[llvm-dev] Relationship between clang, opt and llc
toddy wang via llvm-dev
llvm-dev at lists.llvm.org
Tue Jan 9 00:09:26 PST 2018
Thanks, Craig.
So, clang -Xclang -disable-llvm-passes actually disables all the LLVM
passed populated by clang so that there is no middle-end optimization on bc
files.
clang -O2 LULESH.c //clang is the driver, invoking cc1, cc1as, ld
//options can be passed through to cc1
directly.
//maybe have different names, e.g.
-fvectorize in clang driver and -vectorize-loops in clang -cc1
//options are dumped by clang -help
and clang
--help-hidden
clang -cc1 // c/c++ frontend is also referred as clang
// this is the c/c++
frontend(preprocessor + Lexer + parser) and middle-end ( LLVM-IR optimizer
+ IR-assembly generator)
//controlled by -Xclang <options>,
Xclang options dumped by clang -cc1 -help
//mllvm Options like -unroll-max count
are controlled by -mllvm <options>.
//mllvm Options can be dumped by clang
-v -help -mllvm and clang -v --help-hidden
*//Question: are all mllvm options for
middle-end while Xclang options are for front-end?*
clang -cc1as // assembly-obj assembler
ld/ldd/gold //linker (if -flto is not provided) or
link-time optimizer and linker (if -flto -fuse-ld=lld is provided or -flto
-fuse-ld=gold is provided)
On Tue, Jan 9, 2018 at 2:00 AM, Craig Topper <craig.topper at gmail.com> wrote:
> Yes that is what he meant. "-dce, -adce, etc" are command line options
> consumed by tools/opt/opt.cpp to give to the PassManagerBuilder that it
> creates. The parsing of those options doesn't exist in any of the llvm
> library code that is linked into clang. Clang has its own code for
> populating a PassManagerBuilder in tools/clang/lib/CodeGen/BackendUtil.cpp
>
> ~Craig
>
> On Mon, Jan 8, 2018 at 10:55 PM, toddy wang via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Mehdi,
>>
>> I found -unroll-max-count can be passed w/ -mllvm.
>> -dce, -adce, etc, are also dumped by 'opt --help-hidden'. However, they
>> cannot be passed w/ -mllvm.
>> Is this what "You can't schedule passes this way, only set parameters
>> like -unroll-threshold=<uint> etc." means?
>>
>> [twang15 at c89 temp]$ clang++ -mllvm -unroll-max-count=4 -mllvm -dce
>> -save-temps LULESH.cc
>> clang (LLVM option parsing): Unknown command line argument '-dce'. Try:
>> 'clang (LLVM option parsing) -help'
>> clang (LLVM option parsing): Did you mean '-mv4'?
>>
>>
>> On Mon, Jan 8, 2018 at 12:48 PM, Mehdi AMINI <joker.eph at gmail.com> wrote:
>>
>>>
>>>
>>> 2018-01-08 8:59 GMT-08:00 toddy wang <wenwangtoddy at gmail.com>:
>>>
>>>>
>>>>
>>>> On Mon, Jan 8, 2018 at 11:53 AM, Mehdi AMINI <joker.eph at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> 2018-01-08 8:41 GMT-08:00 toddy wang <wenwangtoddy at gmail.com>:
>>>>>
>>>>>> Hi Medhi,
>>>>>>
>>>>>> It seems -mllvm does not work as expected. Anything wrong?
>>>>>>
>>>>>> [twang15 at c92 temp]$ clang++ -O3 -mllvm *-deadargelim* LULESH.cc
>>>>>> clang (LLVM option parsing): Unknown command line argument
>>>>>> '-deadargelim'. Try: 'clang (LLVM option parsing) -help'
>>>>>> clang (LLVM option parsing): Did you mean '-regalloc'?
>>>>>>
>>>>>> [twang15 at c92 temp]$ clang++ -O3 -mllvm *deadargelim* LULESH.cc
>>>>>> clang (LLVM option parsing): Unknown command line argument
>>>>>> 'deadargelim'. Try: 'clang (LLVM option parsing) -help'
>>>>>>
>>>>>
>>>>> You can't schedule passes this way, only set parameters
>>>>> like -unroll-threshold=<uint> etc.
>>>>>
>>>>> Where can I find options like -unroll-threshold=<uint>? I cannot find
>>>> it in either opt -help or clang -help.
>>>>
>>>
>>> This one shows up in `opt --help-hidden`. Otherwise in the source code
>>> for each transformation.
>>> (remember when I mentioned these are intended for LLVM developers and
>>> not end-user facing?).
