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Release Notes: add a description of -enable-iv-rewrite.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143878 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -834,6 +834,29 @@ Builder.CreateResume(UnwindData);
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</div>
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<!--=========================================================================-->
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<h3>
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<a name="loopoptimization">Loop Optimization Improvements</a>
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</h3>
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<div>
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<p>The induction variable simplification pass in 3.0 only modifies
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induction variables when profitable. Sign and zero extension
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elimination, linear function test replacement, loop unrolling, and
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other simplifications that require induction variable analysis have
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been generalized so they no longer require loops to be rewritten in a
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typically suboptimal form prior to optimization. This new design
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preserves more IR level information, avoids undoing earlier loop
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optimizations (particularly hand-optimized loops), and no longer
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strongly depends on the code generator rewriting loops a second time
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in a now optimal form--an intractable problem.</p>
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<p>The original behavior can be restored with -mllvm -enable-iv-rewrite;
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however, support for this mode will be short lived. As such, bug
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reports should be filed for any significant performance regressions
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when moving from -mllvm -enable-iv-rewrite to the 3.0 default mode.</p>
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</div>
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<!--=========================================================================-->
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<h3>
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<a name="optimizer">Optimizer Improvements</a>
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