<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="">Hi Preston,<div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 8, 2015, at 10:56 PM, Preston Briggs via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">When I compile two different modules using<div class=""><br class=""></div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px" class=""><div class=""><font face="monospace, monospace" class="">clang -O -S -emit-llvm</font></div></blockquote><div class=""><br class=""></div><div class="">I get different .ll files, no surprise.</div><div class=""><br class=""></div><div class="">The first looks like</div><div class=""><br class=""></div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px" class=""><div class=""><div class=""><font face="monospace, monospace" class="">double *v;</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">double zap(long n) {</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> double sum = 0;</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> for (long i = 0; i < n; i++)</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> sum += v[i];</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> return sum;</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">}</font></div></div></blockquote><div class=""><br class=""></div><div class="">yielding</div><div class=""><br class=""></div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px" class=""><div class=""><div class=""><font face="monospace, monospace" class="">@v = common global double* null, align 8</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">; Function Attrs: nounwind readonly uwtable</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">define double @zap(i64 %n) #0 {</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">entry:</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %cmp4 = icmp sgt i64 %n, 0</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> br i1 %cmp4, label %<a href="http://for.body.lr.ph/" class="">for.body.lr.ph</a>, label %for.end</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><a href="http://for.body.lr.ph/" class="">for.body.lr.ph</a>: ; preds = %entry</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %0 = load double** @v, align 8, !tbaa !1</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> br label %for.body</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">for.body: ; preds = %for.body, %<a href="http://for.body.lr.ph/" class="">for.body.lr.ph</a></font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %i.06 = phi i64 [ 0, %<a href="http://for.body.lr.ph/" class="">for.body.lr.ph</a> ], [ %inc, %for.body ]</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %sum.05 = phi double [ 0.000000e+00, %<a href="http://for.body.lr.ph/" class="">for.body.lr.ph</a> ], [ %add, %for.body ]</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %arrayidx = getelementptr inbounds double* %0, i64 %i.06</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %1 = load double* %arrayidx, align 8, !tbaa !5</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %add = fadd double %sum.05, %1</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %inc = add nsw i64 %i.06, 1</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> </font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">%exitcond = icmp eq i64 %inc, %n</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> br i1 %exitcond, label %for.end, label %for.body</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">for.end: ; preds = %for.body, %entry</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %sum.0.lcssa = phi double [ 0.000000e+00, %entry ], [ %add, %for.body ]</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> ret double %sum.0.lcssa</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">}</font></div></div></blockquote><div class=""><br class=""></div><div class="">and the second looks like</div><div class=""><br class=""></div><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px" class=""><div class=""><div class=""><font face="monospace, monospace" class="">double v[10000];</font></div></div><div class=""><div class=""><br class=""></div></div></blockquote><blockquote style="margin:0px 0px 0px 40px;border:none;padding:0px" class=""><div class=""><div class=""><font face="monospace, monospace" class="">double zap(long n) {</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> double sum = 0;</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> for (long i = 0; i < n; i++)</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> sum += v[i];</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> return sum;</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">}</font></div></div></blockquote><div class=""><br class=""></div><div class="">yielding</div><div class=""><br class=""></div><blockquote style="margin:0 0 0 40px;border:none;padding:0px" class=""><div class=""><div class=""><font face="monospace, monospace" class="">; ModuleID = 'z.c'</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-f128:128:128-n8:16:32:64-S128"</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">target triple = "x86_64-unknown-linux-gnu"</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">@v = common global [10000 x