fix: check division and modulo for zero divisors

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

5 files changed, 22 insertions, 9 deletions

diff --git a/AGENTS/SUBSYSTEMS.md b/AGENTS/SUBSYSTEMS.md
index 0075222..d4d1189 100644
--- a/AGENTS/SUBSYSTEMS.md
+++ b/AGENTS/SUBSYSTEMS.md
@@ -51,10 +51,11 @@ FileHandler, is deliberately ignorant of user kinds). The enum range check after
 producer (constants, same-name copies, zero-init, memset-0 arrays) is valid by
 construction, which is why 0-based ordinals are load-bearing.
 
-**Runtime checks** (`RuntimeCheck`): only array bounds checking and the file checks are
-actually wired up. `emitDivZeroCheck` exists but has **zero call sites** — division by
-zero (for `/`, DIV and MOD) is currently unchecked; the TODO sits in
-`ArithmeticHandler.cc`. The public `emitErrorAndExit(Cond, Msg, ArrayRef<Value*>)`
+**Runtime checks** (`RuntimeCheck`): array bounds, file misuse, enum ranges, null
+dereferences and array-parameter bounds contracts are checked at their emit sites.
+`ArithmeticHandler` calls `emitDivZeroCheck` before `/`, DIV and MOD; the check accepts
+i64 and double divisors, including both floating-point signed zeros, and exits with
+a `[Fatal]` diagnostic and status 1. The public `emitErrorAndExit(Cond, Msg, ArrayRef<Value*>)`
 overload exists for failure sites whose printf arguments (line, filename) are runtime
 values — FileHandler's helpers use it; the private compile-time-line form delegates to
 it.
diff --git a/include/cps/ArithmeticHandler.h b/include/cps/ArithmeticHandler.h
index ca6266d..af7394b 100644
--- a/include/cps/ArithmeticHandler.h
+++ b/include/cps/ArithmeticHandler.h
@@ -5,14 +5,18 @@
 
 namespace cps {
 
+class RuntimeCheck;
+
 class ArithmeticHandler {
     llvm::LLVMContext &Context;
     llvm::IRBuilder<> &Builder;
     bool &HadError;
+    RuntimeCheck &RuntimeChecker;
 
 public:
-    ArithmeticHandler(llvm::LLVMContext &Ctx, llvm::IRBuilder<> &B, bool &HadErrorFlag)
-        : Context(Ctx), Builder(B), HadError(HadErrorFlag) {}
+    ArithmeticHandler(llvm::LLVMContext &Ctx, llvm::IRBuilder<> &B, bool &HadErrorFlag,
+                      RuntimeCheck &RC)
+        : Context(Ctx), Builder(B), HadError(HadErrorFlag), RuntimeChecker(RC) {}
 
     llvm::Value *emitBinaryOp(int Op, llvm::Value *LHS, llvm::Value *RHS, int Line);
 };
diff --git a/lib/CodeGen/ArithmeticHandler.cc b/lib/CodeGen/ArithmeticHandler.cc
index fa71e0c..b7f0665 100644
--- a/lib/CodeGen/ArithmeticHandler.cc
+++ b/lib/CodeGen/ArithmeticHandler.cc
@@ -1,5 +1,6 @@
 #include "cps/ArithmeticHandler.h"
 #include "cps/Lexer.h"
+#include "cps/RuntimeCheck.h"
 #include <cstdio>
 
 using namespace llvm;
@@ -32,7 +33,7 @@ Value *ArithmeticHandler::emitBinaryOp(int Op, Value *LHS, Value *RHS, int Line)
             return nullptr;
         }
 
-        // TODO: DIV divide by 0 runtime check
+        RuntimeChecker.emitDivZeroCheck(RVal, Line);
         return Builder.CreateFDiv(LVal, RVal, "divtmp");
     }
 
@@ -42,6 +43,7 @@ Value *ArithmeticHandler::emitBinaryOp(int Op, Value *LHS, Value *RHS, int Line)
             HadError = true;
             return nullptr;
         }
+        RuntimeChecker.emitDivZeroCheck(RHS, Line);
         if (Op == tok_div) return Builder.CreateSDiv(LHS, RHS, "div_int_tmp");
         if (Op == tok_mod) return Builder.CreateSRem(LHS, RHS, "mod_tmp");
     }
diff --git a/lib/CodeGen/CodeGen.cc b/lib/CodeGen/CodeGen.cc
index a6669a6..d5942e9 100644
--- a/lib/CodeGen/CodeGen.cc
+++ b/lib/CodeGen/CodeGen.cc
@@ -85,7 +85,8 @@ CodeGen::CodeGen() {
                                             Symbols,
                                             HadError);
 
-    ArithHandler = std::make_unique<ArithmeticHandler>(*TheContext, *Builder, HadError);
+    ArithHandler = std::make_unique<ArithmeticHandler>(*TheContext, *Builder, HadError,
+                                                     *RuntimeChecker);
     StrHandler = std::make_unique<StringHandler>(*TheContext, *Builder, *TheModule);
     StrConvHandler = std::make_unique<StringConversionHandler>(*TheContext, *Builder, *TheModule);
     Files = std::make_unique<FileHandler>(*TheContext, *Builder, *TheModule, *RuntimeChecker);
diff --git a/lib/CodeGen/RuntimeCheck.cc b/lib/CodeGen/RuntimeCheck.cc
index 40304fd..8075873 100644
--- a/lib/CodeGen/RuntimeCheck.cc
+++ b/lib/CodeGen/RuntimeCheck.cc
@@ -57,7 +57,12 @@ void RuntimeCheck::emitErrorAndExit(Value *Condition, Value *Msg, ArrayRef<Value
 }
 
 void RuntimeCheck::emitDivZeroCheck(Value *Divisor, int Line) {
-    Value *IsZero = Builder.CreateICmpEQ(Divisor, ConstantInt::get(TheContext, APInt(64, 0)), "is_zero");
+    Value *IsZero;
+    if (Divisor->getType()->isDoubleTy()) {
+        IsZero = Builder.CreateFCmpOEQ(Divisor, ConstantFP::get(Divisor->getType(), 0.0), "is_zero");
+    } else {
+        IsZero = Builder.CreateICmpEQ(Divisor, ConstantInt::get(Divisor->getType(), 0), "is_zero");
+    }
     emitErrorAndExit(IsZero, DivZeroMsg, Line);
 }