[llvm] [Transforms] LoopIdiomRecognize recognize strlen and wcslen (PR #108985)

[Michael Kruse via llvm-commits]

================
@@ -1524,6 +1551,276 @@ static Value *matchCondition(BranchInst *BI, BasicBlock *LoopEntry,
   return nullptr;
 }
 
+namespace {
+
+class StrlenVerifier {
+public:
+  explicit StrlenVerifier(const Loop *CurLoop, ScalarEvolution *SE,
+                          const TargetLibraryInfo *TLI)
+      : CurLoop(CurLoop), SE(SE), TLI(TLI) {}
+
+  bool isValidStrlenIdiom() {
+    // Give up if the loop has multiple blocks, multiple backedges, or
+    // multiple exit blocks
+    if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1 ||
+        !CurLoop->getUniqueExitBlock())
+      return false;
+
+    // It should have a preheader and a branch instruction.
+    BasicBlock *Preheader = CurLoop->getLoopPreheader();
+    if (!Preheader)
+      return false;
+
+    BranchInst *EntryBI = dyn_cast<BranchInst>(Preheader->getTerminator());
+    if (!EntryBI)
+      return false;
+
+    // The loop exit must be conditioned on an icmp with 0 the null terminator.
+    // The icmp operand has to be a load on some SSA reg that increments
+    // by 1 in the loop.
+    BasicBlock *LoopBody = *CurLoop->block_begin();
+
+    // Skip if the body is too big as it most likely is not a strlen idiom.
+    if (!LoopBody || LoopBody->size() >= 15)
+      return false;
+
+    BranchInst *LoopTerm = dyn_cast<BranchInst>(LoopBody->getTerminator());
+    Value *LoopCond = matchCondition(LoopTerm, LoopBody);
+    if (!LoopCond)
+      return false;
+
+    LoadInst *LoopLoad = dyn_cast<LoadInst>(LoopCond);
+    if (!LoopLoad || LoopLoad->getPointerAddressSpace() != 0)
+      return false;
+
+    OperandType = LoopLoad->getType();
+    if (!OperandType || !OperandType->isIntegerTy())
+      return false;
+
+    // See if the pointer expression is an AddRec with constant step a of form
+    // ({n,+,a}) where a is the width of the char type.
+    Value *IncPtr = LoopLoad->getPointerOperand();
+    const SCEVAddRecExpr *LoadEv =
+        dyn_cast<SCEVAddRecExpr>(SE->getSCEV(IncPtr));
+    if (!LoadEv || LoadEv->getLoop() != CurLoop || !LoadEv->isAffine())
+      return false;
+    LoadBaseEv = LoadEv->getStart();
+
+    LLVM_DEBUG({
+      dbgs() << "pointer load scev: ";
+      LoadEv->print(outs());
+      dbgs() << "\n";
+    });
+
+    const SCEVConstant *Step =
+        dyn_cast<SCEVConstant>(LoadEv->getStepRecurrence(*SE));
+    if (!Step)
+      return false;
+
+    unsigned StepSize = 0;
+    StepSizeCI = dyn_cast<ConstantInt>(Step->getValue());
+    if (!StepSizeCI)
+      return false;
+    StepSize = StepSizeCI->getZExtValue();
+
+    // Verify that StepSize is consistent with platform char width.
+    OpWidth = OperandType->getIntegerBitWidth();
+    unsigned WcharSize = TLI->getWCharSize(*LoopLoad->getModule());
+    if (OpWidth != StepSize * 8)
+      return false;
+    if (OpWidth != 8 && OpWidth != 16 && OpWidth != 32)
+      return false;
+    if (OpWidth >= 16)
+      if (OpWidth != WcharSize * 8)
+        return false;
+
+    // Scan every instruction in the loop to ensure there are no side effects.
+    for (Instruction &I : *LoopBody)
+      if (I.mayHaveSideEffects())
+        return false;
+
+    BasicBlock *LoopExitBB = CurLoop->getExitBlock();
+    if (!LoopExitBB)
+      return false;
+
+    for (PHINode &PN : LoopExitBB->phis()) {
+      if (!SE->isSCEVable(PN.getType()))
+        return false;
+
+      const SCEV *Ev = SE->getSCEV(&PN);
+      if (!Ev)
+        return false;
+
+      LLVM_DEBUG({
+        dbgs() << "loop exit phi scev: ";
+        Ev->print(dbgs());
+        dbgs() << "\n";
+      });
+
+      // Since we verified that the loop trip count will be a valid strlen
+      // idiom, we can expand all lcssa phi with {n,+,1} as (n + strlen) and use
+      // SCEVExpander materialize the loop output.
+      const SCEVAddRecExpr *AddRecEv = dyn_cast<SCEVAddRecExpr>(Ev);
+      if (!AddRecEv || !AddRecEv->isAffine())
+        return false;
+
+      // We only want RecAddExpr with recurrence step that are constant. This
+      // is good enough for all the idioms we want to recognize. Later we expand
+      // the recurrence as {base,+,a} -> (base + a * strlen) and materialize
+      if (!dyn_cast<SCEVConstant>(AddRecEv->getStepRecurrence(*SE)))
+        return false;
+    }
+
+    return true;
+  }
+
+public:
+  const Loop *CurLoop;
+  ScalarEvolution *SE;
+  const TargetLibraryInfo *TLI;
+
+  unsigned OpWidth;
+  ConstantInt *StepSizeCI;
+  const SCEV *LoadBaseEv;
+  Type *OperandType;
+};
+
+} // namespace
+
+/// The Strlen Idiom we are trying to detect has the following structure
+///
+/// preheader:
+///   ...
+///   br label %body, ...
+///
+/// body:
+///   ... ; %0 is incremented by a gep
+///   %1 = load i8, ptr %0, align 1
+///   %2 = icmp eq i8 %1, 0
+///   br i1 %2, label %exit, label %body
+///
+/// exit:
+///   %lcssa = phi [%0, %body], ...
+///
+/// We expect the strlen idiom to have a load of a character type that
+/// is compared against '\0', and such load pointer operand must have scev
+/// expression of the form {%str,+,c} where c is a ConstantInt of the
+/// appropiate character width for the idiom, and %str is the base of the string
+/// And, that all lcssa phis have the form {...,+,n} where n is a constant,
+///
+/// When transforming the output of the strlen idiom, the lccsa phi are
+/// expanded using SCEVExpander as {base scev,+,a} -> (base scev + a * strlen)
+/// and all subsequent uses are replaced. For example,
+///
+/// \code{.c}
+///     const char* base = str;
+///     while (*str != '\0')
+///         ++str;
+///     size_t result = str - base;
+/// \endcode
+///
+/// will be transformed as as follow: The idiom will be replaced by a strlen
----------------
Meinersbur wrote:

```suggestion
/// will be transformed as follows: The idiom will be replaced by a strlen
```

https://github.com/llvm/llvm-project/pull/108985