double] zeroinitializer, align 16</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">; Function Attrs: nounwind readonly uwtable</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">define double @zap(i64 %n) #0 {</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">entry:</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %cmp4 = icmp sgt i64 %n, 0</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> br i1 %cmp4, label %for.body, label %for.end</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">for.body: ; preds = %entry, %for.body</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %i.06 = phi i64 [ %inc, %for.body ], [ 0, %entry ]</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %sum.05 = phi double [ %add, %for.body ], [ 0.000000e+00, %entry ]</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %arrayidx = getelementptr inbounds [10000 x double]* @v, i64 0, i64 %i.06</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %0 = load double* %arrayidx, align 8, !tbaa !1</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %add = fadd double %sum.05, %0</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %inc = add nsw i64 %i.06, 1</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %exitcond = icmp eq i64 %inc, %n</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> br i1 %exitcond, label %for.end, label %for.body</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">for.end: ; preds = %for.body, %entry</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> %sum.0.lcssa = phi double [ 0.000000e+00, %entry ], [ %add, %for.body ]</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""> ret double %sum.0.lcssa</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">}</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">attributes #0 = { nounwind readonly uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">!llvm.ident = !{!0}</font></div></div><div class=""><div class=""><font face="monospace, monospace" class=""><br class=""></font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">!0 = metadata !{metadata !"Clang Front-End version 3.4.1 (tags/RELEASE_34/final)"}</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">!1 = metadata !{metadata !2, metadata !2, i64 0}</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">!2 = metadata !{metadata !"double", metadata !3, i64 0}</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">!3 = metadata !{metadata !"omnipotent char", metadata !4, i64 0}</font></div></div><div class=""><div class=""><font face="monospace, monospace" class="">!4 = metadata !{metadata !"Simple C/C++ TBAA"}</font></div></div></blockquote><div class=""><br class=""></div><div class="">(I included all the metadata and such for the 2nd case, on the off chance it matters.)</div><div class=""><br class=""></div><div class="">Is there any way I can convince licm (or something) to rip open the GEP and hoist the reference to @v outside the loop, similar to the first example?</div></div></div></blockquote><div><br class=""></div></div><br class=""><div class="">I believe that in the second case, there is no need to load the address of v as it is constant. However you have a constant address to an array, which is represented by [10000 x double]* @v in the IR, which requires to use the two-level GEP. </div></div><div class=""><br class=""></div><div class="">You “could” manage to represent it this way:</div><div class=""><br class=""></div><div class="">define double @zap(i64 %n) #0 {<br class="">entry:<br class=""> %cmp6 = icmp sgt i64 %n, 0<br class=""> %hoisted = bitcast [10000 x double]* @v to double*<br class=""> br i1 %cmp6, label %for.body.preheader, label %for.cond.cleanup<br class=""><br class="">for.body.preheader: ; preds = %entry<br class=""> br label %for.body<br class=""><br class="">for.cond.cleanup.loopexit: ; preds = %for.body<br class=""> %add.lcssa = phi double [ %add, %for.body ]<br class=""> br label %for.cond.cleanup<br class=""><br class="">for.cond.cleanup: ; preds = %for.cond.cleanup.loopexit, %entry<br class=""> %sum.0.lcssa = phi double [ 0.000000e+00, %entry ], [ %add.lcssa, %for.cond.cleanup.loopexit ]<br class=""> ret double %sum.0.lcssa<br class=""><br class="">for.body: ; preds = %for.body.preheader, %for.body<br class=""> %i.08 = phi i64 [ %inc, %for.body ], [ 0, %for.body.preheader ]<br class=""> %sum.07 = phi double [ %add, %for.body ], [ 0.000000e+00, %for.body.preheader ]<br class=""> %arrayidx = getelementptr double, double* %hoisted, i64 %i.08<br class=""> %0 = load double, double* %arrayidx, align 8, !tbaa !2<br class=""> %add = fadd double %sum.07, %0<br class=""> %inc = add nuw nsw i64 %i.08, 1<br class=""> %exitcond = icmp eq i64 %inc, %n<br class=""> br i1 %exitcond, label %for.cond.cleanup.loopexit, label %for.body<br class="">}</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">However instcombine will recanonicalize it like it was originally.</div><div class=""><br class=""></div><div class="">Since it is a GEP that operate on a constant address, this shouldn’t matter, why would you want to split this? </div><div class=""><br class=""></div><div class="">Best,</div><div class=""><br class=""></div><div class="">— </div><div class="">Mehdi</div><div class=""><br class=""></div></body></html>