>>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>
>>>> --
>>>>> Mehdi
>>>>>
>>>>>
>>>>>>
>>>>>> -Tao
>>>>>>
>>>>>> On Mon, Jan 8, 2018 at 11:12 AM, Mehdi AMINI <joker.eph at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> 2018-01-07 23:16 GMT-08:00 toddy wang <wenwangtoddy at gmail.com>:
>>>>>>>
>>>>>>>> -mllvm <value> Additional arguments to forward to LLVM's
>>>>>>>> option processing
>>>>>>>>
>>>>>>>> This is dumped by clang. I am not sure what I am supposed to put as
>>>>>>>> value in order to tune unrolling/inlining threshold.
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> As the help says, this is used to pass argument to LLVM itself. If
>>>>>>> you remember you earlier question about setA (clang options) and setC (opt
>>>>>>> options), this allows to reach setC from the clang command line.
>>>>>>> Any option that you see in the output of `opt --help` can be set
>>>>>>> from clang using `-mllvm`. Same caveat as I mentioned before: these aren't
>>>>>>> supposed to be end-user options.
>>>>>>>
>>>>>>> --
>>>>>>> Mehdi
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> On Mon, Jan 8, 2018 at 2:02 AM, Sean Silva <chisophugis at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> For the types of things that you are looking for, you may just
>>>>>>>>> want to try a bunch of -mllvm options. You can tune inlining and unrolling
>>>>>>>>> threshold like that, for example.
>>>>>>>>>
>>>>>>>>> On Jan 7, 2018 10:33 PM, "toddy wang via llvm-dev" <
>>>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>
>>>>>>>>>> Hi Mehdi,
>>>>>>>>>>
>>>>>>>>>> Now we have 5 pipelines. (In addition to the first 3, which I
>>>>>>>>>> have described in detail above, please refer my latest reply for details)
>>>>>>>>>> 1. clang + opt + gold
>>>>>>>>>> 2. clang + opt + lld
>>>>>>>>>> 3. clang + GNU ld/ gold /lld
>>>>>>>>>>
>>>>>>>>>> 4. clang + opt + llc + clang
>>>>>>>>>> clang -emit-llvm -O1 -Xclang -disable-llvm-passes for c/c++ to
>>>>>>>>>> .bc generation and minimal front-end optimization
>>>>>>>>>> opt for single bc file optimization
>>>>>>>>>> llc single bc file to obj file generation and back-end
>>>>>>>>>> optimization (no link-time optimization is possible, since llc works on 1
>>>>>>>>>> bc file at a time)
>>>>>>>>>> clang again for linking all obj file to generate final
>>>>>>>>>> executable. (although in principle there can be a link-time
>>>>>>>>>> optimization even with all obj files, it requires a lot of work and is
>>>>>>>>>> machine-dependent. This may also be the reason why modern compilers like
>>>>>>>>>> LLVM/GCC/ICC, etc performs LTO not at obj level. But, obj level may yield
>>>>>>>>>> extra benefit even LTO at intermediate level has been applied by compilers,
>>>>>>>>>> because obj level can see more information.)
>>>>>>>>>>
>>>>>>>>>> `clang -Ox` + `opt -Ox` + `llc -Ox` is too coarse-grain.
>>>>>>>>>>
>>>>>>>>>> 5. Modify clang to align with GCC/ICC so that many tunables are
>>>>>>>>>> exposed at clang command line. Not sure how much work is needed, but at
>>>>>>>>>> least requires an overall understanding of compiler internals, which can be
>>>>>>>>>> gradually figured out.
>>>>>>>>>>
>>>>>>>>>> I believe 5 is interesting, but 2 may be good enough. More
>>>>>>>>>> experiments are needed before decision is made.
>>>>>>>>>>
>>>>>>>>>> On Mon, Jan 8, 2018 at 12:56 AM, Mehdi AMINI <joker.eph at gmail.com
>>>>>>>>>> > wrote:
>>>>>>>>>>
>>>>>>>>>>> Hi Toddy,
>>>>>>>>>>>
>>>>>>>>>>> You can achieve what you're looking for with a pipeline based on
>>>>>>>>>>> `clang -Ox` + `opt -Ox` + `llc -Ox` (or lld instead of llc), but this won't
>>>>>>>>>>> be guarantee'd to be well supported across releases of the compiler.
>>>>>>>>>>>
>>>>>>>>>>> Otherwise, if there are some performance-releated (or not...)
