[IR] Add utility to convert constant expression operands (of an instruction) to instructions.

In the situation where we need to replace a constant operand C from a constant expression CE
by an instruction NI, it not possible without converting CE itself into an instruction. This
utility helps to convert the given set of constant expression operands from an instruction I
into a corresponding set of instructions.

The current use-case for this utility is from the patches - https://reviews.llvm.org/D103225
and https://reviews.llvm.org/D103655.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D103661

llvm/include/llvm/IR/ReplaceConstant.h

@@ -16,6 +16,8 @@
 
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Instruction.h"
+#include <map>
+#include <vector>
 
 namespace llvm {
 
@@ -23,6 +25,36 @@ namespace llvm {
 /// it before \p Instr.
 Instruction *createReplacementInstr(ConstantExpr *CE, Instruction *Instr);
 
+/// The given instruction \p I contains given constant expression \p CE as one
+/// of its operands, possibly nested within constant expression trees. Convert
+/// all reachable paths from contant expression operands of \p I to \p CE into
+/// corresponding instructions, insert them before \p I, update operands of \p I
+/// accordingly, and if required, return all such converted instructions at
+/// \p Insts.
+void convertConstantExprsToInstructions(
+    Instruction *I, ConstantExpr *CE,
+    SmallPtrSetImpl<Instruction *> *Insts = nullptr);
+
+/// The given instruction \p I contains constant expression CE within the
+/// constant expression trees of it`s constant expression operands, and
+/// \p CEPaths holds all the reachable paths (to CE) from such constant
+/// expression trees of \p I. Convert constant expressions within these paths
+/// into corresponding instructions, insert them before \p I, update operands of
+/// \p I accordingly, and if required, return all such converted instructions at
+/// \p Insts.
+void convertConstantExprsToInstructions(
+    Instruction *I,
+    std::map<Use *, std::vector<std::vector<ConstantExpr *>>> &CEPaths,
+    SmallPtrSetImpl<Instruction *> *Insts = nullptr);
+
+/// Given an instruction \p I which uses given constant expression \p CE as
+/// operand, either directly or nested within other constant expressions, return
+/// all reachable paths from the constant expression operands of \p I to \p CE,
+/// and return collected paths at \p CEPaths.
+void collectConstantExprPaths(
+    Instruction *I, ConstantExpr *CE,
+    std::map<Use *, std::vector<std::vector<ConstantExpr *>>> &CEPaths);
+
 } // end namespace llvm
 
 #endif // LLVM_IR_REPLACECONSTANT_H

llvm/lib/IR/ReplaceConstant.cpp

@@ -68,4 +68,95 @@ Instruction *createReplacementInstr(ConstantExpr *CE, Instruction *Instr) {
     llvm_unreachable("Unhandled constant expression!\n");
   }
 }
+
+void convertConstantExprsToInstructions(Instruction *I, ConstantExpr *CE,
+                                        SmallPtrSetImpl<Instruction *> *Insts) {
+  // Collect all reachable paths to CE from constant exprssion operands of I.
+  std::map<Use *, std::vector<std::vector<ConstantExpr *>>> CEPaths;
+  collectConstantExprPaths(I, CE, CEPaths);
+
+  // Convert all constant expressions to instructions which are collected at
+  // CEPaths.
+  convertConstantExprsToInstructions(I, CEPaths, Insts);
+}
+
+void convertConstantExprsToInstructions(
+    Instruction *I,
+    std::map<Use *, std::vector<std::vector<ConstantExpr *>>> &CEPaths,
+    SmallPtrSetImpl<Instruction *> *Insts) {
+  for (Use &U : I->operands()) {
+    // The operand U is either not a constant expression operand or the
+    // constant expression paths do not belong to U, ignore U.
+    if (!CEPaths.count(&U))
+      continue;
+
+    // If the instruction I is a PHI instruction, then fix the instruction
+    // insertion point to the entry of the incoming basic block for operand U.
+    auto *BI = I;
+    if (auto *Phi = dyn_cast<PHINode>(I)) {
+      BasicBlock *BB = Phi->getIncomingBlock(U);
+      BI = &(*(BB->getFirstInsertionPt()));
+    }
+
+    // Go through the paths associated with operand U, and convert all the
+    // constant expressions along all paths to corresponding instructions.
+    auto *II = I;
+    auto &Paths = CEPaths[&U];
+    SmallPtrSet<ConstantExpr *, 8> Visited;
+    for (auto &Path : Paths) {
+      for (auto *CE : Path) {
+        if (!Visited.insert(CE).second)
+          continue;
+        auto *NI = CE->getAsInstruction();
+        NI->insertBefore(BI);
+        II->replaceUsesOfWith(CE, NI);
+        CE->removeDeadConstantUsers();
+        BI = II = NI;
+        if (Insts)
+          Insts->insert(NI);
+      }
+    }
+  }
+}
+
+void collectConstantExprPaths(
+    Instruction *I, ConstantExpr *CE,
+    std::map<Use *, std::vector<std::vector<ConstantExpr *>>> &CEPaths) {
+  for (Use &U : I->operands()) {
+    // If the operand U is not a constant expression operand, then ignore it.
+    auto *CE2 = dyn_cast<ConstantExpr>(U.get());
+    if (!CE2)
+      continue;
+
+    // Holds all reachable paths from CE2 to CE.
+    std::vector<std::vector<ConstantExpr *>> Paths;
+
+    // Collect all reachable paths from CE2 to CE.
+    std::vector<ConstantExpr *> Path{CE2};
+    std::vector<std::vector<ConstantExpr *>> Stack{Path};
+    while (!Stack.empty()) {
+      std::vector<ConstantExpr *> TPath = Stack.back();
+      Stack.pop_back();
+      auto *CE3 = TPath.back();
+
+      if (CE3 == CE) {
+        Paths.push_back(TPath);
+        continue;
+      }
+
+      for (auto &UU : CE3->operands()) {
+        if (auto *CE4 = dyn_cast<ConstantExpr>(UU.get())) {
+          std::vector<ConstantExpr *> NPath(TPath.begin(), TPath.end());
+          NPath.push_back(CE4);
+          Stack.push_back(NPath);
+        }
+      }
+    }
+
+    // Associate all the collected paths with U, and save it.
+    if (!Paths.empty())
+      CEPaths[&U] = Paths;
+  }
+}
+
 } // namespace llvm