<div dir="auto"><div>No, I meant LLD, the LLVM linker. This option for LLD is relevant for exploring different pass pipelines for link time optimization.</div><div dir="auto"><br></div><div dir="auto">It is essentially equivalent to the -passes flag for 'opt'.</div><div dir="auto"><br></div><div dir="auto">Such a flag doesn't make much sense for 'llc' because llc mostly runs backend passes, which are much more difficult to construct custom pipelines for (backend passes are often required for correctness or have complex ordering requirements).</div><div dir="auto"><br></div><div dir="auto">-- Sean Silva<br><div class="gmail_extra" dir="auto"><br><div class="gmail_quote">On Jan 6, 2018 7:35 PM, "toddy wang" <<a href="mailto:wenwangtoddy@gmail.com">wenwangtoddy@gmail.com</a>> wrote:<br type="attribution"><blockquote class="quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">@Sean, do you mean llc ?<div>For llc 4.0 and llc 5.0, I cannot find <span style="font-size:12.800000190734863px">-lto-newpm-passes option, is it a hidden one?</span></div></div><div class="elided-text"><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Jan 6, 2018 at 7:37 PM, Sean Silva <span dir="ltr"><<a href="mailto:chisophugis@gmail.com" target="_blank">chisophugis@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="auto"><span><div><br><div class="gmail_extra"><br><div class="gmail_quote">On Jan 5, 2018 11:30 PM, "toddy wang via llvm-dev" <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br type="attribution"><blockquote class="m_-1140391782659658647m_6034123896947059277quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">What I am trying is to compile a program with different sets of optimization flags.<div>If there is no fine-grained control over clang optimization flags, it would be impossible to achieve what I intend.</div></div></blockquote></div></div></div><div dir="auto"><br></div></span><div dir="auto">LLD has -lto-newpm-passes (and the corresponding -lto-newpm-aa-pipeline) which allows you to pass a custom pass pipeline with full control. At one point I was using a similar modification to clang (see <a href="https://reviews.llvm.org/D21954" target="_blank">https://reviews.llvm.org/<wbr>D21954</a>) that never landed.</div><span class="m_-1140391782659658647HOEnZb"><font color="#888888"><div dir="auto"><br></div><div dir="auto">-- Sean Silva</div></font></span><div><div class="m_-1140391782659658647h5"><div dir="auto"><br></div><div dir="auto"><div class="gmail_extra"><div class="gmail_quote"><blockquote class="m_-1140391782659658647m_6034123896947059277quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div><br></div><div>Although there is fine-grained control via opt, for a large-scale projects, clang-opt-llc pipeline may not be a drop-in solution.</div></div><div class="m_-1140391782659658647m_6034123896947059277elided-text"><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Jan 5, 2018 at 10:00 PM, Craig Topper <span dir="ltr"><<a href="mailto:craig.topper@gmail.com" target="_blank">craig.topper@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">I don't think "clang -help" prints options about optimizations. Clang itself doesn't have direct support for fine grained optimization control. Just the flag for levels -O0/-O1/-O2/-O3. This is intended to be simple and sufficient interface for most users who just want to compile their code. So I don't think there's a way to pass just -dse to clang.<div><br></div><div>opt on the other hand is more of a utility for developers of llvm that provides fine grained control of optimizations for testing purposes.</div><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426HOEnZb"><font color="#888888"><div><br></div><div><br></div></font></span></div><div class="gmail_extra"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426HOEnZb"><font color="#888888"><br clear="all"><div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738gmail_signature" data-smartmail="gmail_signature">~Craig</div></div></font></span><div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426h5">
<br><div class="gmail_quote">On Fri, Jan 5, 2018 at 5:41 PM, toddy wang <span dir="ltr"><<a href="mailto:wenwangtoddy@gmail.com" target="_blank">wenwangtoddy@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">Craig, thanks a lot!<div><br></div><div>I'm actually confused by clang optimization flags.</div><div><br></div><div>If I run clang -help, it will show many optimizations (denoted as set A) and non-optimization options (denoted as set B). </div><div>If I run llvm-as < /dev/null | opt -O0/1/2/3 -disable-output -debug-pass=Arguments, it also shows many optimization flags (denote as set C).</div><div><br></div><div>There are many options in set C while not in set A, and also options in set A but not in set C.</div><div><br></div><div>The general question is: what is the relationship between set A and set C, at the same optimization level O0/O1/O2/O3?</div><div>Another question is: how to specify an option in set C as a clang command line option, if it is not in A?</div><div><br></div><div>For example, -dse is in set C but not in set A, how can I specify it as a clang option? Or simply I cannot do that.