[llvm-dev] Some questions about phase ordering in OPT and LLC

[serge guelton via llvm-dev]

On Mon, May 09, 2016 at 01:07:07PM -0700, Mehdi Amini via llvm-dev wrote:
> 
> > On May 9, 2016, at 10:43 AM, Ricardo Nobre via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> > 
> > Hi,
> > 
> > I'm a PhD student doing phase ordering as part of my PhD topic and I would like to ask some questions about LLVM.
> > 
> > Executing the following command to see what passes does OPT execute when targeting a SPARC V8 processor:
> > 
> > /opt/clang+llvm-3.7.1-x86_64-linux-gnu-ubuntu-15.10/bin/llvm-as < /dev/null | /opt/clang+llvm-3.7.1-x86_64-linux-gnu-ubuntu-15.10/bin/opt -O3 -march=sparc -mcpu=v8 -disable-output -debug-pass=Arguments
> > 
> > I get the following output:
> > Pass Arguments:  -tti -no-aa -tbaa -scoped-noalias -assumption-cache-tracker -targetlibinfo -basicaa -verify -simplifycfg -domtree -sroa -early-cse -lower-expect
> > Pass Arguments:  -targetlibinfo -tti -no-aa -tbaa -scoped-noalias -assumption-cache-tracker -basicaa -ipsccp -globalopt -deadargelim -domtree -instcombine -simplifycfg -basiccg -prune-eh -inline-cost -inline -functionattrs -argpromotion -domtree -sroa -early-cse -lazy-value-info -jump-threading -correlated-propagation -simplifycfg -domtree -instcombine -tailcallelim -simplifycfg -reassociate -domtree -loops -loop-simplify -lcssa -loop-rotate -licm -loop-unswitch -instcombine -scalar-evolution -loop-simplify -lcssa -indvars -loop-idiom -loop-deletion -loop-unroll -mldst-motion -domtree -memdep -gvn -memdep -memcpyopt -sccp -domtree -bdce -instcombine -lazy-value-info -jump-threading -correlated-propagation -domtree -memdep -dse -loops -loop-simplify -lcssa -licm -adce -simplifycfg -domtree -instcombine -barrier -float2int -domtree -loops -loop-simplify -lcssa -loop-rotate -branch-prob -block-freq -scalar-evolution -loop-accesses -loop-vectorize -instcombine -scalar-evolution -slp-vectorizer -simplifycfg -domtree -instcombine -loops -loop-simplify -lcssa -scalar-evolution -loop-unroll -instcombine -loop-simplify -lcssa -licm -scalar-evolution -alignment-from-assumptions -strip-dead-prototypes -elim-avail-extern -globaldce -constmerge -verify
> > 
> > Why are there two "Pass Arguments"?
> > What does it mean?
> 
> 
> There are two PassManager instantiated and ran on the IR. I am not aware of a good reason for that (the first one is created with PassManagerBuilder::populateFunctionPassManager() and the second one with PassManagerBuilder::populateModulePassManager()).
> 
> You can look at AddOptimizationPasses() in opt.cpp.

As far as I understand, the two passmanager do not interleave their
passes. It first runs all the function passes and below. Then all the
module passes. So if you specify:

    opt -mymodulepass0 -myfunctionpass -mymodulepass1

What you actually get is:

1. myfunctionpass on each function
2. mymodulepass0 
3. mymodulepass0