[LLVMdev] LTO v. opt

[Duncan P. N. Exon Smith]

Some of those passes can't be specified on the command-line; they're
implicitly added by other passes.

I usually remove the passes that `opt` complains about and things
still reproduce.

> On 2015-Jan-05, at 13:29, David Callahan <dcallahan at fb.com> wrote:
> 
> Thanks to you both. 
> 
> On my Linux (centos6) system, I have reproduce a variant of the bug and learned about 
>    -plugin-opt=-debug-pass=Arguments
> which I infer from comments is intended to built arguments to “opt” however I found that some of the arguments don’t seem to be quite correct. I assume this just minor bit rot. 
> 
>  bin/opt -o pass1.bc -datalayout -notti -basictti -x86tti -targetlibinfo -no-aa -tbaa -scoped-noalias -assumption-tracker -basicaa -verify -verify-di -ipsccp -globalopt -c\
> onstmerge -deadargelim -instcombine -basiccg -inline-cost -inline -prune-eh -globalopt -globaldce -basiccg -argpromotion -instcombine -lazy-value-info -jump-threading -domt\
> ree -sroa -basiccg -functionattrs -globalsmodref-aa -domtree -loops -loop-simplify -lcssa -licm -memdep -mldst-motion -domtree -memdep -gvn -memdep -memcpyopt -dse -loops -\
> scalar-evolution -loop-simplify -lcssa -indvars -loop-deletion -branch-prob -block-freq -loop-vectorize -scalar-evolution -alignment-from-assumptions -instcombine -lazy-val\
> ue-info -jump-threading -simplifycfg -globaldce -verify -verify-di bug17623a.o
> opt: Unknown command line argument '-basictti'.  Try: 'bin/opt -help'
> opt: Did you mean '-basiccg'?
> opt: Unknown command line argument '-x86tti'.  Try: 'bin/opt -help'
> opt: Did you mean '-notti'?
> opt: Unknown command line argument '-verify-di'.  Try: 'bin/opt -help'
> opt: Did you mean '-verify'?
> opt: Unknown command line argument '-verify-di'.  Try: 'bin/opt -help'
> opt: Did you mean '-verify'?
> 
> 
> On 1/5/15, 1:16 PM, "Duncan P. N. Exon Smith" <dexonsmith at apple.com> wrote:
> 
> 
> On 2015-Jan-05, at 01:14, Bill Wendling <isanbard at gmail.com> wrote:
> On Jan 3, 2015, at 11:52 PM, Bill Wendling <isanbard at gmail.com> wrote:
> On Jan 2, 2015, at 8:32 PM, David Callahan <dcallahan at fb.com> wrote:
> Hi,
>   
> I am new to the LLVM dev community so forgive a perhaps obvious question. I am looking at bug 17623 which is an LTO/optimizer interaction bug.  I am working on a Mac with Xcode installed but have also built the 3.6 LLVM binaries (from a few month old local branch).
>   
> The default version of “ld” from Apple supports an option “-save-temps” which I believe saves bitcode both before and after the optimizer (and the bug is visible as a difference between these two)
>   
> ~/llvm-install/bin/clang -flto -O2 -c -o bug17623.o bug17623.c
> "/usr/bin/ld" -demangle -dynamic -arch x86_64 -macosx_version_min 10.10.0 -lSystem /Users/dcallahan/llvm-install/bin/..\
> /lib/clang/3.6.0/lib/darwin/libclang_rt.osx.a -save-temps -o bug36 bug17623.o
>   
> (this produces bug36.lto.bc and bug36.lto.opt.bc)
>   
> However, if I use “opt” to try and duplicate the actions run by “ld”, as in:
>   
> ~/llvm-install/bin/opt -O2 -o bug36.opt.bc bug36.lto.bc
>   
> This generates a different (much cleaner and correct!) output.
>   
> So my questions are:
> 1.     Is it feasible to get “opt” to reproduce the behavior of “ld”?
> The optimizations that occur during LTO (the ‘ld’ command) are different from the “normal” optimizations that opt runs. For one, LLVM has a view of the whole program, not just one compilation unit at a time. If you want to look at some of the optimizations that it performs, you can use the “opt -std-link-opts” command.
> 2.     Assuming I am just missing some parameters to “opt”, how do I determine what they should be?
> The link time optimizations are enabled via the "-std-link-opts” option.
> 
> You might also need to add `-internalize-public-api-list` to prevent
> functions like `main()` from getting internalized (and dead-stripped).
> 
> I forgot to mention that you will need to link all of the .bc files together using 'llvm-link’ before you run ‘opt -std-link-opts’ on it.
> 
> (If you're starting from bug36.lto.bc then they've already been linked.)