libgolang: Teach select to accept inplace tx data

Currently select, via _selcase, requires users to provide pointers to tx
and rx buffers. However if element type itself can fit into a pointer
word, we can put the element directly into _selcase and mark the case
with a flag, that it contains inplace data instead of referring to
external storage. This will be helpful in upcoming patch where we'll
teach pychan to work with several element types, not only pyobject
elements.

This patch does careful introduction of _selcase.flags - in such a way
that the size of _selcase stays the same as it was before by using
bitfields. The .ptxrx pointer is unioned with newly introduced inplace
uint64 .itxrx data, which is used by select instead of .ptxrx if the
flag is set. The usage of uint64 should not increase _selcase size on
64-bit platforms.

Then _selcase.ptx() and .prx() accessors are adapted accordingly and
the rest of the changes are corresponding test and
_chanselect2<onstack=false> adaptation.

This functionality is kind of low-level and is not exposed via any
_selsend() or chan.sends() API changes. Whenever inplace tx should be
used, the case should be prepared either completely manually, or with
e.g. first calling _selsend() and then manually changing .flags and
.itxrx.  Added test serves as the example on how to do it.

Inplace rx is currently forbidden - because supporting that would require
to drop const from casev select argument. However in the future, for
symmetry, we might want to support that as well.

P.S.

Since write to selcase.itxrx requires casting pointers e.g. like this:

	*(int *)&sel[0].itxrx = 12345;

it breaks C99 strict aliasing and by default compiler can generate bad
code on such pattern. To the problem we adapt the build system
to default compiler to no-strict-aliasing (as many other projects do,
e.g. Linux kernel) with the idea that in many cases where strict
aliasing was intended to help it actually does not, because e.g. pointer
types are the same, while explicitly marking pointers with `restrict`
keyword does help indeed.

Nothing new for Python2 here, as it is using -fno-strict-aliasing by
itself. However Python3 is compiling without -fno-strict-aliasing:
https://python.org/dev/peps/pep-3123 .

golang/_golang.pxd

@@ -46,6 +46,7 @@ In addition to Cython/nogil API, golang.pyx provides runtime for golang.py:
 
 from libcpp cimport nullptr_t, nullptr as nil
 from libcpp.utility cimport pair
+from libc.stdint cimport uint64_t
 cdef extern from *:
     ctypedef bint cbool "bool"
 
@@ -103,10 +104,15 @@ cdef extern from "golang/libgolang.h" namespace "golang" nogil:
         _CHANSEND
         _CHANRECV
         _DEFAULT
+    enum _selflags:
+        _INPLACE_DATA
     cppclass _selcase:
-        _chanop op
-        void    *ptxrx
-        cbool   *rxok
+        _chan     *ch
+        _chanop   op
+        unsigned  flags
+        void      *ptxrx
+        uint64_t  itxrx
+        cbool     *rxok
 
         const void *ptx() const
         void *prx() const

golang/_golang_test.pyx

@@ -176,6 +176,7 @@ cdef extern from * nogil:
     extern void _test_close_wakeup_all_vsrecv();
     extern void _test_close_wakeup_all_vsselect();
     extern void _test_select_win_while_queue();
+    extern void _test_select_inplace();
     """
     void _test_chan_cpp_refcount()              except +topyexc
     void _test_chan_cpp()                       except +topyexc
@@ -184,6 +185,7 @@ cdef extern from * nogil:
     void _test_close_wakeup_all_vsrecv()        except +topyexc
     void _test_close_wakeup_all_vsselect()      except +topyexc
     void _test_select_win_while_queue()         except +topyexc
+    void _test_select_inplace()                 except +topyexc
 def test_chan_cpp_refcount():
     with nogil:
         _test_chan_cpp_refcount()
@@ -205,3 +207,6 @@ def test_close_wakeup_all_vsselect():
 def test_select_win_while_queue():
     with nogil:
         _test_select_win_while_queue()
+def test_select_inplace():
+    with nogil:
+        _test_select_inplace()

golang/libgolang.h

@@ -150,12 +150,26 @@ enum _chanop {
     _DEFAULT    = 2,
 };
 
+enum _selflags {
+    // _INPLACE_DATA indicates that select case data is stored in
+    // _selcase.itxrx instead of in *_selcase.ptxrx .
+    // XXX can be used only for send for now. In the future, for symmetry, we
+    // might want to allow _INPLACE_DATA for recv too.
+    _INPLACE_DATA   = 1,
+};
+
+
 // _selcase represents one select case.
 typedef struct _selcase {
-    _chan           *ch;    // channel
-    enum _chanop    op;     // chansend/chanrecv/default
-    void            *ptxrx; // chansend: ptx; chanrecv: prx
-    bool            *rxok;  // chanrecv: where to save ok if !NULL; otherwise not used
+    _chan           *ch;        // channel
+    enum _chanop    op    : 8;  // chansend/chanrecv/default
+    enum _selflags  flags : 8;  // e.g. _INPLACE_DATA
+    unsigned              :16;
+    union {
+        void        *ptxrx;     // chansend: ptx; chanrecv: prx
+        uint64_t     itxrx;     // used instead of .ptxrx if .flags&_INPLACE_DATA != 0
+    };
+    bool            *rxok;      // chanrecv: where to save ok if !NULL; otherwise not used
 
