[llvm] [VPlan] Narrow truncates of an induction plus a constant offset (PR #226035)

[Ramkumar Ramachandra via llvm-commits]

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@@ -929,7 +932,26 @@ getOptimizableIVOf(VPValue *VPV, PredicatedScalarEvolution &PSE) {
     }
     llvm_unreachable("should have been covered by switch above");
   };
-  return IsWideIVInc() ? WideIV : nullptr;
+  if (!PostIncStart)
+    return IsWideIVInc() ? WideIV : nullptr;
+
+  // start + C + i * step stays affine for any constant C, including the step,
+  // so it can be folded into the start value.
+  const APInt *C;
+  APInt Offset;
+  if (match(VPV, m_c_Add(m_Specific(WideIV), m_APInt(C))))
+    Offset = *C;
+  else if (match(VPV, m_Sub(m_Specific(WideIV), m_APInt(C))))
+    Offset = -*C;
+  else
+    return nullptr;
+
+  const APInt *StartC;
+  if (!match(WideIV->getStartValue(), m_APInt(StartC)))
+    return nullptr;
+  *PostIncStart =
+      WideIV->getParent()->getPlan()->getConstantInt(*StartC + Offset);
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artagnon wrote:

Hm, this is a bit unfortunate: don't we have the Plan at all call-sites that we can pass directly? It would avoid the lookup? Alternatively, can we just return the Offset APInt via the argument, and the caller can take care of the rest?

https://github.com/llvm/llvm-project/pull/226035