[llvm-dev] Testcase failing when compiled with clang -O2, but passing using opt

Phil Tomson via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 7 13:41:31 PST 2015

I'm working on LLVM for a new target processor and we've got a testcase
that fails with -O2 optimization is used. So I figured I'd figure out what
optimizations are being applied with -O2 using:

 $TG_BIN/opt -O2 -disable-output -debug-pass=Arguments some.bc

then I figured I'd pass these to opt and remove one for each run until,
though process of elimination, I had found the offending optimization.

However, when I pass the full list of optimizations returned from that
command above, I find that our test passes.

Previously we were running the following in our Makefile to get the .o for
the testcase (that was then later linked):

     $(CC) $(CFLAGS) -fno-builtin -c $< -o $@

(where CC=clang and CFAGS=-O2

Now I'm running:

      $(CC) -emit-llvm $(CFLAGS) -fno-builtin -c $< -o $(basename $<).bc
      $(OPT) $(OPTS) -o $(basename $<).opt.bc $(basename $<).bc
      $(LLC) $(basename $<).bc -o $(basename $<).s
      $(AS) $(basename $<).s -o $@

Where OPTS is:

     -datalayout -notti -no-aa -tbaa -scoped-noalias
-assumption-cache-tracker -targetlibinfo -basicaa -verify -simplifycfg
-domtree -sroa -early-cse -lower-expect -ipsccp -globalopt -deadargelim
-instcombine -basiccg -prune-eh -inline-cost -inline -functionattrs
-lazy-value-info -jump-threading -correlated-propagation -tailcallelim
-reassociate -loops -loop-simplify -lcssa -loop-rotate -licm -loop-unswitch
-scalar-evolution -indvars -loop-idiom -loop-deletion -function_tti
-loop-unroll -memdep -mldst-motion -gvn -memcpyopt -sccp -dse -adce
-barrier -branch-prob -block-freq -loop-vectorize -slp-vectorizer
-alignment-from-assumptions -strip-dead-prototypes -globaldce -constmerge

So I'm wondering why the former (passing the -O2 to clang) is failing while
the latter passes our tests. Is there something I'm missing from the list
of -O2 optimizations?
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