Re: [cfe-users] [llvm-dev] Fine Grained Optimization Control
Hi Navid, comments inlined. On 3/27/21 9:24 PM, Navid Rahimi via llvm-dev wrote: Hi everyone, tl;dr: I want to control which optimization and transformation can and will run on my code. Does Clang/LLVM permit such an approach? There is no unified approach to this as far as I know. The closest I'm aware of was some research prototype: https://compilers.cs.uni-saarland.de/projects/noise/ I am doing this with GCC. But at first, it seems for some reason GCC does not allow optimizations to run unless I am passing -Ox flag (x>=1). The approach I thought would work is using -O3 and disabling all the optimizations one by one with -fno-XXX, then passing each optimization I want with -fXXX. Even after doing that it seems GCC does take the flags seriously. Sometimes it might consider the -fXXX flags, but sometimes it totally ignores. I was investigating this issue more recently due to a project I am involved in. I realized that there are two sets of optimizations and transformation can happen in Clang/LLVM. Clang can do a few optimizations itself on AST and then LLVM will run its own optimizations. Please correct me if I am wrong. I'm not aware of optimizations/transformation we do on the AST, except the things that "have to" happen on that level. Here is a list of few questions I am trying to find an answer for: 1) I am looking for a list of optimizations that Clang might do. Where can I find them? I doubt there are "optimzations" to speak of, constant propagation can happen though. 2) I am looking for a list of optimizations that LLVM might do. Where can I find them? Most passes that exist in LLVM are listed in llvm/lib/Passes/PassRegistry.def There are (outdated) lists online as well. 3) Is there any way to disable/enable specific Clang optimization? Most, if not all, are mandatory. 4) Is there any way to disable/enable specific LLVM optimization? Some, not all, have command line flags to disable them, I would do: opt -help-hidden | grep disable opt -help-hidden | grep enable if I needed a list. 5) Would LLVM/Clang respect specific optimization flags? I don't think you can build your own optimization pipelines via clang but you can emit IR and do it with opt. I appreciate immensely any help regarding these questions. Hope this helps, others might have more information. ~ Johannes Best wishes, Navid. ___ LLVM Developers mailing list llvm-...@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
Re: [cfe-users] [llvm-dev] Fine Grained Optimization Control
Thanks Johannes. That makes this makes it more understandable to me. What can I do for optimization that doesn’t have flag? How should I approach disabling them. On Sat, Mar 27, 2021 at 22:10 Johannes Doerfert wrote: > Hi Navid, > > comments inlined. > > On 3/27/21 9:24 PM, Navid Rahimi via llvm-dev wrote: > > Hi everyone, > > > > tl;dr: I want to control which optimization and transformation can and > will > > run on my code. Does Clang/LLVM permit such an approach? > > There is no unified approach to this as far as I know. The closest > I'm aware of was some research prototype: >https://compilers.cs.uni-saarland.de/projects/noise/ > > > > > > I am doing this with GCC. But at first, it seems for some reason GCC does > > not allow optimizations to run unless I am passing -Ox flag (x>=1). The > > approach I thought would work is using -O3 and disabling all the > > optimizations one by one with -fno-XXX, then passing each optimization I > > want with -fXXX. Even after doing that it seems GCC does take the flags > > seriously. Sometimes it might consider the -fXXX flags, but sometimes it > > totally ignores. > > > > I was investigating this issue more recently due to a project I am > involved > > in. I realized that there are two sets of optimizations and > transformation > > can happen in Clang/LLVM. Clang can do a few optimizations itself on AST > > and then LLVM will run its own optimizations. Please correct me if I am > > wrong. > > I'm not aware of optimizations/transformation we do on the AST, > except the things that "have to" happen on that level. > > > > > > Here is a list of few questions I am trying to find an answer for: > > 1) I am looking for a list of optimizations that Clang might do. Where > can > > I find them? > I doubt there are "optimzations" to speak of, constant propagation > can happen though. > > > > 2) I am looking for a list of optimizations that LLVM might do. Where > can I > > find them? > Most passes that exist in LLVM are listed in >llvm/lib/Passes/PassRegistry.def > > There are (outdated) lists online as well. > > > > 3) Is there any way to disable/enable specific Clang optimization? > > Most, if not all, are mandatory. > > > > 4) Is there any way to disable/enable specific LLVM optimization? > > Some, not all, have command line flags to disable them, I would do: >opt -help-hidden | grep disable >opt -help-hidden | grep enable > > if I needed a list. > > > 5) Would LLVM/Clang respect specific optimization flags? > > I don't think you can build your own optimization pipelines via clang > but you can emit IR and do it with opt. > > > > > > I appreciate immensely any help regarding these questions. > > Hope this helps, others might have more information. > > ~ Johannes > > > > > > Best wishes, > > Navid. > > > > > > ___ > > LLVM Developers mailing list > > llvm-...@lists.llvm.org > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > -- Best wishes, Navid. ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
Re: [cfe-users] [llvm-dev] Fine Grained Optimization Control
