[llvm-dev] flags to reproduce clang -O3 with opt -O3

Thu Jul 2 20:00:14 PDT 2020

Thanks.

My intent is to reduce the overall compile time by eliminating unused
optimizations.

Do you happen to know if there is a list somewhere (or a way to dump /
extract them) of the individual flags which llc uses as part of -O3, so I
can perhaps experiment with removing the unnecessary ones for my code?

On Thu, Jul 2, 2020 at 7:55 PM Mehdi AMINI <joker.eph at gmail.com> wrote:

>
>
> On Thu, Jul 2, 2020 at 7:50 PM Chad Verbowski <chad at verbowski.com> wrote:
>
>> Awesome, thanks!
>>
>> I'd like to have the  last step (llc in your example) not perform
>> additional optimization passes, such as O3, and simply use the O3 pass from
>> opt in the previous line.
>>
>> Do you happen to know if I should use 'llc -O0 foo_o.bc -o foo.exe'
>> instead to achieve this?
>>
>
> No you should use `llc -O3`: this is controlling only the backend part of
> the pipeline.
>
>
>>
>> On Thu, Jul 2, 2020 at 6:35 PM Mehdi AMINI <joker.eph at gmail.com> wrote:
>>
>>>
>>>
>>> On Thu, Jul 2, 2020 at 2:28 PM Chad Verbowski via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hello,
>>>>
>>>> I've been trying to figure out how to reproduce the results of a single
>>>> clang -O3 compilation to a binary with a multi-step process using opt.
>>>>
>>>> Specifically I have:
>>>>
>>>> clang -O3 foo.c -o foo.exe
>>>>
>>>>
>>>> which I want to replicate with:
>>>>
>>>> clang -O0 -c -emit-llvm foo.c
>>>>
>>>>
>>> Using O0 will mark every function in the IR with "optnone" which
>>> prevents `opt` from optimizing it. I'd try `clang -O3  -Xclang
>>> -disable-llvm-passes -c -emit-llvm foo.c`
>>>
>>>
>>>> opt -O3 foo.bc -o foo_o.bc
>>>> clang foo_o.bc -o foo.exe
>>>>
>>>>
>>> This last step won't enable optimizations in the backend, you likely
>>> should try `llc -O3` instead.
>>>
>>> Best,
>>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>
>>>
>>>>
>>>> Any hints / suggestions on what additional flags I need to produce the
>>>> same binary are greatly appreciated!
>>>>
>>>> *What I've tried:*
>>>> I've been reading the archives, and found this
>>>> <http://lists.llvm.org/pipermail/llvm-dev/2017-September/117144.html>,
>>>> which suggests dumping the pass arguments using:
>>>>
>>>> clang -mllvm -debug-pass=Structure -O3 foo.c -o foo.exe
>>>>
>>>> and comparing with:
>>>>
>>>> clang -mllvm -debug-pass=Structure -O0 -c -emit-llvm foo.c
>>>> opt -debug-pass=Structure -O3 foo.bc -o foo_o.bc
>>>> clang -mllvm -debug-pass=Structure foo_o.bc -o foo.exe
>>>>
>>>>
>>>> The first has 30 "Pass Argument" statements though only these 5 are
>>>> distinct. Across these 5 there are 190 distinct flags.  The multi-step
>>>> compilation has only 140 distinct flags. Comparing the flags, 18 from the
>>>> multi-step are missing in the 1pass, and 67 from 1pass are missing in the
>>>> multistep.
>>>>
>>>> These appear to be opt flags, since they cause an error when trying to
>>>> use them with clang (e.g. -x86-fixup-LEAs) and when used with opt
>>>> causes a crash with stack dump and request to submit a bug report. Others
