Thank you, it was what I really was searching for :)
However, I don't know if I well understood. I've find this link in the
second link which seemed what i was looking for:
http://nondot.org/sabre/LLVMNotes/BuiltinUnreachable.txt .If I put around the
code block (inside my function with precondition (x>0 && y>0)) a
contruct like that that use __builtin_unreachable:
int foo(int x, int y){
if(x>0 && y>0){ ...function codeblock... } {
__builtin_unreachable (); }
}
I can get the optimization without really having a branch jump in the machine
code?Thank you again,
Niko Zarzani
> Date: Mon, 22 Oct 2012 23:41:43 -0400
> From: xi.wang at gmail.com
> To: koni10 at hotmail.it
> CC: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] precondition suggestion to LLVM
>
> You may want to check this out:
>
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-October/053924.html
>
> and also
>
> http://llvm.org/PR810
>
> - xi
>
> On 10/22/12 6:05 PM, Niko Zarzani wrote:
> > Hi all,
> > Is there any way to tell LLVM some additional information about the
> > variables in the code in order to make better optimization?
> > For example, if my function has a certain precondition (such as
x>0)
> > then it will be possible to better optimize the code given that
> > information (which the compiler does not know).
> > I am new in this field and I don't know if there are ways to tell
the
> > compiler preconditions (such as by using some preprocessing
directives).
> > Thank you in advance,
> > Niko Zarzani
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20121023/742c410f/attachment.html>
On Mon, Oct 22, 2012 at 9:33 PM, Niko Zarzani <koni10 at hotmail.it> wrote:> Thank you, it was what I really was searching for :) > > However, I don't know if I well understood. I've find this link in the > second link which seemed what i was looking for: > http://nondot.org/sabre/LLVMNotes/BuiltinUnreachable.txt . > If I put around the code block (inside my function with precondition (x>0 && > y>0)) a contruct like that that use __builtin_unreachable: > > int foo(int x, int y){ > > if(x>0 && y>0){ > ...function codeblock... > } > { > __builtin_unreachable (); > } > > } > > I can get the optimization without really having a branch jump in the > machine code?In theory, yes - in practice, we eliminate unreachable blocks too early for any real optimization to occur on the basis of this branch to unreachable. More details here: http://llvm.org/bugs/show_bug.cgi?id=810> Thank you again, > > Niko Zarzani > >> Date: Mon, 22 Oct 2012 23:41:43 -0400 >> From: xi.wang at gmail.com >> To: koni10 at hotmail.it >> CC: llvmdev at cs.uiuc.edu > >> Subject: Re: [LLVMdev] precondition suggestion to LLVM >> >> You may want to check this out: >> >> http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-October/053924.html >> >> and also >> >> http://llvm.org/PR810 >> >> - xi >> >> On 10/22/12 6:05 PM, Niko Zarzani wrote: >> > Hi all, >> > Is there any way to tell LLVM some additional information about the >> > variables in the code in order to make better optimization? >> > For example, if my function has a certain precondition (such as x>0) >> > then it will be possible to better optimize the code given that >> > information (which the compiler does not know). >> > I am new in this field and I don't know if there are ways to tell the >> > compiler preconditions (such as by using some preprocessing directives). >> > Thank you in advance, >> > Niko Zarzani > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >
Niko, There are a number of folks interested in similar things, but to my knowledge nothing has been added to LLVM to explicitly support this use case (yet). Using an unreachable instruction is a good starting point, but as mentioned by David, we prune these too early at this point in time. If you're interested, I've got a hacky implementation which works around the problem by inserting a dummy function, running all passes, dropping the dummy function, and rerunning optimizations. I just posted the code on github[1], but - fair warning - this is basically untested at this point. I've run a couple of toy examples, but that's about it. Yours, Philip Reames [1] https://github.com/preames/llvm-assume-hack On 10/22/2012 09:40 PM, David Blaikie wrote:> On Mon, Oct 22, 2012 at 9:33 PM, Niko Zarzani <koni10 at hotmail.it> wrote: >> Thank you, it was what I really was searching for :) >> >> However, I don't know if I well understood. I've find this link in the >> second link which seemed what i was looking for: >> http://nondot.org/sabre/LLVMNotes/BuiltinUnreachable.txt . >> If I put around the code block (inside my function with precondition (x>0 && >> y>0)) a contruct like that that use __builtin_unreachable: >> >> int foo(int x, int y){ >> >> if(x>0 && y>0){ >> ...function codeblock... >> } >> { >> __builtin_unreachable (); >> } >> >> } >> >> I can get the optimization without really having a branch jump in the >> machine code? > > In theory, yes - in practice, we eliminate unreachable blocks too > early for any real optimization to occur on the basis of this branch > to unreachable. More details here: > http://llvm.org/bugs/show_bug.cgi?id=810 > >> Thank you again, >> >> Niko Zarzani >> >>> Date: Mon, 22 Oct 2012 23:41:43 -0400 >>> From: xi.wang at gmail.com >>> To: koni10 at hotmail.it >>> CC: llvmdev at cs.uiuc.edu >> >>> Subject: Re: [LLVMdev] precondition suggestion to LLVM >>> >>> You may want to check this out: >>> >>> http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-October/053924.html >>> >>> and also >>> >>> http://llvm.org/PR810 >>> >>> - xi >>> >>> On 10/22/12 6:05 PM, Niko Zarzani wrote: >>>> Hi all, >>>> Is there any way to tell LLVM some additional information about the >>>> variables in the code in order to make better optimization? >>>> For example, if my function has a certain precondition (such as x>0) >>>> then it will be possible to better optimize the code given that >>>> information (which the compiler does not know). >>>> I am new in this field and I don't know if there are ways to tell the >>>> compiler preconditions (such as by using some preprocessing directives). >>>> Thank you in advance, >>>> Niko Zarzani >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >
(Apologies if anyone gets duplicates. Resending as 4.5 hours after sending, I haven't seen this come through.) Niko, There are a number of folks interested in similar things, but to my knowledge nothing has been added to LLVM to explicitly support this use case (yet). Using an unreachable instruction is a good starting point, but as mentioned by David, we prune these too early at this point in time. If you're interested, I've got a hacky implementation which works around the problem by inserting a dummy function, running all passes, dropping the dummy function, and rerunning optimizations. I just posted the code on github[1], but - fair warning - this is basically untested at this point. I've run a couple of toy examples, but that's about it. Yours, Philip Reames [1] https://github.com/preames/llvm-assume-hack On 10/22/2012 09:40 PM, David Blaikie wrote:> On Mon, Oct 22, 2012 at 9:33 PM, Niko Zarzani <koni10 at hotmail.it> wrote: >> Thank you, it was what I really was searching for :) >> >> However, I don't know if I well understood. I've find this link in the >> second link which seemed what i was looking for: >> http://nondot.org/sabre/LLVMNotes/BuiltinUnreachable.txt . >> If I put around the code block (inside my function with precondition (x>0 && >> y>0)) a contruct like that that use __builtin_unreachable: >> >> int foo(int x, int y){ >> >> if(x>0 && y>0){ >> ...function codeblock... >> } >> { >> __builtin_unreachable (); >> } >> >> } >> >> I can get the optimization without really having a branch jump in the >> machine code? > > In theory, yes - in practice, we eliminate unreachable blocks too > early for any real optimization to occur on the basis of this branch > to unreachable. More details here: > http://llvm.org/bugs/show_bug.cgi?id=810 > >> Thank you again, >> >> Niko Zarzani >> >>> Date: Mon, 22 Oct 2012 23:41:43 -0400 >>> From: xi.wang at gmail.com >>> To: koni10 at hotmail.it >>> CC: llvmdev at cs.uiuc.edu >> >>> Subject: Re: [LLVMdev] precondition suggestion to LLVM >>> >>> You may want to check this out: >>> >>> http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-October/053924.html >>> >>> and also >>> >>> http://llvm.org/PR810 >>> >>> - xi >>> >>> On 10/22/12 6:05 PM, Niko Zarzani wrote: >>>> Hi all, >>>> Is there any way to tell LLVM some additional information about the >>>> variables in the code in order to make better optimization? >>>> For example, if my function has a certain precondition (such as x>0) >>>> then it will be possible to better optimize the code given that >>>> information (which the compiler does not know). >>>> I am new in this field and I don't know if there are ways to tell the >>>> compiler preconditions (such as by using some preprocessing directives). >>>> Thank you in advance, >>>> Niko Zarzani >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >