[LLVMdev] LICM for function calls

Sun Jul 19 21:14:21 PDT 2015

----- Original Message -----
> From: "Philip Reames" <listmail at philipreames.com>
> To: "Hal Finkel" <hfinkel at anl.gov>, "Xin Tong" <trent.xin.tong at gmail.com>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Friday, July 17, 2015 7:38:31 PM
> Subject: Re: [LLVMdev] LICM for function calls
> 
> 
> 
> On 07/15/2015 07:05 PM, Hal Finkel wrote:
> > ----- Original Message -----
> >> From: "Xin Tong" <trent.xin.tong at gmail.com>
> >> To: "Philip Reames" <listmail at philipreames.com>
> >> Cc: "Hal Finkel" <hfinkel at anl.gov>, llvmdev at cs.uiuc.edu
> >> Sent: Wednesday, July 15, 2015 6:35:11 PM
> >> Subject: Re: [LLVMdev] LICM for function calls
> >>
> >> i think attributes have taken control flow into account. I think
> >> readnone and nounwind functions are not safe to speculative
> >> execute
> >> because the function could run indefinitely, e.g. an infinite
> >> loop.
> > Yes and no. Our readnone is meant to be modeled on GCC's 'pure'
> > attribute, and as documented
> > (https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes)
> > cannot be applied to functions containing infinite loops. It says
> > specifically, "Interesting non-pure functions are functions with
> > infinite loops or those depending on volatile memory or ..."
> >
> > And so, no, readnone functions must return normally in a finite
> > period of time.
> >
> > However, we also have a long-standing issue here, in that, the
> > FunctionAttrs transformation will add readnone to functions that
> > don't have side effects, regardless of whether or not they
> > potentially have infinite loops. This is not wrong for C/C++, etc.
> > where we get to assume termination of loops, but may cause
> > problems for other languages where infinite loops are well
> > defined.
> >
> > However, infrastructure-wise, it is hard to fix this problem,
> > because FunctionAttrs runs well before the loop canonicalization
> > that would make it easy to determine loop trip counts (even
> > assuming the pass manager were fixed to allow a CGSCC pass to use
> > SCEV in the first place). What we really need here, I suspect, is
> > some attribute indicating C-like termination semantics that can be
> > attached to functions from such languages and will allow us to
> > infer things like readnone without actually doing loop analysis
> > when the semantics of the source language allows it.
> Hal, I disagree with your take here.  I agree that the current
> optimizer
> is a bit muddied about how it treats potentially infinite loops, but
> I
> see nothing in the documentation that says we can assume C++
> semantics
> for llvm IR.  In fact, there's an example of an infinite loop given
> in
> the LangRef as an example of valid code.  :) (PHI Instruction
> example)

As you might suppose from the thread I started on infinite loops, I don't disagree that the IR supports infinite loops (or at least should).

> 
> Given the lack of that, I would argue that the current state is that
> readonly is pretty clearly different from gcc's pure attribute.  I
> would
> argue that the bugs is that we DCE readonly functions which might
> contain infinite loops.  :)

I was arguing that, in practice, readonly/readnone are used to represent GCC's pure/const attributes (in Clang) and weakening that relationship must be done carefully as to not regress existing users. FWIW, prior to r44273 (way back in 2007) these attributes were actually named pure and const; and this issue came up in 2010 (http://permalink.gmane.org/gmane.comp.compilers.llvm.devel/31490). Indeed, I agree we should fix this, but we should not do so until we have a mechanism to retain the current semantics for the Clang users who rightfully benefit from them (from their point of view, pure/const are behaving as documented). This was one of the major motivations for starting the thread on infinite loops.

Thanks again,
Hal

> >
> >   -Hal
> >
> >> -Xin
> >>
> >> On Tuesday, July 14, 2015, Philip Reames <
> >> listmail at philipreames.com
> >>> wrote:
> >>
> >> On 07/14/2015 10:25 PM, Hal Finkel wrote:
> >>
> >>
> >> ----- Original Message -----
> >>
> >>
> >> From: "Philip Reames" < listmail at philipreames.com >
> >> To: "Thomas F Raoux" < thomas.f.raoux at intel.com >,
> >> llvmdev at cs.uiuc.edu
> >> Sent: Tuesday, July 14, 2015 11:59:49 PM
> >> Subject: Re: [LLVMdev] LICM for function calls
> >>
> >> On 07/14/2015 07:45 AM, Raoux, Thomas F wrote:
> >>
> >>
> >> Hi,
> >>
> >> Right now in LICM (and many other transformations) we always
> >> assume
> >> it is never safe to speculatively execute a function call.
> >>
> >> The following function always return false for all function calls
> >> except for few intrinsics:
> >> bool llvm::isSafeToSpeculativelyExecute(const Value *V,
> >> const DataLayout *TD) {
> >> ...
> >> case Instruction::Call: {
> >> if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
> >> ...
> >> }
> >> return false; // The called function could have undefined
> >> behavior or
> >> // side-effects, even if marked readnone
> >> nounwind.
> >>
> >> In some cases this could have an important performance impact so
> >> I'm looking for a potential way to improve it.
> >>
> >> Is there any combination of attributes which would guarantee that
> >> a
> >> function is safe to speculatively execute a function call? (As far
> >> as I can tell there isn't.)
> >> In general, no there isn't. The challenge is that a function's
> >> attributes can be control dependent. For example, you can have a
> >> function which is "readnone argmemonly nounwind" which writes the
> >> entire
> >> heap if the global "c" is true. Hoisting a call to such a function
> >> above "if (!c)" would be problematic. I think that in practice
> >> this
> >> is
> >> mostly an issue with call site attributes, but I believe the same
> >> conceptual problem applies to attributes on function declarations
> >> as
> >> well.
> >> I'm fairly certain we already consider declaration attributes to
> >> have
> >> no control dependencies.
> >> If true, this would be great. I wasn't sure what the rules here
> >> were
> >> and was trying to be conservative. If nothing else, we need to
> >> clarify the docs and make sure everyone is actually in agreement
> >> about this.
> >>
> >>
> >>
> >> -Hal
> >>
> >>
> >>
> >> Changing that might be reasonable, but we'd need to spec it
> >> carefully. I think there's room here for improvement, but it'll
> >> likely
> >> be a slow process at first.
> >>
> >> One area you might look into is the "guaranteed to execute" notion
> >> in
> >> LICM. This gives a basis for hoisting a call out of a loop which
> >> doesn't require speculating it past any relevant control
> >> dependence.
> >> You still have to worry about aliasing for safety, but reasoning
> >> about
> >> the semantics of attributes and faults should be a bit more
> >> straight
> >> forward.
> >>
> >>
> >> Does anybody have a suggestion on what would be the best way to
> >> implement such functionality? Or do you think such optimization is
> >> undesirable?
> >>
> >> Cheers,
> >> Thomas
> >>
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> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory