<div dir="ltr"><div><div><div>Back to the discussion on the RFC, I still see some advantage in following the proposed solution. I see two paths forward:<br><br></div>1-
Change canonical form, possibly lower memcpy to non-integer load and
store in InstCombine. Then teach the backends to convert that to integer
load and store if that is more profitable. Notice that we are talking
about loads that have no use other than store. So it is a fairly simple
change in the backends.<br><br></div>2- Do not change the canonical
form. Then for this bug, we need to teach select speculation to see
through bitcasts. We will probably need to teach other optimizations to
see though bitcasts in the future as problems are uncovered. That is
until typeless pointer work is complete. Once the typeless pointer work
is complete, we have some extra code in each optimization for seeing
through bitcasts which is possibly no longer needed.<br><br></div><div>Based
on this I think (1) is the right thing to do. But there might be other
reasons for the current canonical form that I am not aware of. Please
let me know what you think.<br><br></div><div>Thanks<br></div>Ehsan<br></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Mar 16, 2016 at 2:13 PM, David Blaikie <span dir="ltr"><<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote"><span class="">On Wed, Mar 16, 2016 at 11:00 AM, Ehsan Amiri <span dir="ltr"><<a href="mailto:ehsanamiri@gmail.com" target="_blank">ehsanamiri@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div><div>David,<br><br></div>Could you give us an update on the status of typeless pointer work? How much work is left and when you think it might be ready?<br></div></div></div></blockquote><div><br></div></span><div>It's a bit of an onion peel, really - since it will eventually involve generalizing/fixing every optimization that's currently leaning on typed pointers to keep the performance while removing the crutch they're currently leaning on. (in cases where bitcasts are literally just getting in the way, those won't require cleaning up & should just become "free performance wins" once we remove them, though)<br><br>At the moment we can roundtrip every LLVM IR test case through bitcode and textual IR (reading/writing both formats) while using only a narrow whitelist of places that request the type of a pointer (things like the verifier, the parser/printer where it actually needs the typed pointer to verify it matches the explicit type, etc).<br><br>The next thing on the list is probably figuring out the byval/inalloca representation (add an explicit pointee type? just make the number of bytes explicit with no type information?).<br><br>Then start migrating optimizations over - doing the same sort of testing I did for the IR/bitcode roundtripping - assert that the pointee type is not accessed, whitelist places that need it until the bitcasts go away, fix anything else... it'll still be a fair bit of work & I don't really know how much. It should parallelize pretty well (doing any of this work is really helpful, each optimization is indepednent, etc) if anyone wants to/is able to help.<br><br>- Dave</div><div><div class="h5"><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><div><br></div>Thanks<span><font color="#888888"><br></font></span></div><span><font color="#888888">Ehsan<br><br></font></span></div><div><div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Mar 16, 2016 at 1:12 PM, David Blaikie <span dir="ltr"><<a href="mailto:dblaikie@gmail.com" target="_blank">dblaikie@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote"><span>On Wed, Mar 16, 2016 at 8:34 AM, Mehdi Amini via llvm-dev <span dir="ltr"><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word">Hi,<div><br></div><div>How do it interact with the "typeless pointers" work?</div></div></blockquote><div><br></div></span><div>Right - the goal of the typeless pointer work is to fix all these bugs related to "didn't look through bitcasts" in optimizations. Sometimes that's going to mean more work (because the code is leaning on the absence of bitcasts & the presence of convenient (but not guaranteed) type information to inform optimization decisions) but if we remove typed pointers while keeping optimization quality in the cases we have today, then we should've also fixed the cases that were broken because the type information didn't end up aligning to produce the optimal output.