</div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div></div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738HOEnZb"><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738h5"><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Jan 5, 2018 at 7:55 PM, Craig Topper <span dir="ltr"><<a href="mailto:craig.topper@gmail.com" target="_blank">craig.topper@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">O0 didn't start applying optnone until r304127 in May 2017 which is after the 4.0 family was branched. So only 5.0, 6.0, and trunk have that behavior. Commit message copied below<div><br></div><div>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p1" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201)"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures">Author: Mehdi Amini <<a href="mailto:joker.eph@gmail.com" target="_blank">joker.eph@gmail.com</a>></span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p1" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201)"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures">Date: <span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-Apple-converted-space"> </span>Mon May 29 05:38:20 2017 +0000</span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p2" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201);min-height:13px"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"></span><br></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p1" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201)"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-Apple-converted-space"> </span>IRGen: Add optnone attribute on function during O0</span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p2" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201);min-height:13px"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p1" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201)"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-Apple-converted-space"> </span>Amongst other, this will help LTO to correctly handle/honor files</span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p1" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201)"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-Apple-converted-space"> </span>compiled with O0, helping debugging failures.</span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p1" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201)"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-Apple-converted-space"> </span>It also seems in line with how we handle other options, like how</span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p1" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201)"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-Apple-converted-space"> </span>-fnoinline adds the appropriate attribute as well.</span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p2" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201);min-height:13px"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-p1" style="margin:0px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:11px;line-height:normal;font-family:Menlo;color:rgb(255,255,255);background-color:rgb(43,102,201)"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-s1" style="font-variant-ligatures:no-common-ligatures"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail-Apple-converted-space"> </span>Differential Revision: <a href="https://reviews.llvm.org/D28404" target="_blank">https://reviews.llvm.org/D2840<wbr>4</a></span></p><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911HOEnZb"><font color="#888888"><div><br></div><div><br></div></font></span></div></div><div class="gmail_extra"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911HOEnZb"><font color="#888888"><br clear="all"><div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168gmail_signature" data-smartmail="gmail_signature">~Craig</div></div></font></span><div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911h5">
<br><div class="gmail_quote">On Fri, Jan 5, 2018 at 4:49 PM, toddy wang <span dir="ltr"><<a href="mailto:wenwangtoddy@gmail.com" target="_blank">wenwangtoddy@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>@Zhaopei, thanks for the clarification.</div><div><br></div>@Craig and @Michael, for clang 4.0.1, -Xclang -disable-O0-optnone gives the following error message. From which version -disable-O0-optnone gets supported?<div><br></div><div><div>[twang15@c89 temp]$ clang++ -O0 -Xclang -disable-O0-optnone -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc LULESH.cc</div><div>error: unknown argument: '-disable-O0-optnone'</div><div><br></div><div>[twang15@c89 temp]$ clang++ --version</div><div>clang version 4.0.1 (tags/RELEASE_401/final)</div><div>Target: x86_64-unknown-linux-gnu</div></div></div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168HOEnZb"><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168h5"><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Jan 5, 2018 at 4:45 PM, Craig Topper <span dir="ltr"><<a href="mailto:craig.topper@gmail.com" target="_blank">craig.topper@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">If you pass -O0 to clang, most functions will be tagged with an optnone function attribute that will prevent opt and llc even if you pass -O3 to opt and llc. This is the mostly likely cause for the slow down in 2.<div><br></div><div>You can disable the optnone function attribute behavior by passing "-Xclang -disable-O0-optnone" to clang</div></div><div class="gmail_extra"><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789HOEnZb"><font color="#888888"><br clear="all"><div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759gmail_signature" data-smartmail="gmail_signature">~Craig</div></div></font></span><div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789h5">
