<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=""><br class=""><div><blockquote type="cite" class=""><div class="">On Dec 9, 2015, at 7:58 AM, Preston Briggs <<a href="mailto:briggs@reservoir.com" class="">briggs@reservoir.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">I'm trying to make the IR "better", in a machine-independent fashion,<div class="">without having to do any lowering.</div></div></div></blockquote><div><br class=""></div><div><div>The question is “would the IR be more canonical” with the representation you suggest? Why would the optimizer benefit from this representation instead of the current one in general?</div></div><div>Right now this GEP reads as an offset from a constant global, which seems pretty optimal to me.</div><div><br class=""></div><div><div><div>My impression is that when you reach a point where the “better” is target specific, this is part of the lowering (I’m using lowering in the sense that you go away from the canonical representation the optimizer expects). I believe it is pretty common that targets need to do this kind of lowering.</div><div><br class=""></div></div></div><div>— </div><div>Mehdi</div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">I've written code that rewrites GEPs as simple adds and multiplies,</div><div class="">which helps a lot, but there's still some sort of re-canonicalization</div><div class="">that's getting in my way. Is there perhaps a way to suppress it?</div></div></div></blockquote><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Preston</div><div class=""><br class=""></div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Wed, Dec 9, 2015 at 7:47 AM, Mehdi Amini <span dir="ltr" class=""><<a href="mailto:mehdi.amini@apple.com" target="_blank" class="">mehdi.amini@apple.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="">I guess is has to be done as part of the lowering for such a target, either during CodegenPrepare or during something like MachineLICM.<div class=""><br class=""></div><div class="">— </div><span class="HOEnZb"><font color="#888888" class=""><div class="">Mehdi</div></font></span><div class=""><div class="h5"><div class=""><br class=""></div><div class=""><div class=""><br class=""></div><div class=""><br class=""><div class=""><blockquote type="cite" class=""><div class="">On Dec 9, 2015, at 7:13 AM, Preston Briggs <<a href="mailto:briggs@reservoir.com" target="_blank" class="">briggs@reservoir.com</a>> wrote:</div><br class=""><div class=""><div dir="ltr" class="">On some targets with limited addressing modes,<div class="">getting that 64-bit relocatable but loop-invariant value into a register</div><div class="">requires several instructions. I'd like those several instruction outside</div><div class="">the loop, where they belong.</div><div class=""><br class=""></div><div class="">Yes, my experience is that something (I assume instcombine) recanonicalizes.</div><div class=""><br class=""></div><div class="">Thanks,</div><div class="">Preston</div><div class=""><br class=""></div></div><div class="gmail_extra"><br class=""><div class="gmail_quote">On Tue, Dec 8, 2015 at 11:21 PM, Mehdi Amini <span dir="ltr" class=""><<a href="mailto:mehdi.amini@apple.com" target="_blank" class="">mehdi.amini@apple.com</a>></span> wrote:<br class=""><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class="">Hi Preston,<div class=""><div class=""><div class=""><br class=""><div class=""><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" target="_blank" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class=""><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/" target="_blank" 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/" target="_blank" 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/" target="_blank" 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/" target="_blank" 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/" target="_blank" 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 class=""><br class=""></div></div><br class=""></div></div><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=""><span class="">define double @zap(i64 %n) #0 {<br class="">entry:<br class=""></span> %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<span class=""><br class=""> %exitcond = icmp eq i64 %inc, %n<br class=""></span> 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><span class=""><font color="#888888" class=""><div class="">Mehdi</div><div class=""><br class=""></div></font></span></div></blockquote></div><br class=""></div>
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