<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>Hi,</div><div><br></div><div>Here is a patch to help llvm turning a code like this:</div><div>if (foo)</div><div> free(foo)</div><div><br></div><div>into that:</div><div>free(foo)</div><div><br></div><div>It is legal and safe, since free is already checking its argument against NULL internally (I may find the reference in the Standard if needed :)).</div><div><br></div><div>The proposed patch moves a call to free from basic block FB into FB's predecessor, P, when the path from P to FB is taken only if the argument of free is not equal to NULL.</div><div><br></div><div>It also has more restriction on P and FB to be sure that this code motion is profitable. Namely:</div><div>1. FB must have only one predecessor P</div><div>2. FB must contain only the call to free plus an unconditional branch to S</div><div>3. P successors are FB and S.</div><div><br></div><div>Because of 1., we will not increase the code size when moving the call to free from FB to P.</div><div>Because of 2., FB will be empty after the move.</div><div>Because of 2. and 3., P's branch instruction becomes useless, so as FB (simplifycfg will do the job).</div><div><br></div><div>We can extend this optimization if we are able to derive a good cost/profitability model. Currently, I do not have anything in mind, so I've kept it simple.</div><div><br></div><div>The patch also contains a testcase just for the code motion part, not the total simplification.</div><br><div apple-content-edited="true">
<div style="color: rgb(0, 0, 0); font-family: Helvetica; font-size: medium; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">-Quentin</div>
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