[PATCH] D152278: [SCEV] Compute SCEV for ashr(add(shl(x, n), c), m) instr triplet

[Vedant Paranjape via Phabricator via llvm-commits]

vedant-amd added inline comments.


================
Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:7893
+          // type, so create a new Constant with type same as TruncTy
+          int64_t AddOperand = AddOperandCI->getSExtValue();
+          const SCEV *AddConstant = getConstant(TruncTy, AddOperand >> AShrAmt);
----------------
efriedma wrote:
> Is there some reason to expect that "c" fits into an int64_t?
Right, I need to add a check like this I guess ?

```
      if (CI->getValue().uge(BitWidth))

```


================
Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:7894
+          int64_t AddOperand = AddOperandCI->getSExtValue();
+          const SCEV *AddConstant = getConstant(TruncTy, AddOperand >> AShrAmt);
+
----------------
efriedma wrote:
> I guess the reason AddOperandCI has to be a constant is that this shift needs to constant-fold?
Theoretically, it doesn't have to be a constant. But, I don't see where such a IR will be emitted. I can handle this case in a future patch.

This IR is usually emitted for the following C code. InstCombine does the optimization.

```
int64_t a = 10;
int32_t b = a - 1;
printf("%d", arr[b]);
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D152278/new/

https://reviews.llvm.org/D152278