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

[Clement Courbet via llvm-dev]

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.


*On the clang side*:
The clang part is quite trivial as the infrastructure is already in place
to emit dynamic ubsan guards: D99248 <https://reviews.llvm.org/D99248>

*On the LLVM Side:*
Alternatives:
*(A)* - (annotation or assume options) Simply enable scev-aa which knows
how to handle value ranges in general. IIUC it's not enabled in clang
because it has issues with invalidation when code changes, and is therefore
not cacheable. This makes it too slow to be practical.
*(B) *- (annotation or assume options) Teach BasicAA to honor !range
metadata (D99247 <https://reviews.llvm.org/D99247>)
*(C)* - (inrange option) Teach BasicAA to honor inrange attributes of GEP.

I was leaning towards (1) and (B) because:
 - BasicAA already has basic support for value range analysis
(zero/nonzero), this is a small and natural extension.
 - The BasicAA improvement already benefits some existing code (as
evidenced by the test changes in D99247 <https://reviews.llvm.org/D99247>)
 - Using range metadata rather than the `inrange` attribute means that
BasicAA will automatically benefit from improvements in value tracking in
the future.

Opinions ?
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