[PATCH] Add a dereferencable attribute

[Hal Finkel]

----- Original Message -----
> From: "Hal Finkel" <hfinkel at anl.gov>
> To: "Nick Lewycky" <nlewycky at google.com>
> Cc: "Chandler Carruth" <chandlerc at gmail.com>, aschwaighofer at apple.com, "Philip Reames" <listmail at philipreames.com>,
> me+llvm at luqman.ca, "Commit Messages and Patches for LLVM" <llvm-commits at cs.uiuc.edu>,
> reviews+D4449+public+728896e6e6399a87 at reviews.llvm.org
> Sent: Thursday, July 10, 2014 8:59:10 AM
> Subject: Re: [PATCH] Add a dereferencable attribute
> 
> ----- Original Message -----
> > From: "Nick Lewycky" <nlewycky at google.com>
> > To: reviews+D4449+public+728896e6e6399a87 at reviews.llvm.org
> > Cc: "Hal Finkel" <hfinkel at anl.gov>, "Chandler Carruth"
> > <chandlerc at gmail.com>, aschwaighofer at apple.com, "Philip
> > Reames" <listmail at philipreames.com>, me+llvm at luqman.ca, "Commit
> > Messages and Patches for LLVM"
> > <llvm-commits at cs.uiuc.edu>
> > Sent: Thursday, July 10, 2014 3:50:54 AM
> > Subject: Re: [PATCH] Add a dereferencable attribute
> > 
> > 
> > 
> > 
> > On 9 July 2014 20:19, hfinkel at anl.gov < hfinkel at anl.gov > wrote:
> > 
> > 
> > Hi chandlerc, aschwaighofer, nlewycky, reames, luqmana,
> > 
> > This patch adds a dereferencable attribute. In some sense, this is
> > a
> > companion to the nonnull attribute, but specifies that the pointer
> > is known to be dereferencable in the same sense as a pointer
> > generated by alloca is known to be dereferencable.
> > 
> > 
> > 
> > Please don't make it rely on the llvm type like this. We pick llvm
> > types to cause ABI decisions in the backend, we may pad them out to
> > a multiple of alignment just because we happen to know they're on
> > the stack, we do all manner of silliness with llvm types. I do
> > understand how Value::isDereferenceablePointer works, but it
> > doesn't
> > have to deal with the language splice; that the C++ language (or
> > other language) may have a spec that guarantees different bytes are
> > dereferenceable than the type would imply. It sees an alloca tied
> > to
> > an llvm type and knows it's okay, not just because of the type and
> > not just because of the alloca.
> > 
> > 
> > Instead, there is really nifty support for llvm attributes which
> > contain values, either as integers or as strings. Please make the
> > frontend encode the number of bytes dereferenceable into the
> > attribute and make llvm look at that value. Combined with
> > DataLayout
> > we may show that it covers the whole llvm type as passed in, but
> > even if it doesn't we can analyze offsetting GEPs and such
> > individually.
> 
> Good point. I'll do that!

Another benefit of encoding the size is that we can handle the C99's int x[static 3] function parameters. ;)

 -Hal

> 
>  -Hal
> 
> > 
> > 
> > Nick
> > 
> > 
> > 
> > 
> > My primary use case for this is C++ references that are passes as
> > function parameters and used in loops. For example,
> > 
> > void foo(int * __restrict__ a, int * __restrict__ b, int &c, int n)
> > {
> > for (int i = 0; i < n; ++i)
> > if (a[i] > 0)
> > a[i] = c*b[i];
> > }
> > 
> > Currently, we generate a load of 'c' in every loop iteration
> > because
> > we can't prove that there is no control dependence on the
> > dereferencability of 'c' from the if condition. But because 'c' is
> > a
> > reference, we know it must point to something, and since nothing
> > else in the loop can alias it, it is safe to load it in the
> > preheader.
> > 
> > (there is a simple companion Clang patch, which I'll also post.)
> > 
> > http://reviews.llvm.org/D4449
> > 
> > Files:
> > docs/LangRef.rst
> > include/llvm-c/Core.h
> > include/llvm/Bitcode/LLVMBitCodes.h
> > include/llvm/IR/Argument.h
> > include/llvm/IR/Attributes.h
> > lib/AsmParser/LLLexer.cpp
> > lib/AsmParser/LLParser.cpp
> > lib/AsmParser/LLToken.h
> > lib/Bitcode/Reader/BitcodeReader.cpp
> > lib/Bitcode/Writer/BitcodeWriter.cpp
> > lib/IR/Attributes.cpp
> > lib/IR/Function.cpp
> > lib/IR/Value.cpp
> > test/Bitcode/attributes.ll
> > test/Transforms/LICM/hoist-deref-load.ll
> > 
> > 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 

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