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?