Update to use witx tagged unions

[pchickey]

Implements changes proposed in WebAssembly/WASI#220.

Changes to generated code for union types

$subscription

Current Master

/**
 * The contents of a $subscription.
 */
typedef union __wasi_subscription_u_t {
    /**
     * When type is `eventtype::clock`:
     */
    __wasi_subscription_clock_t clock;

    /**
     * When type is `eventtype::fd_read` or `eventtype::fd_write`:
     */
    __wasi_subscription_fd_readwrite_t fd_readwrite;

} __wasi_subscription_u_t;

_Static_assert(sizeof(__wasi_subscription_u_t) == 32, "witx calculated size");
_Static_assert(_Alignof(__wasi_subscription_u_t) == 8, "witx calculated align");

/**
 * Subscription to an event.
 */
typedef struct __wasi_subscription_t {
    /**
     * User-provided value that is attached to the subscription in the
     * implementation and returned through `event::userdata`.
     */
    __wasi_userdata_t userdata;

    /**
     * The type of the event to which to subscribe.
     */
    __wasi_eventtype_t type;

    /**
     * The contents of the subscription.
     */
    __wasi_subscription_u_t u;

} __wasi_subscription_t;

_Static_assert(sizeof(__wasi_subscription_t) == 48, "witx calculated size");
_Static_assert(_Alignof(__wasi_subscription_t) == 8, "witx calculated align");
_Static_assert(offsetof(__wasi_subscription_t, userdata) == 0, "witx calculated offset");
_Static_assert(offsetof(__wasi_subscription_t, type) == 8, "witx calculated offset");
_Static_assert(offsetof(__wasi_subscription_t, u) == 16, "witx calculated offset");

Tagged Unions

/**
 * The contents of a $subscription.
 */
typedef union __wasi_subscription_u_u_t {
    __wasi_subscription_clock_t clock;
    __wasi_subscription_fd_readwrite_t fd_read;
    __wasi_subscription_fd_readwrite_t fd_write;
} __wasi_subscription_u_u_t;
typedef struct __wasi_subscription_u_t {
    __wasi_eventtype_t tag;
    __wasi_subscription_u_u_t u;
} __wasi_subscription_u_t;

_Static_assert(sizeof(__wasi_subscription_u_t) == 40, "witx calculated size");
_Static_assert(_Alignof(__wasi_subscription_u_t) == 8, "witx calculated align");
_Static_assert(offsetof(__wasi_subscription_u_t, u) == 8, "witx calculated union offset");
_Static_assert(sizeof(__wasi_subscription_u_u_t) == 32, "witx calculated union size");
_Static_assert(_Alignof(__wasi_subscription_u_u_t) == 8, "witx calculated union align");

/**
 * Subscription to an event.
 */
typedef struct __wasi_subscription_t {
    /**
     * User-provided value that is attached to the subscription in the
     * implementation and returned through `event::userdata`.
     */
    __wasi_userdata_t userdata;

    /**
     * The type of the event to which to subscribe, and its contents
     */
    __wasi_subscription_u_t u;

} __wasi_subscription_t;

_Static_assert(sizeof(__wasi_subscription_t) == 48, "witx calculated size");
_Static_assert(_Alignof(__wasi_subscription_t) == 8, "witx calculated align");
_Static_assert(offsetof(__wasi_subscription_t, userdata) == 0, "witx calculated offset");
_Static_assert(offsetof(__wasi_subscription_t, u) == 8, "witx calculated offset");

$event

Current Master

/**
 * The contents of an $event.
 */
typedef union __wasi_event_u_t {
    /**
     * When type is `eventtype::fd_read` or `eventtype::fd_write`:
     */
    __wasi_event_fd_readwrite_t fd_readwrite;

} __wasi_event_u_t;

_Static_assert(sizeof(__wasi_event_u_t) == 16, "witx calculated size");
_Static_assert(_Alignof(__wasi_event_u_t) == 8, "witx calculated align");

/**
 * An event that occurred.
 */
typedef struct __wasi_event_t {
    /**
     * User-provided value that got attached to `subscription::userdata`.
     */
    __wasi_userdata_t userdata;

    /**
     * If non-zero, an error that occurred while processing the subscription request.
     */
    __wasi_errno_t error;

    /**
     * The type of the event that occurred.
     */
    __wasi_eventtype_t type;

    /**
     * The contents of the event.
     */
    __wasi_event_u_t u;

} __wasi_event_t;

_Static_assert(sizeof(__wasi_event_t) == 32, "witx calculated size");
_Static_assert(_Alignof(__wasi_event_t) == 8, "witx calculated align");
_Static_assert(offsetof(__wasi_event_t, userdata) == 0, "witx calculated offset");
_Static_assert(offsetof(__wasi_event_t, error) == 8, "witx calculated offset");
_Static_assert(offsetof(__wasi_event_t, type) == 10, "witx calculated offset");
_Static_assert(offsetof(__wasi_event_t, u) == 16, "witx calculated offset");

