[llvm] [LAA][LV]Allow recognition of strided pointers with constant stride (PR #171151)

[Florian Hahn via llvm-commits]

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@@ -2975,6 +2975,8 @@ static const SCEV *getStrideFromPointer(Value *Ptr, ScalarEvolution *SE, Loop *L
   if (isa<SCEVUnknown>(V))
     return V;
 
+  // Look through multiplies that scale a stride by a constant.
+  match(V, m_scev_Mul(m_SCEVConstant(), m_SCEV(V)));
   if (auto *C = dyn_cast<SCEVIntegralCastExpr>(V))
     if (isa<SCEVUnknown>(C->getOperand()))
       return V;
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fhahn wrote:

> How is this correct to return V as stride if we stripped multiplication by a constant above?

The result should only be used to add a SCEV assumption `%stride == 1`, and the full pointer SCEV we use contructed from the original pointer via `PSE.getSCEV(Ptr)`

> Are you trying to speculate V to be one because you want some memory dependences checks to be emitted at all and not because you want some access to become unit-striped?
In cases where the constant multiplier matches the access size, this should result unit-strided accesses, like in the test case for the patch, right?

https://github.com/llvm/llvm-project/pull/171151