[llvm-dev] [RFC] Adding range metadata to array subscripts.

[Florian Hahn via llvm-dev]

> On Mar 24, 2021, at 15:16, Johannes Doerfert via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> On 3/24/21 9:06 AM, Clement Courbet wrote:
>> On Wed, Mar 24, 2021 at 2:20 PM Johannes Doerfert <
>> johannesdoerfert at gmail.com> wrote:
>> 
>>> I really like encoding more (range) information in the IR,
>>> more thoughts inlined.
>>> 
>>> On 3/24/21 4:14 AM, Clement Courbet via llvm-dev wrote:
>>>> Hi everyone,
>>>> 
>>>> tl;dr: I would like to teach clang to output range metadata so that LLVM
>>>> can do better alias analysis. I have a proposal as D99248
>>>> <https://reviews.llvm.org/D99248> (clang part) and D99247
>>>> <https://reviews.llvm.org/D99247> (llvm part). But there are other
>>> possible
>>>> options that I'm detailing below.
>>>> 
>>>> Consider the following code, adapted from brotli
>>>> <https://en.wikipedia.org/wiki/Brotli>:
>>>> 
>>>> ```
>>>> 
>>>> struct Histogram {
>>>> 
>>>>    int values[256];
>>>> 
>>>>    int total;
>>>> 
>>>> };
>>>> 
>>>> Histogram DoIt(const int* image, int size) {
>>>> 
>>>>    Histogram histogram;
>>>> 
>>>>    for (int i = 0; i < size; ++i) {
>>>> 
>>>>      ++histogram.values[image[i]];  // (A)
>>>> 
>>>>      ++histogram.total;             // (B)
>>>> 
>>>>    }
>>>> 
>>>>    return histogram;
>>>> 
>>>> }
>>>> ```
>>>> 
>>>> In this code, the compiler does not know anything about the values of
>>>> images[i], so it assumes that 256 is a possible value for it. In that
>>> case,
>>>> (A) would change the value of histogram.total, so (B) has to load, add
>>> one
>>>> and store [godbolt <https://godbolt.org/z/KxE343>].
>>>> 
>>>> Fortunately, C/C++ has a rule that it is invalid (actually, UB) to use
>>>> values to form a pointer to total and dereference it. What valid C/C++
>>> code
>>>> is allowed to do with values is:
>>>>   - Form any pointer in [values, values + 256].
>>>>   - Form and dereference any pointer in [values, values + 256)
>>>> 
>>>> Note that the LLVM memory model is much laxer than that of C/C++. It has
>>> no
>>>> notion of types. In particular, given an LLVM aggregate definition:
>>>> 
>>>> ```
>>>> %struct.S = type { [42 x i32], i32, i32 }
>>>> ```
>>>> 
>>>> It is perfectly valid to use an address derived from a GEP(0,0,%i)  [gep
>>>> reference] representing indexing into the [42 x i32] array to load the
>>> i32
>>>> member at index 2. It is also valid for %i to be 43 (though not 44 if an
>>>> inbound GEP is used).
>>>> So clang has to give LLVM more information about the C/C++ rules.
>>>> 
>>>> *IR representation:*
>>>> LLVM has several ways of representing ranges of values:
>>>>   - *!range* metadata can be attached to integer call and load
>>> instructions
>>>> to indicate the allowed range of values of the result. LLVM's
>>> ValueTracking
>>>> provides a function for querying the range for any llvm::Variable.
>>>>   - The *llvm.assume* intrinsic takes a boolean condition that can also
>>> be
>>>> used by ValueTracking to infer range of values.
>>>>   - The *inrange* attribute of GEP can be used to indicate C-like
>>> semantics
>>>> for the structure field marked with the inrange attribute. It can only be
>>>> used for GEP constantexprs (ie.e. GEPs defined inline), but not for
>>>> standalone GEPs defining instructions.  relevant discussion
>>>> <https://reviews.llvm.org/D22793?id=65626#inline-194653>.
>>>> 
>>>> Alternatives:
>>>> *(1) *Annotate each array subscript index value with a range, e.g.:
>>>> ```
>>>> %i = i64 …
>>>> %ri =  call i64 @llvm.annotation.i64(%index), !range !0
>>>> %gep1 = getelementptr inbounds %struct.S, %struct.S* %s, i64 0, i32 0,
>>> i32
>>>> %ri
>>>> ...
>>>> !0 = !{i64 0, i64 42}
>>>> ```
>>>> *(2) *(variant of 1) relax the constraint that !range metadata can only
>>> be
>>>> set on call and load instructions, and set the !range metadata on the
>>> index
>>>> expression. We still need annotations for function parameters though:
>>>> ```
>>>> %i = i64 … , !range !0
>>>> %gep1 = getelementptr inbounds %struct.S, %struct.S* %s, i64 0, i32 0,
>>> i32
>>>> %i
>>>> ...
>>>> !0 = !{i64 0, i64 42}
>>>> ```
>>>> This is slightly more compact.
>>>> 
>>>> *(3)* Same as (1), with llvm.assume. This feels inferior to annotations.
>>>> *(4)* Extend inrange to non-constantexprs GEPs. It is unclear how this
>>> will
>>>> interfere with optimizations.
>>> I would very much like not to introduce another way to encode
>>> assumptions other than `llvm.assume`. If you want to avoid the extra
>>> instructions, use `llvm.assume(i1 true) ["range"(%val, %lb, %ub)]`,
>>> which is in line with our move towards operand bundle use.
>>> 
>> Thanks, I did not know about that. I've just tried it but it appears that
>> tags have to be attribute names, and `!range` is not a valid attribute,
>> it's a metadata node. Is there a way to encode this ?
> 
> We need to get rid of that assertion. There are other non-attributes
> to be used in assume operand bundles in the (near) future, so the this
> work has to be done anyway.


+1 on trying to use assume, rather than adding another way. 

But are value ranges special for assumes, so that we need to handle them in a bundle? Is that just so we can easier skip ‘artificial’ assume users?

Cheers,
Florian
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