From d6ef35600e7093e06dfc7a3fb5ce9a69232ca74e Mon Sep 17 00:00:00 2001
From: Marek Chalupa <mchqwerty@gmail.com>
Date: Mon, 20 Jun 2022 13:36:30 +0200
Subject: [PATCH] llvm-slicer-crit: improve the precision of matching without
 PTA

We can handle also constant expressions and global variables.
---
 tools/llvm-slicer-crit.cpp | 51 +++++++++++++++++++++++++++++++++++---
 1 file changed, 47 insertions(+), 4 deletions(-)

diff --git a/tools/llvm-slicer-crit.cpp b/tools/llvm-slicer-crit.cpp
index d9f2294e5..f7eaf7450 100644
--- a/tools/llvm-slicer-crit.cpp
+++ b/tools/llvm-slicer-crit.cpp
@@ -38,15 +38,49 @@ static inline bool isNumber(const std::string &s) {
 }
 
 static inline bool mayBeTheVar(const llvm::Value *val, const std::string &var) {
+    // global variables have names, just compare it
+    if (auto *G = llvm::dyn_cast<llvm::GlobalVariable>(val)) {
+        return G->getName() == var;
+    }
+
+    // for other cases, we must relay on the information about allocas
     auto name = valuesToVariables.find(val);
     if (name != valuesToVariables.end() && name->second != var) {
         return false;
     }
+
     // either the var matches or we do not know the var,
     // which we must take as a match
     return true;
 }
 
+static llvm::Value *constExprVar(const llvm::ConstantExpr *CE) {
+    using namespace llvm;
+    Value *var = nullptr;
+    Instruction *Inst = CE->getAsInstruction();
+
+    switch (Inst->getOpcode()) {
+    case Instruction::GetElementPtr:
+        var = cast<GetElementPtrInst>(Inst)->getPointerOperand();
+        break;
+    case Instruction::BitCast:
+        var = Inst->getOperand(0);
+        break;
+    default:
+        break;
+    }
+
+#if LLVM_VERSION_MAJOR < 5
+    delete Inst;
+#else
+    Inst->deleteValue();
+#endif
+
+    if (var && isa<ConstantExpr>(var))
+        var = constExprVar(cast<ConstantExpr>(var));
+    return var;
+}
+
 static bool usesTheVariable(const llvm::Instruction &I, const std::string &var,
                             bool isglobal = false,
                             LLVMPointerAnalysis *pta = nullptr) {
@@ -56,17 +90,26 @@ static bool usesTheVariable(const llvm::Instruction &I, const std::string &var,
     if (!pta) {
         // try basic cases that we can decide without PTA
         using namespace llvm;
+        const Value *operand = nullptr;
         if (auto *S = dyn_cast<StoreInst>(&I)) {
             auto *A = S->getPointerOperand()->stripPointerCasts();
-            if (isa<AllocaInst>(A) && !mayBeTheVar(A, var)) {
-                return false;
+            if (auto *C = dyn_cast<ConstantExpr>(A)) {
+                operand = constExprVar(C);
+            } else if ((isa<AllocaInst>(A) || isa<GlobalVariable>(A))) {
+                operand = A;
             }
         } else if (auto *L = dyn_cast<LoadInst>(&I)) {
             auto *A = L->getPointerOperand()->stripPointerCasts();
-            if (isa<AllocaInst>(A) && !mayBeTheVar(A, var)) {
-                return false;
+            if (auto *C = dyn_cast<ConstantExpr>(A)) {
+                operand = constExprVar(C);
+            } else if ((isa<AllocaInst>(A) || isa<GlobalVariable>(A))) {
+                operand = A;
             }
         }
+
+        if (operand && !mayBeTheVar(operand, var)) {
+            return false;
+        }
         return true;
     }