stage2: elemPtr for slices

* Restructure elemPtr a bit
* New AIR instruction: slice_elem_ptr, which returns a pointer to an element of a slice
* Value: adapt elemPtr to work on slices

src/codegen/c.zig

@@ -1083,6 +1083,7 @@ fn genBody(f: *Function, body: []const Air.Inst.Index) error{ AnalysisFail, OutO
             .ptr_elem_val       => try airPtrElemVal(f, inst, "["),
             .ptr_elem_ptr       => try airPtrElemPtr(f, inst),
             .slice_elem_val     => try airSliceElemVal(f, inst, "["),
+            .slice_elem_ptr     => try airSliceElemPtr(f, inst),
             .array_elem_val     => try airArrayElemVal(f, inst),
 
             .unwrap_errunion_payload     => try airUnwrapErrUnionPay(f, inst),
@@ -1165,6 +1166,24 @@ fn airSliceElemVal(f: *Function, inst: Air.Inst.Index, prefix: []const u8) !CVal
     return local;
 }
 
+fn airSliceElemPtr(f: *Function, inst: Air.Inst.Index) !CValue {
+    if (f.liveness.isUnused(inst))
+        return CValue.none;
+    const ty_pl = f.air.instructions.items(.data)[inst].ty_pl;
+    const bin_op = f.air.extraData(Air.Bin, ty_pl.payload).data;
+
+    const slice = try f.resolveInst(bin_op.lhs);
+    const index = try f.resolveInst(bin_op.rhs);
+    const writer = f.object.writer();
+    const local = try f.allocLocal(f.air.typeOfIndex(inst), .Const);
+    try writer.writeAll(" = &");
+    try f.writeCValue(writer, slice);
+    try writer.writeByte('[');
+    try f.writeCValue(writer, index);
+    try writer.writeAll("];\n");
+    return local;
+}
+
 fn airArrayElemVal(f: *Function, inst: Air.Inst.Index) !CValue {
     if (f.liveness.isUnused(inst)) return CValue.none;
 

src/codegen/llvm.zig

@@ -1760,6 +1760,7 @@ pub const FuncGen = struct {
 
                 .array_elem_val     => try self.airArrayElemVal(inst),
                 .slice_elem_val     => try self.airSliceElemVal(inst),
+                .slice_elem_ptr     => try self.airSliceElemPtr(inst),
                 .ptr_elem_val       => try self.airPtrElemVal(inst),
                 .ptr_elem_ptr       => try self.airPtrElemPtr(inst),
 
@@ -2157,12 +2158,20 @@ pub const FuncGen = struct {
 
         const slice = try self.resolveInst(bin_op.lhs);
         const index = try self.resolveInst(bin_op.rhs);
-        const base_ptr = self.builder.buildExtractValue(slice, 0, "");
-        const indices: [1]*const llvm.Value = .{index};
-        const ptr = self.builder.buildInBoundsGEP(base_ptr, &indices, indices.len, "");
+        const ptr = self.sliceElemPtr(slice, index);
         return self.load(ptr, slice_ty);
     }
 
+    fn airSliceElemPtr(self: *FuncGen, inst: Air.Inst.Index) !?*const llvm.Value {
+        if (self.liveness.isUnused(inst)) return null;
+        const ty_pl = self.air.instructions.items(.data)[inst].ty_pl;
+        const bin_op = self.air.extraData(Air.Bin, ty_pl.payload).data;
+
+        const slice = try self.resolveInst(bin_op.lhs);
+        const index = try self.resolveInst(bin_op.rhs);
+        return self.sliceElemPtr(slice, index);
+    }
+
     fn airArrayElemVal(self: *FuncGen, inst: Air.Inst.Index) !?*const llvm.Value {
         if (self.liveness.isUnused(inst)) return null;
 
@@ -3575,6 +3584,16 @@ pub const FuncGen = struct {
         return self.builder.buildBitCast(union_field_ptr, result_llvm_ty, "");
     }
 
+    fn sliceElemPtr(
+        self: *FuncGen,
+        slice: *const llvm.Value,
+        index: *const llvm.Value,
+    ) *const llvm.Value {
+        const base_ptr = self.builder.buildExtractValue(slice, 0, "");
+        const indices: [1]*const llvm.Value = .{index};
+        return self.builder.buildInBoundsGEP(base_ptr, &indices, indices.len, "");
+    }
+
     fn getIntrinsic(self: *FuncGen, name: []const u8) *const llvm.Value {
         const id = llvm.lookupIntrinsicID(name.ptr, name.len);
         assert(id != 0);