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Opt level

With version 3.8 the passes are as follow:

https://stackoverflow.com/questions/15548023/clang-optimization-levels http://clang.llvm.org/docs/CommandGuide/clang.html#cmdoption-O1

  • baseline (-O0): default

opt sets : -targetlibinfo -tti -verify clang adds : -mdisable-fp-elim -mrelax-all

  • －O1 is based on -O0

opt adds: -globalopt -demanded-bits -branch-prob -inferattrs -ipsccp -dse -loop-simplify -scoped-noalias -barrier -adce -deadargelim -memdep -licm -globals-aa -rpo-functionattrs -basiccg -loop-idiom -forceattrs -mem2reg -simplifycfg -early-cse -instcombine -sccp -loop-unswitch -loop-vectorize -tailcallelim -functionattrs -loop-accesses -memcpyopt -loop-deletion -reassociate -strip-dead-prototypes -loops -basicaa -correlated-propagation -lcssa -domtree -always-inline -aa -block-freq -float2int -lower-expect -sroa -loop-unroll -alignment-from-assumptions -lazy-value-info -prune-eh -jump-threading -loop-rotate -indvars -bdce -scalar-evolution -tbaa -assumption-cache-tracker clang adds : -momit-leaf-frame-pointer clang drops : -mdisable-fp-elim -mrelax-all

  • -O2 is based on -O1

opt adds: -elim-avail-extern -mldst-motion -slp-vectorizer -gvn -inline -globaldce -constmerge opt drops: -always-inline clang adds: -vectorize-loops -vectorize-slp

  • -O3 is based on -O2

opt adds: -argpromotion -Ofast is based on -O3, valid in clang but not in opt

clang adds: -fno-signed-zeros -freciprocal-math -ffp-contract=fast -menable-unsafe-fp-math -menable-no-nans -menable-no-infs

  • -Os is the same as -O2

  • -Oz is based on -Os

opt drops: -slp-vectorizer clang drops: -vectorize-loops

Text Only
–Ox flags: Specify which optimization level to use. -O0 means "no optimization":
this level compiles the fastest and generates the most debuggable code. -O2 is a
moderate level of optimization which enables most optimizations. -Os is like -O2
with extra optimizations to reduce code size. -Oz is like -Os (and thus -O2), but
reduces code size further. -O3 is like -O2, except that it enables optimizations that
take longer to perform or that may generate larger code (in an attempt to make the
program run faster). On supported platforms, -O4 enables link-time optimization;
object files are stored in the LLVM bitcode file format and whole program
optimization is done at link time. -O1 is somewhere between -O0 and -O2.
Bash
    -O0 Means “no optimization”: this level compiles the fastest and generates the most debuggable code.

    -O1 Somewhere between -O0 and -O2.

    -O2 Moderate level of optimization which enables most optimizations.

    -O3 Like -O2, except that it enables optimizations that take longer to perform or that may generate larger code (in an attempt to make the program run faster).

    -Ofast Enables all the optimizations from -O3 along with other aggressive optimizations that may violate strict compliance with language standards.

    -Os Like -O2 with extra optimizations to reduce code size.

    -Oz Like -Os (and thus -O2), but reduces code size further.

    -Og Like -O1. In future versions, this option might disable different optimizations in order to improve debuggability.

    -O Equivalent to -O2.

    -O4 and higher

        Currently equivalent to -O3

gcc -O is equal gcc -O1.

How to use

the opt is defaut use the ll file or bc file. And the output is bc file. If you want to the output is ll file, you should use the -S.

Text Only
opt -O3 sum.ll -o sum.O3.ll
opt -O3 sum.ll -o sum.O3.ll -S

GAS
  --O0                                               - Optimization level 0. Similar to clang -O0
  --O1                                               - Optimization level 1. Similar to clang -O1
  --O2                                               - Optimization level 2. Similar to clang -O2
  --O3                                               - Optimization level 3. Similar to clang -O3
  --Os                                               - Like -O2 with extra optimizations for size. Similar to clang -Os
  --Oz                                               - Like -Os but reduces code size further. Similar to clang -Oz
  -S                                                 - Write output as LLVM assembly