<br><div class="gmail_quote">On Fri, Jan 5, 2018 at 1:19 PM, toddy wang 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 dir="ltr">I tried the following on LULESH1.0 serial version (<a href="https://codesign.llnl.gov/lulesh/LULESH.cc" target="_blank">https://codesign.llnl.gov/lul<wbr>esh/LULESH.cc</a>)<div><br></div><div>1. clang++ -O3 LULESH.cc; ./a.out 20</div><div>Runtime: 9.487353 second</div><div><br></div><div>2. clang++ -O0 -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc LULESH.cc; opt -O3 a.bc -o b.bc; llc -O3 -filetype=obj b.bc -o b.o ; clang++ b.o -o b.out; ./b.out 20</div><div>Runtime: 24.15 seconds</div><div><br></div><div>3. clang++ -O3 -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc LULESH.cc; opt -O3 a.bc -o b.bc; llc -O3 -filetype=obj b.bc -o b.o ; clang++ b.o -o b.out; ./b.out 20</div><div>Runtime: 9.53 seconds</div><div><br></div><div>1 and 3 have almost the same performance, while 2 is significantly worse, while I expect 1, 2 ,3 should have trivial difference.</div><div><br></div><div>Is this a wrong expectation?</div><div><br></div><div>@Peizhao, what did you try in your last post?</div></div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759HOEnZb"><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759h5"><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Apr 11, 2017 at 12:15 PM, Peizhao Ou 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 dir="ltr">It's really nice of you pointing out the -Xclang option, it makes things much easier. I really appreciate your help!<div><br></div><div>Best,</div><div>Peizhao</div></div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759m_9146112996686254187HOEnZb"><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759m_9146112996686254187h5"><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Apr 10, 2017 at 10:12 PM, Mehdi Amini <span dir="ltr"><<a href="mailto:mehdi.amini@apple.com" target="_blank">mehdi.amini@apple.com</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"><br><div><span><blockquote type="cite"><div>On Apr 10, 2017, at 5:21 PM, Craig Topper via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759m_9146112996686254187m_-1208587949809955168m_-6624049905980539258Apple-interchange-newline"><div><div dir="ltr">clang -O0 does not disable all optimization passes modify the IR.; In fact it causes most functions to get tagged with noinline to prevent inlinining</div></div></blockquote><div><br></div></span><div>It also disable lifetime instrinsics emission and TBAA, etc.</div><span><div><br></div><br><blockquote type="cite"><div><div dir="ltr"><div><br></div><div>What you really need to do is</div><div><br></div><div>clang -O3 -c emit-llvm -o source.bc -v</div><div><br></div><div>Find the -cc1 command line from that output. Execute that command with --disable-llvm-passes. leave the -O3 and everything else.</div></div></div></blockquote><div><br></div></span><div>That’s a bit complicated: CC1 options can be passed through with -Xclang, for example here just adding to the regular clang invocation ` -Xclang -disable-llvm-passes`</div><div><br></div><div>Best,</div><div><br></div><div>— </div><span class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759m_9146112996686254187m_-1208587949809955168HOEnZb"><font color="#888888"><div>Mehdi</div></font></span><div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759m_9146112996686254187m_-1208587949809955168h5"><div><br></div><div><br></div><br><blockquote type="cite"><div><div dir="ltr"><div><br></div><div>You should be able to feed the output from that command to opt/llc and get consistent results.</div><div><br></div><div><br></div><div><br></div></div><div class="gmail_extra"><br clear="all"><div><div class="m_-1140391782659658647m_6034123896947059277m_5023718488316049426m_2174135488509756738m_5669078264214194911m_-4997143994764681168m_2034229146786603789m_-8339188297650556759m_9146112996686254187m_-1208587949809955168m_-6624049905980539258gmail_signature" data-smartmail="gmail_signature">~Craig</div></div>
<br><div class="gmail_quote">On Mon, Apr 10, 2017 at 4:57 PM, Peizhao Ou 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 dir="ltr">Hi folks,<div><br></div><div>I am wondering about the relationship clang, opt and llc. I understand that this has been asked, e.g., <a href="http://stackoverflow.com/questions/40350990/relationship-between-clang-opt-llc-and-llvm-linker" target="_blank">http://stackoverflow.com<wbr>/questions/40350990/relationsh<wbr>ip-between-clang-opt-llc-and-l<wbr>lvm-linker</a>. Sorry for posting a similar question again, but I still have something that hasn't been resolved yet.</div><div><br></div><div>More specifically I am wondering about the following two approaches compiling optimized executable:</div><div><br></div><div>1. clang -O3 -c source.c -o source.o</div><div> ...</div><div> clang a.o b.o c.o ... -o executable</div><div><br></div><div>2. clang -O0 -c -emit-llvm -o source.bc</div><div> opt -O3 source.bc -o source.bc</div><div> llc -O3 -filetype=obj source.bc -o source.o</div><div> ...</div><div> clang a.o b.o c.o ... -o executable<br></div><div><br></div><div>I took a look at the source code of the clang tool and the opt tool, they both seem to use the PassManagerBuilder::populateMo<wbr>dulePassManager() and PassManagerBuilder::populateFu<wbr>nctionPassManager() functions to add passes to their optimization pipeline; and for the backend, the clang and llc both use the addPassesToEmitFile() function to generate object code.</div><div><br></div><div>So presumably the above two approaches to generating optimized executable file should do the same thing. However, I am seeing that the second approach is around 2% slower than the first approach (which is the way developers usually use) pretty consistently.</div><div><br></div><div>Can anyone point me to the reasons why this happens? Or even correct my wrong understanding of the relationship between these two approaches?</div><div><br></div><div>PS: I used the -debug-pass=Structure option to print out the passes, they seem the same except that the first approach has an extra pass called "-add-discriminator", but I don't think that's the reason.</div><div><br></div><div>Peizhao</div>
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