Support simplification of phi instruction in ScalarEvolution

This change is to extend supported patterns for creating SCEVs iteratively in ScalarEvolution (to reflect the recursive algorithm exactly). These patterns are related to one common source value of the phi instruction and this can be resolved from this value.

Author: krystian-andrzejewski

external/llvm/releases/14.0.0/patches_external/0003-ScalarEvolutionStackOverflowWindows.patch

@@ -17,7 +17,7 @@ SPDX-License-Identifier: Apache-2.0 with LLVM-exception
 # TODO: Once upstreamed, update with LLORG revision & adjust per community review
 
 diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
-index e765a4d54..518683c29 100644
+index e765a4d54..8712103b5 100644
 --- a/llvm/lib/Analysis/ScalarEvolution.cpp
 +++ b/llvm/lib/Analysis/ScalarEvolution.cpp
 @@ -5576,12 +5576,14 @@ const SCEV *ScalarEvolution::createAddRecFromPHI(PHINode *PN) {
@@ -73,37 +73,54 @@ index e765a4d54..518683c29 100644
        // Queue CurV for SCEV creation, followed by its's operands which need to
        // be constructed first.
        Stack.emplace_back(CurV, true);
-@@ -7248,21 +7251,19 @@ ScalarEvolution::getOperandsToCreate(Value *V, SmallVectorImpl<Value *> &Ops) {
- 
+@@ -7249,20 +7252,43 @@ ScalarEvolution::getOperandsToCreate(Value *V, SmallVectorImpl<Value *> &Ops) {
    case Instruction::PHI: {
      PHINode *PN = cast<PHINode>(U);
--
--    if (PN->getNumIncomingValues() == 2) {
--      std::pair<Value *, Value *> StartAndBEValueV =
--          getStartAndBackEdgeValues(PN);
+ 
++    // This phi's operands must be attached due to range computations even if it is redundant to determine its SCEV expression.
++    for (auto &Op : PN->operands())
++      Ops.push_back(Op);
++
++    // Check if it can be classified as a special scalar evolution expression like:
++    // * addrec,
++    // * phi-like-select.
+     if (PN->getNumIncomingValues() == 2) {
+       std::pair<Value *, Value *> StartAndBEValueV =
+           getStartAndBackEdgeValues(PN);
 -      if (StartAndBEValueV.first && StartAndBEValueV.second)
 -        // resolve recursively - may create cycles
--        return nullptr;
++      Value *BEValueV = StartAndBEValueV.second,
++            *StartValueV = StartAndBEValueV.first;
++      if (BEValueV && StartValueV)
++        // This phi instruction is initially defined as undefined and can be
++        // replaced with an addrec expression if it meets requirements.
++        // This may create cycles.
++        return getUnknown(V);
++
++      if (BrPHIToSelect(DT, PN))
++        // This phi instruction might be classified as a phi-like-select
++        // expression. Any initial definition cannot be applied since this
++        // pattern doesn't support such a path as dedicated to addrec
++        // expressions.
+         return nullptr;
 -      else if (BrPHIToSelect(DT, PN))
 -        // resolve iteratively - does not create cycles
 -        for (auto &Op : PN->operands())
 -          Ops.push_back(Op);
--    }
--
+     }
+ 
 -    // resolve iteratively - does not create SCEV cycles (trivially resolved)
 -    return nullptr;
-+    // This phi's operands must be attached due to range computations even if it is redundant
-+    // to determine its SCEV expression.
-+    for (auto &Op : PN->operands())
-+      Ops.push_back(Op);
-+    if (BrPHIToSelect(DT, PN))
-+      // This phi instruction might be classified as a phi-like-select expression.
-+      // Any initial definition cannot be applied since this pattern doesn't support
-+      // such a path as dedicated to addrec expressions.
-+      return nullptr;
-+    // This phi instruction is initially defined as undefined and can be replaced
-+    // with an addrec expression if it meets requirements. Otherwise,
-+    // this will be preserved.
++    // This means that this phi instruction has one common input value.
++    // This cannot create cycles due to SSA properties (an usage of the value
++    // cannot be before its definition and this leads to a conclusion that it must
++    // dominate over this phi instruction to meet the initial assumptions).
++    if (Value *V = SimplifyInstruction(PN, {getDataLayout(), &TLI, &DT, &AC}))
++      if (LI.replacementPreservesLCSSAForm(PN, V))
++        return nullptr;
++
++    // This phi instruction is classified as undefined due to
++    // an unsupported pattern. This may create cycles.
 +    return getUnknown(V);
    }