[llvm-dev] Disable combining of loads and stores in instcombine
Neil Ryan via llvm-dev
llvm-dev at lists.llvm.org
Fri Apr 19 13:53:56 PDT 2019
Sorry, I meant that if I have a series of character loads that get combined into a single word load, I don’t know of a way to 1) know that the word load was originally comprised of character loads and 2) decompose the word load back into character loads.
Granted, if I have (1), (2) just a matter of inserting the right ops. I’ve been digging into MachineMemOperand — I’m not entirely sure what methods in this class can get me this information. Any pointers would be much appreciated.
On Apr 18, 2019, 1:45 PM -0700, Arsenault, Matthew <Matthew.Arsenault at amd.com>, wrote:
> I’m not sure what you mean by this. The type in memory doesn’t really mean anything, and no information is lost. You can still tell (for optimizations) some information about the underlying IR object from the MachineMemOperand, which is mostly used for alias analysis.
> From: Neil Ryan <neilryan at cs.washington.edu>
> Date: Thursday, April 18, 2019 at 9:59 PM
> To: llvm-dev <llvm-dev at lists.llvm.org>, "tstellar at redhat.com" <tstellar at redhat.com>, "Arsenault, Matthew" <Matthew.Arsenault at amd.com>
> Subject: Re: [llvm-dev] Disable combining of loads and stores in instcombine
> IIRC it’s not strictly possible to determine what array a load/store is based on. I don’t believe the decomposition is always possible, as information is lost when accesses are combined.
> On Apr 17, 2019, 12:31 PM -0700, Arsenault, Matthew <Matthew.Arsenault at amd.com>, wrote:
> > This is really a codegen problem. You can decompose the load/store however you like in the backend. InstCombine should still combine the loads as a canonicalization.
> > -Matt
> > From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of llvm-dev <llvm-dev at lists.llvm.org>
> > Reply-To: Neil Ryan <neilryan at cs.washington.edu>
> > Date: Wednesday, April 17, 2019 at 9:28 PM
> > To: llvm-dev <llvm-dev at lists.llvm.org>, "tstellar at redhat.com" <tstellar at redhat.com>
> > Subject: Re: [llvm-dev] Disable combining of loads and stores in instcombine
> > I’m writing a pass for some custom hardware — we’d like to split arrays across hardware elements; this doesn’t work if consecutive writes to characters get combined to a word.
> > On Apr 16, 2019, 8:17 PM -0700, Tom Stellard <tstellar at redhat.com>, wrote:
> > > On 04/16/2019 11:38 AM, Neil Ryan via llvm-dev wrote:
> > > > LLVM's optimizer combines stores to consecutive characters into a write of a single word. For instance, if I have char A and I write some static value to each element, these writes would be combined into a single 32-bit word write. I found this thread <http://llvm.1065342.n5.nabble.com/disabling-combining-load-stores-in-optimizer-td37560.html> from 2009 -- it seems like it wasn't possible then. Has anything changed since?
> > >
> > > Why do you want to disable this optimization?
> > >
> > > -Tom
> > >
> > >
> > > > Neil
> > > >
> > > >
> > > > _______________________________________________
> > > > LLVM Developers mailing list
> > > > llvm-dev at lists.llvm.org
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> > >
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