>>>>>>>>>>> command line options you think clang is missing / would benefit, I invite
>>>>>>>>>>> you to propose adding them to cfe-dev at lists.llvm.org and submit
>>>>>>>>>>> a patch!
>>>>>>>>>>>
>>>>>>>>>>> Best,
>>>>>>>>>>>
>>>>>>>>>>> --
>>>>>>>>>>> Mehdi
>>>>>>>>>>>
>>>>>>>>>>> 2018-01-07 21:03 GMT-08:00 toddy wang <wenwangtoddy at gmail.com>:
>>>>>>>>>>>
>>>>>>>>>>>> Thanks a lot, Mehdi.
>>>>>>>>>>>>
>>>>>>>>>>>> For GCC, there are around 190 optimization flags exposed as
>>>>>>>>>>>> command-line options.
>>>>>>>>>>>> For Clang/LLVM, the number is 40, and many important
>>>>>>>>>>>> optimization parameters are not exposed at all, such as loop unrolling
>>>>>>>>>>>> factor, inline function size parameters.
>>>>>>>>>>>>
>>>>>>>>>>>> I understand there is very different idea for whether or not
>>>>>>>>>>>> expose many flags to end-user.
>>>>>>>>>>>> Personally, I believe it is a reasonable to keep end-user
>>>>>>>>>>>> controllable command-line options minimal for user-friendliness.
>>>>>>>>>>>> However, for users who care a lot for a tiny bit performance
>>>>>>>>>>>> improvement, like HPC community, it may be better to expose as many
>>>>>>>>>>>> fine-grained tunables in the form of command line options as possible. Or,
>>>>>>>>>>>> at least there should be a way to achieve this fairly easy.
>>>>>>>>>>>>
>>>>>>>>>>>> I am curious about which way is the best for my purpose.
>>>>>>>>>>>> Please see my latest reply for 3 possible fine-grained
>>>>>>>>>>>> optimization pipeline.
>>>>>>>>>>>> Looking forward to more discussions.
>>>>>>>>>>>>
>>>>>>>>>>>> Thanks a lot!
>>>>>>>>>>>>
>>>>>>>>>>>> On Sun, Jan 7, 2018 at 10:11 AM, Mehdi AMINI <
>>>>>>>>>>>> joker.eph at gmail.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> Hi,
>>>>>>>>>>>>>
>>>>>>>>>>>>> "SetC" options are LLVM cl::opt options, they are intended for
>>>>>>>>>>>>> LLVM developer and experimentations. If a settings is intended to be used
>>>>>>>>>>>>> as a public API, there is usually a programmatic way of setting it in LLVM.
>>>>>>>>>>>>> "SetA" is what clang as a C++ compiler exposes to the
>>>>>>>>>>>>> end-user. Internally clang will (most of the time) use one or multiple LLVM
>>>>>>>>>>>>> APIs to propagate a settings.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Best,
>>>>>>>>>>>>>
>>>>>>>>>>>>> --
>>>>>>>>>>>>> Mehdi
>>>>>>>>>>>>>
>>>>>>>>>>>>> 2018-01-05 17:41 GMT-08:00 toddy wang via llvm-dev <
>>>>>>>>>>>>> llvm-dev at lists.llvm.org>:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> Craig, thanks a lot!
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> I'm actually confused by clang optimization flags.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> If I run clang -help, it will show many optimizations
>>>>>>>>>>>>>> (denoted as set A) and non-optimization options (denoted as set B).
>>>>>>>>>>>>>> If I run llvm-as < /dev/null | opt -O0/1/2/3 -disable-output
>>>>>>>>>>>>>> -debug-pass=Arguments, it also shows many optimization flags (denote as set
>>>>>>>>>>>>>> C).
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> There are many options in set C while not in set A, and also
>>>>>>>>>>>>>> options in set A but not in set C.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> The general question is: what is the relationship between
>>>>>>>>>>>>>> set A and set C, at the same optimization level O0/O1/O2/O3?
>>>>>>>>>>>>>> Another question is: how to specify an option in set C as a
>>>>>>>>>>>>>> clang command line option, if it is not in A?
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> For example, -dse is in set C but not in set A, how can I
>>>>>>>>>>>>>> specify it as a clang option? Or simply I cannot do that.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> On Fri, Jan 5, 2018 at 7:55 PM, Craig Topper <
>>>>>>>>>>>>>> craig.topper at gmail.com> wrote:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> O0 didn't start applying optnone until r304127 in May 2017
>>>>>>>>>>>>>>> which is after the 4.0 family was branched. So only 5.0, 6.0, and trunk
>>>>>>>>>>>>>>> have that behavior. Commit message copied below
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Author: Mehdi Amini <joker.eph at gmail.com>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Date: Mon May 29 05:38:20 2017 +0000
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> IRGen: Add optnone attribute on function during O0
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Amongst other, this will help LTO to correctly
>>>>>>>>>>>>>>> handle/honor files
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> compiled with O0, helping debugging failures.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> It also seems in line with how we handle other options,
>>>>>>>>>>>>>>> like how
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> -fnoinline adds the appropriate attribute as well.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Differential Revision: https://reviews.llvm.org/D28404
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> ~Craig
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On Fri, Jan 5, 2018 at 4:49 PM, toddy wang <
>>>>>>>>>>>>>>> wenwangtoddy at gmail.com> wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> @Zhaopei, thanks for the clarification.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> @Craig and @Michael, for clang 4.0.1, -Xclang
>>>>>>>>>>>>>>>> -disable-O0-optnone gives the following error message. From which
>>>>>>>>>>>>>>>> version -disable-O0-optnone gets supported?
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> [twang15 at c89 temp]$ clang++ -O0 -Xclang
>>>>>>>>>>>>>>>> -disable-O0-optnone -Xclang -disable-llvm-passes -c -emit-llvm -o a.bc
>>>>>>>>>>>>>>>> LULESH.cc
>>>>>>>>>>>>>>>> error: unknown argument: '-disable-O0-optnone'
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> [twang15 at c89 temp]$ clang++ --version
>>>>>>>>>>>>>>>> clang version 4.0.1 (tags/RELEASE_401/final)
>>>>>>>>>>>>>>>> Target: x86_64-unknown-linux-gnu
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> On Fri, Jan 5, 2018 at 4:45 PM, Craig Topper <
>>>>>>>>>>>>>>>> craig.topper at gmail.com> wrote:
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> If you pass -O0 to clang, most functions will be tagged
>>>>>>>>>>>>>>>>> with an optnone function attribute that will prevent opt and llc even if
>>>>>>>>>>>>>>>>> you pass -O3 to opt and llc. This is the mostly likely cause for the slow
>>>>>>>>>>>>>>>>> down in 2.
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> You can disable the optnone function attribute behavior by
>>>>>>>>>>>>>>>>> passing "-Xclang -disable-O0-optnone" to clang
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> ~Craig
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On Fri, Jan 5, 2018 at 1:19 PM, toddy wang via llvm-dev <
>>>>>>>>>>>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I tried the following on LULESH1.0 serial version (
>>>>>>>>>>>>>>>>>> https://codesign.llnl.gov/lulesh/LULESH.cc)
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> 1. clang++ -O3 LULESH.cc; ./a.out 20
>>>>>>>>>>>>>>>>>> Runtime: 9.487353 second
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> 2. clang++ -O0 -Xclang -disable-llvm-passes -c -emit-llvm
>>>>>>>>>>>>>>>>>> -o a.bc LULESH.cc; opt -O3 a.bc -o b.bc; llc -O3 -filetype=obj b.bc -o b.o
>>>>>>>>>>>>>>>>>> ; clang++ b.o -o b.out; ./b.out 20
>>>>>>>>>>>>>>>>>> Runtime: 24.15 seconds
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> 3. clang++ -O3 -Xclang -disable-llvm-passes -c -emit-llvm
>>>>>>>>>>>>>>>>>> -o a.bc LULESH.cc; opt -O3 a.bc -o b.bc; llc -O3 -filetype=obj b.bc -o b.o
>>>>>>>>>>>>>>>>>> ; clang++ b.o -o b.out; ./b.out 20
>>>>>>>>>>>>>>>>>> Runtime: 9.53 seconds
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> 1 and 3 have almost the same performance, while 2 is
>>>>>>>>>>>>>>>>>> significantly worse, while I expect 1, 2 ,3 should have trivial difference.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Is this a wrong expectation?
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> @Peizhao, what did you try in your last post?
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> On Tue, Apr 11, 2017 at 12:15 PM, Peizhao Ou via llvm-dev
>>>>>>>>>>>>>>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> It's really nice of you pointing out the -Xclang option,
>>>>>>>>>>>>>>>>>>> it makes things much easier. I really appreciate your help!
