stage2: introduce store_safe AIR instruction

store:
The value to store may be undefined, in which case the destination
memory region has undefined bytes after this instruction is
evaluated. In such case ignoring this instruction is legal
lowering.

store_safe:
Same as `store`, except if the value to store is undefined, the
memory region should be filled with 0xaa bytes, and any other
safety metadata such as Valgrind integrations should be notified of
this memory region being undefined.

2 files changed, 33 insertions, 54 deletions

diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index 5efe2ee1d6..b60f3553a2 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -2914,7 +2914,8 @@ fn genBodyInner(f: *Function, body: []const Air.Inst.Index) error{ AnalysisFail,
             .load             => try airLoad(f, inst),
             .ret              => try airRet(f, inst, false),
             .ret_load         => try airRet(f, inst, true),
-            .store            => try airStore(f, inst),
+            .store            => try airStore(f, inst, false),
+            .store_safe       => try airStore(f, inst, true),
             .loop             => try airLoop(f, inst),
             .cond_br          => try airCondBr(f, inst),
             .br               => try airBr(f, inst),
@@ -3565,19 +3566,7 @@ fn airBoolToInt(f: *Function, inst: Air.Inst.Index) !CValue {
     return local;
 }
 
-fn storeUndefined(f: *Function, lhs_child_ty: Type, dest_ptr: CValue) !CValue {
-    if (f.wantSafety()) {
-        const writer = f.object.writer();
-        try writer.writeAll("memset(");
-        try f.writeCValue(writer, dest_ptr, .FunctionArgument);
-        try writer.print(", {x}, sizeof(", .{try f.fmtIntLiteral(Type.u8, Value.undef)});
-        try f.renderType(writer, lhs_child_ty);
-        try writer.writeAll("));\n");
-    }
-    return .none;
-}
-
-fn airStore(f: *Function, inst: Air.Inst.Index) !CValue {
+fn airStore(f: *Function, inst: Air.Inst.Index, safety: bool) !CValue {
     // *a = b;
     const bin_op = f.air.instructions.items(.data)[inst].bin_op;
 
@@ -3588,18 +3577,19 @@ fn airStore(f: *Function, inst: Air.Inst.Index) !CValue {
     const ptr_val = try f.resolveInst(bin_op.lhs);
     const src_ty = f.air.typeOf(bin_op.rhs);
 
-    // TODO Sema should emit a different instruction when the store should
-    // possibly do the safety 0xaa bytes for undefined.
-    const src_val_is_undefined =
-        if (f.air.value(bin_op.rhs)) |v| v.isUndefDeep() else false;
-    if (src_val_is_undefined) {
-        if (ptr_info.host_size == 0) {
-            try reap(f, inst, &.{ bin_op.lhs, bin_op.rhs });
-            return try storeUndefined(f, ptr_info.pointee_type, ptr_val);
-        } else if (!f.wantSafety()) {
-            try reap(f, inst, &.{ bin_op.lhs, bin_op.rhs });
-            return .none;
+    const val_is_undef = if (f.air.value(bin_op.rhs)) |v| v.isUndefDeep() else false;
+
+    if (val_is_undef) {
+        try reap(f, inst, &.{ bin_op.lhs, bin_op.rhs });
+        if (safety and ptr_info.host_size == 0) {
+            const writer = f.object.writer();
+            try writer.writeAll("memset(");
+            try f.writeCValue(writer, ptr_val, .FunctionArgument);
+            try writer.writeAll(", 0xaa, sizeof(");
+            try f.renderType(writer, ptr_info.pointee_type);
+            try writer.writeAll("));\n");
         }
+        return .none;
     }
 
