<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi all,<div class=""><br class=""></div><div class="">For my custom architecture, I want to relax the CFG simplification pass, and any other passes replacing conditional branches.</div><div class=""><br class=""></div><div class="">I found that the replacement of conditional branches by “select" and other instructions is often too aggressive, and this causes inefficient code for my target as in most cases branches would be cheaper.</div><div class=""><br class=""></div><div class="">For example, considering the following c code:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""><span style="color: #ba2da2" class="">long</span> test (<span style="color: #ba2da2" class="">long</span> a, <span style="color: #ba2da2" class="">long</span> b)</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class="">{</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> <span style="color: #ba2da2" class="">int</span> neg = <span style="color: #272ad8" class="">0</span>;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> <span style="color: #ba2da2" class="">long</span> res;</div><div style="margin: 0px; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> <span style="color: #ba2da2" class="">if</span> (a < <span style="color: #272ad8" class="">0</span>) </div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> {</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> a = -a; </div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> neg = <span style="color: #272ad8" class="">1</span>;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> }</div><div style="margin: 0px; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; color: rgb(0, 132, 0); background-color: rgb(255, 255, 255);" class=""><span style="color: #000000" class=""> res = a*b;</span></div><div style="margin: 0px; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> <span style="color: #ba2da2" class="">if</span> (neg)</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> res = -res;</div><div style="margin: 0px; line-height: normal; background-color: rgb(255, 255, 255); min-height: 14px;" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""> <span style="color: #ba2da2" class="">return</span> res;</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class="">}</div></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""><br class=""></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""><span style="font-family: Helvetica; font-size: 12px;" class="">This code can be simplified in c, but it’s just an example to show the point.</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""><span style="font-family: Helvetica; font-size: 12px;" class=""><br class=""></span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""><span style="font-family: Helvetica; font-size: 12px;" class="">The code above gets compiled like this (-Oz flag):</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Monaco; background-color: rgb(255, 255, 255);" class=""><span style="font-family: Helvetica; font-size: 12px;" class=""><br class=""></span></div><div style="margin: 0px; line-height: normal; background-color: rgb(255, 255, 255);" class=""><div style="margin: 0px; line-height: normal;" class=""><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class="">; Function Attrs: minsize norecurse nounwind optsize readnone</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class="">define dso_local i32 @test(i32 %a, i32 %b) local_unnamed_addr #0 {</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class="">entry:</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""> %cmp = icmp slt i32 %a, 0</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""> %sub = sub nsw i32 0, %a</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""> %a.addr.0 = select i1 %cmp, i32 %sub, i32 %a</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""> %mul = mul nsw i32 %a.addr.0, %b</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""> %sub2 = sub nsw i32 0, %mul</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""> %res.0 = select i1 %cmp, i32 %sub2, i32 %mul</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""> ret i32 %res.0</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class="">}</div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""><br class=""></div><div style="font-family: Monaco; font-size: 11px; margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">All branching was removed and replaced by ‘select’ instructions. For my architecture, it would be desirable to keep the original branches in most cases, because even simple 32 bit operations are too expensive to speculatively execute them, and branches are cheap.</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">Setting 'phi-node-folding-threshold’ to 1 or even 0 (instead of the default 2), definitely improves the situation in many cases, but Clang still creates many instances of ‘select’ instructions, which are detrimental to my target. I am unsure about where are they created, as I believe that the simplifycfg pass does not longer create them. </div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">So the question is: Are there any other hooks in clang, or custom code that I can implement, to relax the creation of ’select’ instructions and make it preserve branches in the original c code?</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">Thanks,</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">John</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div></div></div></div>_______________________________________________<br class="">LLVM Developers mailing list<br class=""><a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a><br class="">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<br class=""></body></html>