[LLVMdev] LTO v. opt

[Bill Wendling]

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.

-bw

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