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<div>Thanks all for replies. </div>
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<div>Does LLVM have a separate VBE pass? I couldn't find one. Although VBE is primarily for a code size, I see a case that VBE can affect following optimizations to reduce redundant moves in the binary. </div>
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<div>Daniel, great to hear that you are working on a more advanced GVN implementation. I wonder if you have a timeline to upstream yours. </div>
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<div>Best,</div>
<div>Taewook</div>
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<span style="font-weight:bold">From: </span>Daniel Berlin <<a href="mailto:dberlin@dberlin.org">dberlin@dberlin.org</a>><br>
<span style="font-weight:bold">Date: </span>Tuesday, February 9, 2016 at 12:28 PM<br>
<span style="font-weight:bold">To: </span>Taewook Oh <<a href="mailto:twoh@fb.com">twoh@fb.com</a>><br>
<span style="font-weight:bold">Cc: </span>mats petersson <<a href="mailto:mats@planetcatfish.com">mats@planetcatfish.com</a>>, "<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>" <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [llvm-dev] [GVN] same sequence of instructions in if and else branch<br>
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<div dir="ltr">and by "right thing" i mean it can hoist if you want and it can prove it will not extend the live range.
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<div>Note that VBE (very busy expressions) is a code size optimization only. It does not save time.</div>
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<div class="gmail_quote">On Tue, Feb 9, 2016 at 12:26 PM, Daniel Berlin <span dir="ltr">
<<a href="mailto:dberlin@dberlin.org" target="_blank">dberlin@dberlin.org</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">This GVN does not do that, this is correct. It is a very simple GVN. All phi nodes are considered to have unique values.
<div>GCC's GVN, and the one i'm developing for LLVM at <a href="https://github.com/dberlin/llvm-gvn-rewrite" target="_blank">
https://github.com/dberlin/llvm-gvn-rewrite</a> does the right thing.</div>
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<div class="gmail_quote">On Tue, Feb 9, 2016 at 11:42 AM, Taewook Oh 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>
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<div>There is a phi-node "%phi = phi i64 [%cast1, %if], [%cast2, %else]" in the common successor. The actual control flow is a bit more complex, but there is a common successor block, and %cast1 and %cast2 are the two values that the phi node in the common
successor takes.</div>
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<div>Does the existence of the phi node affect optimization?</div>
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<div>Thanks,</div>
<div>Taewook</div>
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<span style="font-weight:bold">From: </span><<a href="mailto:mats.o.petersson@googlemail.com" target="_blank">mats.o.petersson@googlemail.com</a>> on behalf of mats petersson <<a href="mailto:mats@planetcatfish.com" target="_blank">mats@planetcatfish.com</a>><br>
<span style="font-weight:bold">Date: </span>Tuesday, February 9, 2016 at 11:34 AM<br>
<span style="font-weight:bold">To: </span>Taewook Oh <<a href="mailto:twoh@fb.com" target="_blank">twoh@fb.com</a>><br>
<span style="font-weight:bold">Cc: </span>"<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br>
<span style="font-weight:bold">Subject: </span>Re: [llvm-dev] [GVN] same sequence of instructions in if and else branch<br>
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<div>And there's no PHI node after this that depends on the condition?<br>
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Mats<br>
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<div class="gmail_quote">On 9 February 2016 at 19:30, Taewook Oh 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>
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<p><span>Hello, </span></p>
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<p><span>I found that GVN doesn't promote identical sequence of instructions in if and else branch to their common predecessors. For example, for the following code snippet</span></p>
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<p><span>pred:</span></p>
<p><span><span></span>…</span></p>
<p><span><span></span>br i1 %cmp, label %if, label %else</span></p>
<p><span>if:</span></p>
<p><span><span></span>%incdec.ptr.1 = getelementptr inbounds i8, i8* %ptr, i64 1</span></p>
<p><span><span></span>%cast1 = ptrtoint i8* %incdec.ptr.1 to i64</span></p>
<p><span> <span> </span>…</span></p>
<p><span>else:</span></p>
<p><span><span></span>%incdec.ptr.2 = getelementptr inbounds i8, i8* %ptr, i64 1</span></p>
<p><span><span></span>%cast2 = ptrtoint i8* %incdec.ptr.2 to i64</span></p>
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<p><span>GVN doesn't move instructions in 'if' and 'else' blocks to 'pred' block even though it knows that incdec.ptr.1/case1 has a same value number with incdec.ptr.2/cast2. I see a case where this kind of redundancy confuses following optimization passes
and ends up generating suboptimal code. </span></p>
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<p><span>From the GVN implementation, it seems that transformation is performed only when the "leader" value dominates the other value, so it cannot handle a case like the above example. I wonder if this is by design or just a missing feature. </span></p>
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<p><span>Thanks,</span></p>
<p><span>Taewook</span></p>
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