srctree

Andrew Kelley parent 956f53be 1c9bb6a7
C backend: avoid memcpy when len=0

As of Clang 18, calling memcpy() with a misaligned pointer trips UBSAN,even if the length is zero. This unfortunately includes any call to@memcpy when source or destination are undefined and the length iszero.

This patch makes the C backend avoid calling memcpy when the length iszero, thereby avoiding undefined behavior.

A zig1.wasm update will be needed in the llvm18 branch to activate thiscode.

inline split

src/codegen/c.zig added: 17, removed: 7, total 10

@@ -6799,11 +6799,27 @@ fn airMemcpy(f: *Function, inst: Air.Inst.Index) !CValue {

const src_ty = f.typeOf(bin_op.rhs);

const writer = f.object.writer();

if (dest_ty.ptrSize(zcu) != .One) {

try writer.writeAll("if (");

try writeArrayLen(f, writer, dest_ptr, dest_ty);

try writer.writeAll(" != 0) ");

}

try writer.writeAll("memcpy(");

try writeSliceOrPtr(f, writer, dest_ptr, dest_ty);

try writer.writeAll(", ");

try writeSliceOrPtr(f, writer, src_ptr, src_ty);

try writer.writeAll(", ");

try writeArrayLen(f, writer, dest_ptr, dest_ty);

try writer.writeAll(" * sizeof(");

try f.renderType(writer, dest_ty.elemType2(zcu));

try writer.writeAll("));\n");

try reap(f, inst, &.{ bin_op.lhs, bin_op.rhs });

return .none;

}

fn writeArrayLen(f: *Function, writer: ArrayListWriter, dest_ptr: CValue, dest_ty: Type) !void {

const zcu = f.object.dg.zcu;

switch (dest_ty.ptrSize(zcu)) {

.One => try writer.print("{}", .{

try f.fmtIntLiteral(try zcu.intValue(Type.usize, dest_ty.childType(zcu).arrayLen(zcu))),

@@ -6811,12 +6827,6 @@ fn airMemcpy(f: *Function, inst: Air.Inst.Index) !CValue {

.Many, .C => unreachable,

.Slice => try f.writeCValueMember(writer, dest_ptr, .{ .identifier = "len" }),

}

try writer.writeAll(" * sizeof(");

try f.renderType(writer, dest_ty.elemType2(zcu));

try writer.writeAll("));\n");

try reap(f, inst, &.{ bin_op.lhs, bin_op.rhs });

return .none;

}

fn airSetUnionTag(f: *Function, inst: Air.Inst.Index) !CValue {