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<pre><code class="cpp color1 bold">is_sorted.cpp
bool</code> <code class="cpp plain">is_sorted(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">*a, </code><code class="cpp color1 bold">int</code> <code class="cpp plain">n) {</code><code class="cpp spaces">
</code><code class="cpp keyword bold">for</code> <code class="cpp plain">(</code><code class="cpp color1 bold">int</code> <code class="cpp plain">i = 0; i < n - 1; i++)</code></pre>
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<div class="line number3 index2 alt2"><code class="cpp spaces"> </code><code
class="cpp keyword bold">if</code> <code class="cpp plain">(a[i]
> a[i + 1])</code></div>
<div class="line number4 index3 alt1"><code class="cpp spaces"> </code><code
class="cpp keyword bold">return</code> <code class="cpp
keyword bold">false</code><code class="cpp plain">;</code></div>
<div class="line number5 index4 alt2"><code class="cpp spaces"> </code><code
class="cpp keyword bold">return</code> <code class="cpp
keyword bold">true</code><code class="cpp plain">;</code></div>
<div class="line number6 index5 alt1"><code class="cpp plain">}</code><br>
</div>
</div>
<pre><a class="moz-txt-link-freetext" href="https://blog.regehr.org/archives/1605">https://blog.regehr.org/archives/1605</a> How Clang Compiles a Function
<a class="moz-txt-link-freetext" href="https://blog.regehr.org/archives/1603">https://blog.regehr.org/archives/1603</a> How LLVM Optimizes a Function
clang version 10.0.0, Xubuntu 19.04
clang is_sorted.cpp -S -emit-llvm -o is_sorted_.ll
clang is_sorted.cpp -O0 -S -emit-llvm -o is_sorted_O0.ll
clang is_sorted.cpp -O0 -Xclang -disable-llvm-passes -S -emit-llvm -o is_sorted_disable.ll
No difference in the prior three ll files.
clang is_sorted.cpp -O1 -S -emit-llvm -o is_sorted_O1.ll
Many differences between is_sorted_O1.ll and is_sorted_.ll.
opt -O3 -S is_sorted_.ll -o is_sorted_optO3.ll
clang is_sorted.cpp -mllvm -debug-pass=Arguments -O3 -S -emit-llvm -o is_sorted_O3arg.ll
opt <optimization sequence obtained in prior step> -S is_sorted_.ll -o is_sorted_opt_parms.ll
No difference between is_sorted_optO3.ll and is_sorted_opt_parms.ll, the last two opt runs.
Many differences between is_sorted_O3arg.ll and is_sorted_opt_parms.ll, the last two runs,
clang and opt.
Conclusions:
Given my current understanding, the ll files from the first three clang runs
are before any optimizations. Those ll files are from the front-end phase (CFE).
But this is a simple program and it may be that for a more complex program that
the ll files could be different.
Whether or not we use a -O3 optimization or use the parameters provided by clang for a
-03 optimization, we obtain the same result.
The difference in question is why an opt run using the CFE ll before optimization
obtains a different ll than a CFE run that includes optimization. That is, for this case,
it is not the expansion of the -O3 parameters that is the difference.
Initially, it would be interesting to have an ll listing before optimization from the
clang run that includes optimization to compare with the ll from the clang run without
optimization.
Neil Nelson
On 10/19/19 11:48 AM, Mehdi AMINI via llvm-dev wrote:
</pre>
<blockquote type="cite"
cite="mid:CANF-O=b=KMffc_NmQ8wuTpPvJrFLnhZx7JiaPed9qFwQsNWAJg@mail.gmail.com">
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<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, Oct 17, 2019 at
11:22 AM David Greene via llvm-dev <<a
href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">hameeza
ahmed via llvm-dev <<a
href="mailto:llvm-dev@lists.llvm.org" target="_blank"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>>
writes:<br>
<br>
> Hello,<br>
> I want to study the individual O3 optimizations. For
this I am using<br>
> following commands, but unable to replicate O3
behavior.<br>
><br>
> 1.
Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang
-O1<br>
> -Xclang -disable-llvm-passes -emit-llvm -S vecsum.c
-o vecsum-noopt.ll<br>
><br>
> 2.
Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang
-O3<br>
> -mllvm -debug-pass=Arguments -emit-llvm -S vecsum.c<br>
><br>
> 3.
Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/opt<br>
> <optimization sequence obtained in step 2> -S
vecsum-noopt.ll -S -o<br>
> o3-chk.ll<br>
><br>
> Why the IR obtained by above step i.e individual O3
sequences, is not same<br>
> when O3 is passed?<br>
><br>
> Where I am doing mistake?<br>
</blockquote>
<div><br>
</div>
<div>If you could provide the full reproducer, it could help
to debug this.</div>
<div> <br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><br>
I think you need to turn off LLVM optimizations when doing
the<br>
-emit-llvm dump. Something like this:<br>
<br>
Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O3 \<br>
-mllvm -debug-pass=Arguments -Xclang
-disable-llvm-optzns -emit-llvm \<br>
-S vecsum.c<br>
<br>
Otherwise you are effectively running the O3 pipeline
twice, as clang<br>
will emit LLVM IR after optimization, not before (this
confused me too<br>
when I first tried it).<br>
</blockquote>
<div><br>
</div>
<div>This is the common pitfall indeed!</div>
<div>I think they are doing it correctly in step 1 though by
including: `-Xclang -disable-llvm-passes`.</div>
<div><br>
</div>
<div><br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">That
said, I'm not sure you will get the same IR out of opt as
with<br>
clang -O3 even with the above. For example, clang sets<br>
TargetTransformInfo for the pass pipeline and the detailed
information<br>
it uses may or may not be transmitted via the IR it dumps
out. I have<br>
not personally tried to do this kind of thing in a while.</blockquote>
<div><br>
</div>
<div>I struggled as well to setup TTI and TLI the same way
clang does :(</div>
<div>It'd be nice to revisit our PassManagerBuilder setup
and the opt integration to provide reproducibility (maybe
could be a starter project for someone?).</div>
<div><br>
</div>
<div>-- </div>
<div>Mehdi</div>
<div><br>
</div>
</div>
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