     const target = f.object.dg.module.getTarget();
diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index e8ed5745d6..ac105606e8 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -4649,7 +4649,8 @@ pub const FuncGen = struct {
                 .not            => try self.airNot(inst),
                 .ret            => try self.airRet(inst),
                 .ret_load       => try self.airRetLoad(inst),
-                .store          => try self.airStore(inst),
+                .store          => try self.airStore(inst, false),
+                .store_safe     => try self.airStore(inst, true),
                 .assembly       => try self.airAssembly(inst),
                 .slice_ptr      => try self.airSliceField(inst, 0),
                 .slice_len      => try self.airSliceField(inst, 1),
@@ -8115,48 +8116,36 @@ pub const FuncGen = struct {
         return buildAllocaInner(self.context, self.builder, self.llvm_func, self.di_scope != null, llvm_ty, alignment, self.dg.module.getTarget());
     }
 
-    fn airStore(self: *FuncGen, inst: Air.Inst.Index) !?*llvm.Value {
+    fn airStore(self: *FuncGen, inst: Air.Inst.Index, safety: bool) !?*llvm.Value {
         const bin_op = self.air.instructions.items(.data)[inst].bin_op;
         const dest_ptr = try self.resolveInst(bin_op.lhs);
         const ptr_ty = self.air.typeOf(bin_op.lhs);
         const operand_ty = ptr_ty.childType();
 
-        // TODO Sema should emit a different instruction when the store should
-        // possibly do the safety 0xaa bytes for undefined.
         const val_is_undef = if (self.air.value(bin_op.rhs)) |val| val.isUndefDeep() else false;
         if (val_is_undef) {
-            {
-                // TODO let's handle this in AIR rather than by having each backend
-                // check the optimization mode of the compilation because the plan is
-                // to support setting the optimization mode at finer grained scopes
-                // which happens in Sema. Codegen should not be aware of this logic.
-                // I think this comment is basically the same as the other TODO comment just
-                // above but I'm leaving them both here to make it look super messy and
-                // thereby bait contributors (or let's be honest, probably myself) into
-                // fixing this instead of letting it rot.
-                const safety = switch (self.dg.module.comp.bin_file.options.optimize_mode) {
-                    .ReleaseSmall, .ReleaseFast => false,
-                    .Debug, .ReleaseSafe => true,
-                };
-                if (!safety) {
-                    return null;
-                }
-            }
+            // Even if safety is disabled, we still emit a memset to undefined since it conveys
+            // extra information to LLVM. However, safety makes the difference between using
+            // 0xaa or actual undefined for the fill byte.
+            const u8_llvm_ty = self.context.intType(8);
+            const fill_byte = if (safety)
+                u8_llvm_ty.constInt(0xaa, .False)
+            else
+                u8_llvm_ty.getUndef();
             const target = self.dg.module.getTarget();
             const operand_size = operand_ty.abiSize(target);
-            const u8_llvm_ty = self.context.intType(8);
-            const fill_char = u8_llvm_ty.constInt(0xaa, .False);
-            const dest_ptr_align = ptr_ty.ptrAlignment(target);
             const usize_llvm_ty = try self.dg.lowerType(Type.usize);
             const len = usize_llvm_ty.constInt(operand_size, .False);
-            _ = self.builder.buildMemSet(dest_ptr, fill_char, len, dest_ptr_align, ptr_ty.isVolatilePtr());
-            if (self.dg.module.comp.bin_file.options.valgrind) {
+            const dest_ptr_align = ptr_ty.ptrAlignment(target);
+            _ = self.builder.buildMemSet(dest_ptr, fill_byte, len, dest_ptr_align, ptr_ty.isVolatilePtr());
+            if (safety and self.dg.module.comp.bin_file.options.valgrind) {
                 self.valgrindMarkUndef(dest_ptr, len);
             }
-        } else {
-            const src_operand = try self.resolveInst(bin_op.rhs);
-            try self.store(dest_ptr, ptr_ty, src_operand, .NotAtomic);
+            return null;
         }
+
+        const src_operand = try self.resolveInst(bin_op.rhs);
+        try self.store(dest_ptr, ptr_ty, src_operand, .NotAtomic);
         return null;
     }