontend cycles idle   
 ( +-  2.39% )
        35,003,292 stalled-cycles-backend    #    1.05% backend  cycles idle   
 ( +-  1.14% )
     6,978,870,054 instructions              #    2.10  insns per cycle        
                                             #    0.01  stalled cycles per insn
 ( +-  0.00% )
       356,371,371 branches                  #  406.036 M/sec                  
 ( +-  0.00% )
           200,790 branch-misses             #    0.06% of all branches        
 ( +-  0.04% )

       0.879302485 seconds time elapsed                                        
 ( +-  0.09% )


Note the 50% stalled cycles on the new one!!!


I'm working on getting two A/B inputs to trunk 'opt' that exhibit the behavior,
lacking any good ideas about why its actually happening.


Note that I've checked -- top of tree and r200067 behave exactly the same. The
change is only in the patch committed with r200067.

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