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  1. Apr 30, 2013
  2. Apr 29, 2013
    • Arnold Schwaighofer's avatar
      SimplifyCFG: If convert single conditional stores · 474df6d3
      Arnold Schwaighofer authored
      This resurrects r179957, but adds code that makes sure we don't touch
      atomic/volatile stores:
      
      This transformation will transform a conditional store with a preceeding
      uncondtional store to the same location:
      
       a[i] =
       may-alias with a[i] load
       if (cond)
         a[i] = Y
      
      into an unconditional store.
      
       a[i] = X
       may-alias with a[i] load
       tmp = cond ? Y : X;
       a[i] = tmp
      
      We assume that on average the cost of a mispredicted branch is going to be
      higher than the cost of a second store to the same location, and that the
      secondary benefits of creating a bigger basic block for other optimizations to
      work on outway the potential case where the branch would be correctly predicted
      and the cost of the executing the second store would be noticably reflected in
      performance.
      
      hmmer's execution time improves by 30% on an imac12,2 on ref data sets. With
      this change we are on par with gcc's performance (gcc also performs this
      transformation). There was a 1.2 % performance improvement on a ARM swift chip.
      Other tests in the test-suite+external seem to be mostly uninfluenced in my
      experiments:
      This optimization was triggered on 41 tests such that the executable was
      different before/after the patch. Only 1 out of the 40 tests (dealII) was
      reproducable below 100% (by about .4%). Given that hmmer benefits so much I
      believe this to be a fair trade off.
      
      llvm-svn: 180731
      474df6d3
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