[PATCH] D98147: [SCEV] Improve modelling for pointer constants

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:1076
+    // Does the operand happen to be a (pointer) constant? If so, see if we can
+    // produce an integer constant (but *NOT* an ptr2int constant expression!).
+    if (auto *PC = dyn_cast<Constant>(U->getValue()))
----------------
lebedev.ri wrote:
> Meinersbur wrote:
> > typo is still there
> > 
> > A `GlobalValue`would but such a constant, but why handle it differently than a local value?
> Because if we allow `ConstantExpr::getPtrToInt()` to return `ptrtoint` constant expression,
> `getSCEV()` will immediately ask createSCEV() to model it as a SCEV expression,
> which will result in calling `ScalarEvolution::getPtrToIntExpr()`,
> which will then ask `ConstantExpr::getPtrToInt()`, which will then return `ptrtoint` constant expression,
> `getSCEV()` will immediately ask createSCEV() to model it as a SCEV expression,
> which will result in calling `ScalarEvolution::getPtrToIntExpr()`,
> which will then ask `ConstantExpr::getPtrToInt()`, which will then return `ptrtoint` constant expression,
> `getSCEV()` will immediately ask createSCEV() to model it as a SCEV expression,
> which will result in calling `ScalarEvolution::getPtrToIntExpr()`,
> which will then ask `ConstantExpr::getPtrToInt()`, which will then return `ptrtoint` constant expression,
> `getSCEV()` will immediately ask createSCEV() to model it as a SCEV expression,
> which will result in calling `ScalarEvolution::getPtrToIntExpr()`,
> which will then ask `ConstantExpr::getPtrToInt()`, which will then return `ptrtoint` constant expression,
> `getSCEV()` will immediately ask createSCEV() to model it as a SCEV expression,
> which will result in calling `ScalarEvolution::getPtrToIntExpr()`,
Edit: 

> IIUC, this is to reduce ptr2int(int2ptr(x)) -> x?

Hm, that's funny. I thought i replied to that question yesterday, but the inline comment is gone.**bold text**

> A GlobalValuewould but such a constant, but why handle it differently than a local value?

Mainly we want this so that if we call `ScalarEvolution::getPtrToIntExpr()`
on a `SCEVUnknown` that is `ConstantPointerNull`, we don't end up with a
`SCEVPtrToIntCast` of an `SCEVUnknown` of a `ConstantPointerNull`,
but with a nice `SCEVConstant` with a value of 0.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D98147/new/

https://reviews.llvm.org/D98147