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C backend: avoid memcpy when len=0

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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 is
zero.

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

A zig1.wasm update will be needed in the llvm18 branch to activate this
code.
This commit is contained in:
Andrew Kelley 2024-04-29 19:58:09 -07:00
parent 595a7f8b08
commit fa5011aa31
1 changed files with 16 additions and 6 deletions
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22
src/codegen/c.zig
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@ -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 {