Tagged Unions

/**
 * An event that occurred.
 */
typedef struct __wasi_event_t {
    /**
     * User-provided value that got attached to `subscription::userdata`.
     */
    __wasi_userdata_t userdata;

    /**
     * If non-zero, an error that occurred while processing the subscription request.
     */
    __wasi_errno_t error;

    /**
     * The type of event that occured
     */
    __wasi_eventtype_t type;

    /**
     * The contents of the event, if it is an `eventtype::fd_read` or
     * `eventtype::fd_write`. `eventtype::clock` events ignore this field.
     */
    __wasi_event_fd_readwrite_t fd_readwrite;

} __wasi_event_t;

_Static_assert(sizeof(__wasi_event_t) == 32, "witx calculated size");
_Static_assert(_Alignof(__wasi_event_t) == 8, "witx calculated align");
_Static_assert(offsetof(__wasi_event_t, userdata) == 0, "witx calculated offset");
_Static_assert(offsetof(__wasi_event_t, error) == 8, "witx calculated offset");
_Static_assert(offsetof(__wasi_event_t, type) == 10, "witx calculated offset");
_Static_assert(offsetof(__wasi_event_t, fd_readwrite) == 16, "witx calculated offset");

$prestat

Current Master

/**
 * The contents of an $prestat.
 */
typedef union __wasi_prestat_u_t {
    /**
     * When type is `preopentype::dir`:
     */
    __wasi_prestat_dir_t dir;

} __wasi_prestat_u_t;

_Static_assert(sizeof(__wasi_prestat_u_t) == 4, "witx calculated size");
_Static_assert(_Alignof(__wasi_prestat_u_t) == 4, "witx calculated align");

/**
 * Information about a pre-opened capability.
 */
typedef struct __wasi_prestat_t {
    /**
     * The type of the pre-opened capability.
     */
    __wasi_preopentype_t pr_type;

    /**
     * The contents of the information.
     */
    __wasi_prestat_u_t u;

} __wasi_prestat_t;

_Static_assert(sizeof(__wasi_prestat_t) == 8, "witx calculated size");
_Static_assert(_Alignof(__wasi_prestat_t) == 4, "witx calculated align");
_Static_assert(offsetof(__wasi_prestat_t, pr_type) == 0, "witx calculated offset");
_Static_assert(offsetof(__wasi_prestat_t, u) == 4, "witx calculated offset");

Tagged Unions

/**
 * Information about a pre-opened capability.
 */
typedef union __wasi_prestat_u_t {
    __wasi_prestat_dir_t dir;
} __wasi_prestat_u_t;
typedef struct __wasi_prestat_t {
    __wasi_preopentype_t tag;
    __wasi_prestat_u_t u;
} __wasi_prestat_t;

_Static_assert(sizeof(__wasi_prestat_t) == 8, "witx calculated size");
_Static_assert(_Alignof(__wasi_prestat_t) == 4, "witx calculated align");
_Static_assert(offsetof(__wasi_prestat_t, u) == 4, "witx calculated union offset");
_Static_assert(sizeof(__wasi_prestat_u_t) == 4, "witx calculated union size");
_Static_assert(_Alignof(__wasi_prestat_u_t) == 4, "witx calculated union align");

[pchickey]

I don't understand why the Azure windows build failed but the github windows build succeeded. Do we need to keep the Azure pipelines around anymore?

[pchickey]

Rebased on master, and submodule points at WASI master - ready to go @sunfishcode

[sunfishcode]

This PR contains a change to remove __wasilibc_unmodified_upstream markers. I discussed this with @pchickey offline, but I want to mention it here too. These markers were more useful when they were simpler and when we were first setting up wasi-libc and figuring out what parts of cloudlibc we needed and which we didn't. But as WASI has evolved the __wasilibc_unmodified_upstream markers have become increasingly complex and awkward to maintain, and we have a clearer picture of what wasi-libc's role is now, so it seems a good time to start dropping them.

The Azure Pipeline rules build wasi-libc, which the Github Action rules don't do yet. If we moved that over to Github Actions, then we could drop the Azure Pipelines.

@pchickey And just to confirm, this change doesn't change the ABI, right?

[sbc100]

Are the markers still useful for the musl parts of the code (where we will most likely want to periodically update)?

[pchickey]

The markers may be useful in musl (libc-top-half) but as we've made more and more changes in the witx files and code generator, I agree with Dan that they're not useful in the bottom half anymore.

Correct, there is no ABI change, thats what you're supposed to be able to convince yourself from the changes summarized in the description.

[pchickey]

#168 merges this PR and #167, demonstrating that the windows build issue in azure here is something thats not the fault of this PR.

[pchickey]

rebased after #167 merged to master

[sunfishcode]

I agree; let's keep the __wasilibc_unmodified_upstream markers in musl for now, but removing them in libc-bottom-half now makes sense.