>>>> like -attributor appear to work with opt.
>>>>
>>>> I'm currently blindly trying to add the 67 different flags to the opt
>>>> step to see which work, and hopefully that subset will produce the same
>>>> result as clang -O3.
>>>>
>>>> It seems like there must be an easier / more exact way of getting the
>>>> opt -O3 multi-step to match the clang -O3 result.
>>>>
>>>> Any thoughts or insights are appreciated. Below is a sorted list of the
>>>> flags missing from each for completeness.
>>>>
>>>> not contained in 1pass O3 (count=18)
>>>>
>>>> -aa-scalar-evolution
>>>>
>>>> -always-inline
>>>>
>>>> -callsite-splitting
>>>>
>>>> -inject-tli-mappings
>>>>
>>>> -ipsccp
>>>>
>>>> -jump-threading-correlated-propagation
>>>>
>>>> -livedebugvalues
>>>>
>>>> -loops-loop-simplify
>>>>
>>>> -memdep-lazy-branch-prob
>>>>
>>>> -openmpopt
>>>>
>>>> -opt-remark-emitter-instcombine
>>>>
>>>> -regallocfast
>>>>
>>>> -speculative-execution
>>>>
>>>> -stackmap-liveness
>>>>
>>>> -tbaa-scoped-noalias
>>>>
>>>> -vector-combine
>>>>
>>>> -verify
>>>>
>>>> -write-bitcode
>>>>
>>>> not contained in multi O3 (count=67)
>>>>
>>>> -attributor
>>>>
>>>> -block-freq-loop-simplify
>>>>
>>>> -branch-folder
>>>>
>>>> -break-false-deps
>>>>
>>>> -callsite-splitting-ipsccp
>>>>
>>>> -codegenprepare
>>>>
>>>> -consthoist
>>>>
>>>> -dead-mi-elimination
>>>>
>>>> -detect-dead-lanes
>>>>
>>>> -early-ifcvt
>>>>
>>>> -early-machinelicm
>>>>
>>>> -early-tailduplication
>>>>
>>>> -expandmemcmp
>>>>
>>>> -greedy
>>>>
>>>> -interleaved-access
>>>>
>>>> -iv-users
>>>>
>>>> -lazy-block-freq-opt-remark-emitter
>>>>
>>>> -livedebugvars
>>>>
>>>> -liveintervals
>>>>
>>>> -liveregmatrix
>>>>
>>>> -livestacks
>>>>
>>>> -livevars
>>>>
>>>> -loop-reduce
>>>>
>>>> -loop-simplify-lcssa-verification
>>>>
>>>> -lrshrink
>>>>
>>>> -machine-block-freq
>>>>
>>>> -machine-combiner
>>>>
>>>> -machine-cp
>>>>
>>>> -machine-cse
>>>>
>>>> -machinedomtree-machine-loops
>>>>
>>>> -machinelicm
>>>>
>>>> -machine-loops
>>>>
>>>> -machinepostdomtree
>>>>
>>>> -machinepostdomtree-block-placement
>>>>
>>>> -machine-scheduler
>>>>
>>>> -machine-sink
>>>>
>>>> -machine-trace-metrics
>>>>
>>>> -mergeicmps
>>>>
>>>> -objc-arc-contract
>>>>
>>>> -opt-phis
>>>>
>>>> -partially-inline-libcalls
>>>>
>>>> -peephole-opt
>>>>
>>>> -postra-machine-sink
>>>>
>>>> -post-RA-sched
>>>>
>>>> -processimpdefs
>>>>
>>>> -reaching-deps-analysis
>>>>
>>>> -rename-independent-subregs
>>>>
>>>> -shrink-wrap
>>>>
>>>> -simple-register-coalescing
>>>>
>>>> -slotindexes
>>>>
>>>> -spill-code-placement
>>>>
>>>> -stack-coloring
>>>>
>>>> -stackmap-liveness-livedebugvalues
>>>>
>>>> -stack-slot-coloring
>>>>
>>>> -tailduplication
>>>>
>>>> -unreachable-mbb-elimination
>>>>
>>>> -virtregmap
>>>>
>>>> -virtregrewriter
>>>>
>>>> -x86-avoid-SFB
>>>>
>>>> -x86-cf-opt
>>>>
>>>> -x86-cmov-conversion
>>>>
>>>> -x86-domain-reassignment
>>>>
>>>> -x86-evex-to-vex-compress
>>>>
>>>> -x86-execution-domain-fix
>>>>
>>>> -x86-fixup-bw-insts
>>>>
>>>> -x86-fixup-LEAs
>>>>
>>>> -x86-optimize-LEAs
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200702/58c57ab6/attachment-0001.html>