[llvm-dev] Replicate Individual O3 optimizations

Neil Nelson via llvm-dev llvm-dev at lists.llvm.org
Sun Oct 20 20:24:18 PDT 2019

|is_sorted.cpp bool|  |is_sorted(||int|  |*a, ||int|  |n) {||||for|  |(||int|  |i = 0; i < n - 1; i++)|

|||if| |(a[i] > a[i + 1])|
|||return| |false||;|
|||return| |true||;|

https://blog.regehr.org/archives/1605 How Clang Compiles a Function
https://blog.regehr.org/archives/1603 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.


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

Neil Nelson

On 10/19/19 11:48 AM, Mehdi AMINI via llvm-dev wrote:

> On Thu, Oct 17, 2019 at 11:22 AM David Greene via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>     hameeza ahmed via llvm-dev <llvm-dev at lists.llvm.org
>     <mailto:llvm-dev at lists.llvm.org>> writes:
>     > Hello,
>     > I want to study the individual O3 optimizations. For this I am using
>     > following commands, but unable to replicate O3 behavior.
>     >
>     > 1.
>     Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O1
>     > -Xclang -disable-llvm-passes -emit-llvm -S vecsum.c -o
>     vecsum-noopt.ll
>     >
>     > 2.
>     Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O3
>     > -mllvm -debug-pass=Arguments -emit-llvm -S vecsum.c
>     >
>     > 3. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/opt
>     > <optimization sequence obtained in step 2> -S vecsum-noopt.ll -S -o
>     > o3-chk.ll
>     >
>     > Why the IR obtained by above step i.e individual O3 sequences,
>     is not same
>     > when O3 is passed?
>     >
>     > Where I am doing mistake?
> If you could provide the full reproducer, it could help to debug this.
>     I think you need to turn off LLVM optimizations when doing the
>     -emit-llvm dump.  Something like this:
>     Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang
>     -O3 \
>       -mllvm -debug-pass=Arguments -Xclang -disable-llvm-optzns
>     -emit-llvm \
>       -S vecsum.c
>     Otherwise you are effectively running the O3 pipeline twice, as clang
>     will emit LLVM IR after optimization, not before (this confused me too
>     when I first tried it).
> This is the common pitfall indeed!
> I think they are doing it correctly in step 1 though by including: 
> `-Xclang -disable-llvm-passes`.
>     That said, I'm not sure you will get the same IR out of opt as with
>     clang -O3 even with the above.  For example, clang sets
>     TargetTransformInfo for the pass pipeline and the detailed information
>     it uses may or may not be transmitted via the IR it dumps out.  I have
>     not personally tried to do this kind of thing in a while.
> I struggled as well to setup TTI and TLI the same way clang does :(
> It'd be nice to revisit our PassManagerBuilder setup and the opt 
> integration to provide reproducibility (maybe could be a starter 
> project for someone?).
> -- 
> Mehdi
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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