[llvm-dev] Any indispensable passes?

[Sean Silva via llvm-dev]

On Wed, Mar 1, 2017 at 12:53 PM, John Criswell via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 3/1/17 2:54 PM, Peizhao Ou via llvm-dev wrote:
>
> Hi everyone,
>
> I am currently testing out a combination of IR->IR passes with opt to
> benchmark how they affect performance. The source code works fine if simply
> use the clang (-O0/-O3) to directly compile to object files and link them.
> However, when I use opt with a select set of passes and then use llc to
> compile them to binary, the compiled binary is wrong.
>
>
> Can you clarify what you mean by "binary is wrong"?  Does the program not
> link, or does it produce incorrect results when executed?
>
> You might try running mem2reg before any other passes in your random list
> of optimizations.  mem2reg does SSA construction; I'd bet that you're more
> likely to trigger a bug in other passes if you haven't run mem2reg first.
> That said, I don't think any passes are needed for correctness.
>

SROA does this too (among other things).

-- Sean Silva


>
> You might also try using bugpoint to reduce the bug that you're seeing.
> As I said before, you may be triggering bugs that don't usually show up in
> the standard pass pipeline.  Reducing the bug (and fixing it if you are
> willing and able) would help improve the quality of the code.
>
> Regards,
>
> John Criswell
>
>
>
> That makes me wonder if there are any IR->IR passes that are indispensable
> and they are to guarantee the semantics?
>
> Here's my workflow just in case:
> ************************************************************
> *********************
> 1. Compile the source code to unoptimized IRs:
> clang -c -emit-llvm -O0 test.c -o test.bc
> 2. Run opt with a set of IR->IR passes, e.g.,
> opt -simplifycfg -sroa -inferattrs -globalopt -instcombine -simplifycfg
> -prune-eh -inline  -tailcallelim -simplifycfg -loop-simplify -lcssa
> -loop-rotate -licm -gvn -verify test.bc -o test.bc
> 3. Run llc with -O0:
> llc -file-type=obj test.bc -o test.o
> 4. Last link all the object files:
> clang -O0 test1.o test2.o -o test
> ************************************************************
> *********************
>
> Thanks,
> Peizhao
>
>
>
> _______________________________________________
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>
>
>
> --
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochesterhttp://www.cs.rochester.edu/u/criswell
>
>
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>
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