<br><br>& I know I've been off the typeless pointer stuff for a few months working on llvm-dwp - but happy for any help (the next immediate piece is probably figuring out teh right representation for byval and inalloca - there were some code reviews sent out for that that I'll need to come back around to - but also any optimizations people want to help rework/improve would be great too & I can provide some techniques/tools to help people approach those)<br><br>- Dave</div><div><div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><div><br></div><div>Thanks,</div><div><br></div><div>-- </div><div>Mehdi</div><div><br><div><blockquote type="cite"><div><div><div>On Mar 16, 2016, at 6:41 AM, Ehsan Amiri via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:</div><br></div></div><div><div><div><div dir="ltr">=== PROBLEM === (See this bug <a href="https://llvm.org/bugs/show_bug.cgi?id=26445" target="_blank">https://llvm.org/bugs/show_bug.cgi?id=26445</a>)<br><br>IR contains code for loading a float from float * and storing it to a float * address. After canonicalization of load in InstCombine [1], new bitcasts are added to the IR (see bottom of the email for code samples). This prevents select speculation in SROA to work. Also after SROA we have bitcasts from int32 to float. (Whereas originally after instCombine, bitcasts are only done on pointer types).<br><br>=== PROPOSED SOLUTION===<br><br>[1] implies that we need load canonicalization when we load a value only to store it again. The reason is to avoid generating slightly different code (due to different ways of adding bitcasts), in different situations. In all examples presented in [1] there is a non-zero number of bitcasts. I think when we load a value of type T from a T* address and store it as a type T value to one or more T* address (and there is no other use or store), we can redefine canonical form to mean there should not be any bitcasts. So we still have a canonical form, but its definition is slightly different.<br><br>=== REASONS FOR / AGAINST===<br><br>Hal Finkel warns that while this may be useful for power pc, this may hurt more than one other platform and become a very large project. Despite this he is fine with bringing up the issue to the mailing list to get feedback, mostly because this seems inline with our future direction of having a unique type for all pointers. (Hal please correct me if I misunderstood your comment)<br><br>This is a much simpler fix compared to alternatives. (ignoring potential regressions)<br><br>=== ALTERNATIVE SOLUTION ===<br><br>Fix select speculation in SROA to see through bitcasts. Handle remaining bitcasts during code gen. Other alternative solutions are welcome.<br><br>Should I implement the proposed solution or is it too risky? I understand that we may need to undo it if it breaks too many things. Comments are welcome.<br><br><br>[1] <a href="http://lists.llvm.org/pipermail/llvm-dev/2015-January/080956.html" target="_blank">http://lists.llvm.org/pipermail/llvm-dev/2015-January/080956.html</a> r226781 git commit id: b778cbc0c8<br><br><br><br>Code Samples (only relevant part is copied):<br><br>-------------------- Before Canonicalization (contains call to std::max): -------------------- <br>entry:<br> %max_value = alloca float, align 4<br> %1 = load float, float* %input, align 4, !tbaa !1<br> store float %1, float* %max_value, align 4, !tbaa !1<br><br>for.body:<br> %call = call dereferenceable(4) float* @_ZSt3maxIfERKT_S2_S2_(float* dereferenceable(4) %max_value, float* dereferenceable(4) %arrayidx1)<br> %3 = load float, float* %call, align 4, !tbaa !1<br> store float %3, float* %max_value, align 4, !tbaa !1<br><br>-------------------- After Canonicalization (contains call to std::max):-------------------- <br><br>entry:<br> %max_value = alloca float, align 4<br> %1 = bitcast float* %input to i32*<br> %2 = load i32, i32* %1, align 4, !tbaa !1<br> %3 = bitcast float* %max_value to i32*<br> store i32 %2, i32* %3, align 4, !tbaa !1<br><br>for.body:<br> %call = call dereferenceable(4) float* @_ZSt3maxIfERKT_S2_S2_(float* nonnull dereferenceable(4) %max_value, float* dereferenceable(4) %arrayidx1)<br> %5 = bitcast float* %call to i32*<br> %6 = load i32, i32* %5, align 4, !tbaa !1<br> %7 = bitcast float* %max_value to i32*<br> store i32 %6, i32* %7, align 4, !tbaa !1<br><br>-------------------- After SROA (the call to std::max is inlined now):-------------------- <br>entry:<br> %max_value.sroa.0 = alloca i32<br> %0 = bitcast float* %input to i32*<br> %1 = load i32, i32* %0, align 4, !tbaa !1<br> store i32 %1, i32* %max_value.sroa.0<br><br>for.body: <br> %max_value.sroa.0.0.max_value.sroa.0.0.6 = load i32, i32* %max_value.sroa.0<br> %3 = bitcast i32 %max_value.sroa.0.0.max_value.sroa.0.0.6 to float<br> %max_value.sroa.0.0.max_value.sroa_cast8 = bitcast i32* %max_value.sroa.0 to float*<br> %__b.__a.i = select i1 %cmp.i, float* %arrayidx1, float* %max_value.sroa.0.0.max_value.sroa_cast8<br> %5 = bitcast float* %__b.__a.i to i32*<br> %6 = load i32, i32* %5, align 4, !tbaa !1<br> store i32 %6, i32* %max_value.sroa.0<br><br>-------------------- After SROA when Canonicalization is turned off-------------------- <br>entry:<br> %0 = load float, float* %input, align 4, !tbaa !1<br><br>for.cond: ; preds = %for.body, %entry<br> %max_value.0 = phi float [ %0, %entry ], [ %.sroa.speculated, %for.body ]<br><br>for.body:<br> %1 = load float, float* %arrayidx1, align 4, !tbaa !1<br> %cmp.i = fcmp olt float %max_value.0, %1<br> %.sroa.speculate.load.true = load float, float* %arrayidx1, align 4, !tbaa !1<br> %.sroa.speculated = select i1 %cmp.i, float %.sroa.speculate.load.true, float %max_value.0<br></div></div></div>
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