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<p class="MsoNormal">Hello,<br>
<br>
The Machine LICM pass has this comment:<o:p></o:p></p>
<p class="MsoNormal">// If this is done before regalloc, only visit outer-most preheader-sporting<o:p></o:p></p>
<p class="MsoNormal">// loops.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I was wondering why there is such a restriction? This prevents hoisting from the deeply-nested inner loops into the outer loop body, when the hoisted instruction is loop invariant everywhere except the outermost loop. I’ve encountered a
problem that would be solved by such hoisting, however I couldn’t figure out the rationale behind LLVM’s MLICM decision. This change appears to be introduced in 2009 (<a href="https://github.com/llvm/llvm-project/commit/79618d1de89e76b1a23a02e8146057a6a21260db">https://github.com/llvm/llvm-project/commit/79618d1de89e76b1a23a02e8146057a6a21260db</a>)
but the only reason stated in the commit message seems to be just simplification/performance related.<br>
<br>
Has anyone tried enabling MLICM for nested loops? Is it a good idea? What are the possible issues?<br>
<br>
Thanks<o:p></o:p></p>
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<p class="MsoNormal">--<o:p></o:p></p>
<p class="MsoNormal">Danila<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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