[IR] Change representation of getelementptr inrange (#84341)

As part of the migration to ptradd
(https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699),
we need to change the representation of the `inrange` attribute, which
is used for vtable splitting.

Currently, inrange is specified as follows:

```
getelementptr inbounds ({ [4 x ptr], [4 x ptr] }, ptr @vt, i64 0, inrange i32 1, i64 2)
```

The `inrange` is placed on a GEP index, and all accesses must be "in
range" of that index. The new representation is as follows:

```
getelementptr inbounds inrange(-16, 16) ({ [4 x ptr], [4 x ptr] }, ptr @vt, i64 0, i32 1, i64 2)
```

This specifies which offsets are "in range" of the GEP result. The new
representation will continue working when canonicalizing to ptradd
representation:

```
getelementptr inbounds inrange(-16, 16) (i8, ptr @vt, i64 48)
```

The inrange offsets are relative to the return value of the GEP. An
alternative design could make them relative to the source pointer
instead. The result-relative format was chosen on the off-chance that we
want to extend support to non-constant GEPs in the future, in which case
this variant is more expressive.

This implementation "upgrades" the old inrange representation in bitcode
by simply dropping it. This is a very niche feature, and I don't think
trying to upgrade it is worthwhile. Let me know if you disagree.

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index fd211f7..a1ee029 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -2751,9 +2751,10 @@ void ModuleBitcodeWriter::writeConstants(unsigned FirstVal, unsigned LastVal,
         Code = bitc::CST_CODE_CE_GEP;
         const auto *GO = cast<GEPOperator>(C);
         Record.push_back(VE.getTypeID(GO->getSourceElementType()));
-        if (std::optional<unsigned> Idx = GO->getInRangeIndex()) {
-          Code = bitc::CST_CODE_CE_GEP_WITH_INRANGE_INDEX;
-          Record.push_back((*Idx << 1) | GO->isInBounds());
+        if (std::optional<ConstantRange> Range = GO->getInRange()) {
+          Code = bitc::CST_CODE_CE_GEP_WITH_INRANGE;
+          Record.push_back(GO->isInBounds());
+          emitConstantRange(Record, *Range);
         } else if (GO->isInBounds())
           Code = bitc::CST_CODE_CE_INBOUNDS_GEP;
         for (unsigned i = 0, e = CE->getNumOperands(); i != e; ++i) {