[LLVMdev] LTO v. opt

[Duncan P. N. Exon Smith]

> On 2015-Jan-05, at 12:59, Sean Silva <chisophugis at gmail.com> wrote:
> 
> tools/llvm-lto should get you pretty close. 

Be aware that unless you tell `llvm-lto` to hang on to specific
symbols, LTO can dead-strip them after -internalize.

Try adding -exported-symbol=_main if you're hitting this.

> On Fri, 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”?
> 
> 2.     Assuming I am just missing some parameters to “opt”, how do I determine what they should be?
> 
> Thanks
> 
> David
> 
> 
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