I recommend adding such a flag to the pass you want to disable. Whenever `runOnXXX` is called, check the flag and exit if set. ~ Johannes On 3/28/21 5:27 PM, Navid Rahimi wrote: Thanks Johannes. That makes this makes it more understandable to me. What can I do for optimization that doesn’t have flag? How should I approach disabling them. On Sat, Mar 27, 2021 at 22:10 Johannes Doerfert wrote: Hi Navid, comments inlined. On 3/27/21 9:24 PM, Navid Rahimi via llvm-dev wrote: Hi everyone, tl;dr: I want to control which optimization and transformation can and will run on my code. Does Clang/LLVM permit such an approach? There is no unified approach to this as far as I know. The closest I'm aware of was some research prototype: https://compilers.cs.uni-saarland.de/projects/noise/ I am doing this with GCC. But at first, it seems for some reason GCC does not allow optimizations to run unless I am passing -Ox flag (x>=1). The approach I thought would work is using -O3 and disabling all the optimizations one by one with -fno-XXX, then passing each optimization I want with -fXXX. Even after doing that it seems GCC does take the flags seriously. Sometimes it might consider the -fXXX flags, but sometimes it totally ignores. I was investigating this issue more recently due to a project I am involved in. I realized that there are two sets of optimizations and transformation can happen in Clang/LLVM. Clang can do a few optimizations itself on AST and then LLVM will run its own optimizations. Please correct me if I am wrong. I'm not aware of optimizations/transformation we do on the AST, except the things that "have to" happen on that level. Here is a list of few questions I am trying to find an answer for: 1) I am looking for a list of optimizations that Clang might do. Where can I find them? I doubt there are "optimzations" to speak of, constant propagation can happen though. 2) I am looking for a list of optimizations that LLVM might do. Where can I find them? Most passes that exist in LLVM are listed in llvm/lib/Passes/PassRegistry.def There are (outdated) lists online as well. 3) Is there any way to disable/enable specific Clang optimization? Most, if not all, are mandatory. 4) Is there any way to disable/enable specific LLVM optimization? Some, not all, have command line flags to disable them, I would do: opt -help-hidden | grep disable opt -help-hidden | grep enable if I needed a list. 5) Would LLVM/Clang respect specific optimization flags? I don't think you can build your own optimization pipelines via clang but you can emit IR and do it with opt. I appreciate immensely any help regarding these questions. Hope this helps, others might have more information. ~ Johannes Best wishes, Navid. ___ LLVM Developers mailing list llvm-...@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
Re: [cfe-users] [llvm-dev] Fine Grained Optimization Control
Makes sense thank you. I have to look at the dependencies between different optimization passes too I guess. On Sun, Mar 28, 2021 at 17:33 Johannes Doerfert wrote: > I recommend adding such a flag to the pass you want to disable. > Whenever `runOnXXX` is called, check the flag and exit if set. > > ~ Johannes > > > On 3/28/21 5:27 PM, Navid Rahimi wrote: > > Thanks Johannes. That makes this makes it more understandable to me. What > > can I do for optimization that doesn’t have flag? How should I approach > > disabling them. > > > > On Sat, Mar 27, 2021 at 22:10 Johannes Doerfert < > johannesdoerf...@gmail.com> > > wrote: > > > >> Hi Navid, > >> > >> comments inlined. > >> > >> On 3/27/21 9:24 PM, Navid Rahimi via llvm-dev wrote: > >>> Hi everyone, > >>> > >>> tl;dr: I want to control which optimization and transformation can and > >> will > >>> run on my code. Does Clang/LLVM permit such an approach? > >> There is no unified approach to this as far as I know. The closest > >> I'm aware of was some research prototype: > >> https://compilers.cs.uni-saarland.de/projects/noise/ > >> > >> > >>> I am doing this with GCC. But at first, it seems for some reason GCC > does > >>> not allow optimizations to run unless I am passing -Ox flag (x>=1). The > >>> approach I thought would work is using -O3 and disabling all the > >>> optimizations one by one with -fno-XXX, then passing each optimization > I > >>> want with -fXXX. Even after doing that it seems GCC does take the flags > >>> seriously. Sometimes it might consider the -fXXX flags, but sometimes > it > >>> totally ignores. > >>> > >>> I was investigating this issue more recently due to a project I am > >> involved > >>> in. I realized that there are two sets of optimizations and > >> transformation > >>> can happen in Clang/LLVM. Clang can do a few optimizations itself on > AST > >>> and then LLVM will run its own optimizations. Please correct me if I am > >>> wrong. > >> I'm not aware of optimizations/transformation we do on the AST, > >> except the things that "have to" happen on that level. > >> > >> > >>> Here is a list of few questions I am trying to find an answer for: > >>> 1) I am looking for a list of optimizations that Clang might do. Where > >> can > >>> I find them? > >> I doubt there are "optimzations" to speak of, constant propagation > >> can happen though. > >> > >> > >>> 2) I am looking for a list of optimizations that LLVM might do. Where > >> can I > >>> find them? > >> Most passes that exist in LLVM are listed in > >> llvm/lib/Passes/PassRegistry.def > >> > >> There are (outdated) lists online as well. > >> > >> > >>> 3) Is there any way to disable/enable specific Clang optimization? > >> Most, if not all, are mandatory. > >> > >> > >>> 4) Is there any way to disable/enable specific LLVM optimization? > >> Some, not all, have command line flags to disable them, I would do: > >> opt -help-hidden | grep disable > >> opt -help-hidden | grep enable > >> > >> if I needed a list. > >> > >>> 5) Would LLVM/Clang respect specific optimization flags? > >> I don't think you can build your own optimization pipelines via clang > >> but you can emit IR and do it with opt. > >> > >> > >>> I appreciate immensely any help regarding these questions. > >> Hope this helps, others might have more information. > >> > >> ~ Johannes > >> > >> > >>> Best wishes, > >>> Navid. > >>> > >>> > >>> ___ > >>> LLVM Developers mailing list > >>> llvm-...@lists.llvm.org > >>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > -- Best wishes, Navid. ___ cfe-users mailing list cfe-users@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