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Best,
>>>>>>>>>>>>>>>>>>> Peizhao
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> On Mon, Apr 10, 2017 at 10:12 PM, Mehdi Amini <
>>>>>>>>>>>>>>>>>>> mehdi.amini at apple.com> wrote:
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> On Apr 10, 2017, at 5:21 PM, Craig Topper via llvm-dev <
>>>>>>>>>>>>>>>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> clang -O0 does not disable all optimization passes
>>>>>>>>>>>>>>>>>>>> modify the IR.; In fact it causes most functions to get tagged with
>>>>>>>>>>>>>>>>>>>> noinline to prevent inlinining
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> It also disable lifetime instrinsics emission and TBAA,
>>>>>>>>>>>>>>>>>>>> etc.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> What you really need to do is
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> clang -O3 -c emit-llvm -o source.bc -v
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Find the -cc1 command line from that output. Execute
>>>>>>>>>>>>>>>>>>>> that command with --disable-llvm-passes. leave the -O3 and everything else.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> That’s a bit complicated: CC1 options can be passed
>>>>>>>>>>>>>>>>>>>> through with -Xclang, for example here just adding to the regular clang
>>>>>>>>>>>>>>>>>>>> invocation ` -Xclang -disable-llvm-passes`
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> Best,
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> —
>>>>>>>>>>>>>>>>>>>> Mehdi
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> You should be able to feed the output from that command
>>>>>>>>>>>>>>>>>>>> to opt/llc and get consistent results.
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> ~Craig
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> On Mon, Apr 10, 2017 at 4:57 PM, Peizhao Ou via
>>>>>>>>>>>>>>>>>>>> llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> Hi folks,
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> I am wondering about the relationship clang, opt and
>>>>>>>>>>>>>>>>>>>>> llc. I understand that this has been asked, e.g.,
>>>>>>>>>>>>>>>>>>>>> http://stackoverflow.com/questions/40350990/relationsh
>>>>>>>>>>>>>>>>>>>>> ip-between-clang-opt-llc-and-llvm-linker. Sorry for
>>>>>>>>>>>>>>>>>>>>> posting a similar question again, but I still have something that hasn't
>>>>>>>>>>>>>>>>>>>>> been resolved yet.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> More specifically I am wondering about the following
>>>>>>>>>>>>>>>>>>>>> two approaches compiling optimized executable:
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> 1. clang -O3 -c source.c -o source.o
>>>>>>>>>>>>>>>>>>>>> ...
>>>>>>>>>>>>>>>>>>>>> clang a.o b.o c.o ... -o executable
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> 2. clang -O0 -c -emit-llvm -o source.bc
>>>>>>>>>>>>>>>>>>>>> opt -O3 source.bc -o source.bc
>>>>>>>>>>>>>>>>>>>>> llc -O3 -filetype=obj source.bc -o source.o
>>>>>>>>>>>>>>>>>>>>> ...
>>>>>>>>>>>>>>>>>>>>> clang a.o b.o c.o ... -o executable
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> I took a look at the source code of the clang tool and
>>>>>>>>>>>>>>>>>>>>> the opt tool, they both seem to use the PassManagerBuilder::populateModulePassManager()
>>>>>>>>>>>>>>>>>>>>> and PassManagerBuilder::populateFunctionPassManager()
>>>>>>>>>>>>>>>>>>>>> functions to add passes to their optimization pipeline; and for the
>>>>>>>>>>>>>>>>>>>>> backend, the clang and llc both use the addPassesToEmitFile() function to
>>>>>>>>>>>>>>>>>>>>> generate object code.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> So presumably the above two approaches to generating
>>>>>>>>>>>>>>>>>>>>> optimized executable file should do the same thing. However, I am seeing
>>>>>>>>>>>>>>>>>>>>> that the second approach is around 2% slower than the first approach (which
>>>>>>>>>>>>>>>>>>>>> is the way developers usually use) pretty consistently.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> Can anyone point me to the reasons why this happens?
>>>>>>>>>>>>>>>>>>>>> Or even correct my wrong understanding of the relationship between these
>>>>>>>>>>>>>>>>>>>>> two approaches?
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> PS: I used the -debug-pass=Structure option to print
>>>>>>>>>>>>>>>>>>>>> out the passes, they seem the same except that the first approach has an
>>>>>>>>>>>>>>>>>>>>> extra pass called "-add-discriminator", but I don't think that's the reason.
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> Peizhao
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>> _______________________________________________
>>>>>>>>>>>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>>>>>>>>>>>> http://lists.llvm.org/cgi-bin/
>>>>>>>>>>>>>>>>>>>>> mailman/listinfo/llvm-dev
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> _______________________________________________
>>>>>>>>>>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> _______________________________________________
>>>>>>>>>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> _______________________________________________
>>>>>>>>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> _______________________________________________
>>>>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>> _______________________________________________
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>>
>
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