 #ifdef __cplusplus
     // ptx returns pointer to data to send for this case.
@@ -176,6 +190,7 @@ _selcase _selsend(_chan *ch, const void *ptx) {
     _selcase _ = {
         .ch     = ch,
         .op     = _CHANSEND,
+        .flags  = (enum _selflags)0,
         .ptxrx  = (void *)ptx,
         .rxok   = NULL,
     };
@@ -188,6 +203,7 @@ _selcase _selrecv(_chan *ch, void *prx) {
     _selcase _ = {
         .ch     = ch,
         .op     = _CHANRECV,
+        .flags  = (enum _selflags)0,
         .ptxrx  = prx,
         .rxok   = NULL,
     };
@@ -200,6 +216,7 @@ _selcase _selrecv_(_chan *ch, void *prx, bool *pok) {
     _selcase _ = {
         .ch     = ch,
         .op     = _CHANRECV,
+        .flags  = (enum _selflags)0,
         .ptxrx  = prx,
         .rxok   = pok,
     };

golang/pyx/build.py

@@ -150,10 +150,15 @@ def Extension(name, sources, **kw):
     lang = kw.setdefault('language', 'c++')
 
     # default to C++11 (chan[T] & co require C++11 features)
+    ccdefault = []
     if lang == 'c++':
-        ccextra = kw.get('extra_compile_args', [])[:]
-        ccextra.insert(0, '-std=c++11') # if another -std=... was already there - it will override us
-        kw['extra_compile_args'] = ccextra
+        ccdefault.append('-std=c++11')
+    # default to no strict-aliasing
+    ccdefault.append('-fno-strict-aliasing')
+
+    _ = kw.get('extra_compile_args', [])[:]
+    _[0:0] = ccdefault              # if another e.g. -std=... was already there -
+    kw['extra_compile_args'] = _    # - it will override us
 
     # some depends to workaround a bit lack of proper dependency tracking in
     # setuptools/distutils.

golang/runtime/libgolang.cpp

@@ -859,6 +859,7 @@ void _chan::_dataq_popleft(void *prx) {
 const _selcase _default = {
     .ch     = NULL,
     .op     = _DEFAULT,
+    .flags  = (_selflags)0,
     .ptxrx  = NULL,
     .rxok   = NULL,
 };
@@ -867,13 +868,15 @@ const void *_selcase::ptx() const {
     const _selcase *cas = this;
     if (cas->op != _CHANSEND)
         panic("_selcase: ptx: op != send");
-    return cas->ptxrx;
+    return cas->flags & _INPLACE_DATA ? &cas->itxrx : cas->ptxrx;
 }
 
 void *_selcase::prx() const {
     const _selcase *cas = this;
     if (cas->op != _CHANRECV)
         panic("_selcase: prx: op != recv");
+    if (cas->flags & _INPLACE_DATA)
+        panic("_selcase: prx: recv with inplace data");
     return cas->ptxrx;
 }
 
@@ -943,6 +946,12 @@ int _chanselect(const _selcase *casev, int casec) {
         // recv
         else if (cas->op == _CHANRECV) {
             if (ch != NULL) {   // nil chan is never ready
+                // recv into inplace data is not supported
+                // ( in the future we might want to support it for symmetry with
+                //   send, but it will requre to drop const from casev )
+                if (cas->flags & _INPLACE_DATA)
+                    panic("select: recv into inplace data");
+
                 ch->_mu.lock();
                 if (1) {
                     bool ok, done = ch->_tryrecv(cas->prx(), &ok);
@@ -1000,7 +1009,7 @@ template<> int _chanselect2</*onstack=*/false>(const _selcase *casev, int casec,
     unsigned rxmax=0, txtotal=0;
 
     // reallocate chan .tx / .rx to heap; adjust casev
-    // XXX avoid doing this if all .tx and .rx are on heap?
+    // XXX avoid doing this if all .tx and .rx are on heap or have inplace data?
     unique_ptr<_selcase[]>  casev_onheap (new _selcase[casec]);
     for (i = 0; i < casec; i++) {
         const _selcase *cas = &casev[i];
@@ -1008,7 +1017,8 @@ template<> int _chanselect2</*onstack=*/false>(const _selcase *casev, int casec,
         if (cas->ch == NULL) // nil chan
             continue;
         if (cas->op == _CHANSEND) {
-            txtotal += cas->ch->_elemsize;
+            if (!(cas->flags & _INPLACE_DATA))
+                txtotal += cas->ch->_elemsize;
         }
         else if (cas->op == _CHANRECV) {
             rxmax = max(rxmax, cas->ch->_elemsize);
@@ -1032,9 +1042,11 @@ template<> int _chanselect2</*onstack=*/false>(const _selcase *casev, int casec,
         if (cas->ch == NULL) // nil chan
             continue;
         if (cas->op == _CHANSEND) {
-            memcpy(ptx, cas->ptxrx, cas->ch->_elemsize);
-            cas->ptxrx = ptx;
-            ptx += cas->ch->_elemsize;
+            if (!(cas->flags & _INPLACE_DATA)) {
+                memcpy(ptx, cas->ptxrx, cas->ch->_elemsize);
+                cas->ptxrx = ptx;
+                ptx += cas->ch->_elemsize;
+            }
         }
         else if (cas->op == _CHANRECV) {
             cas->ptxrx = rxtxdata;

golang/runtime/libgolang_test.cpp

@@ -391,3 +391,73 @@ void _test_select_win_while_queue() {
     for (i=0; i<N; i++)
         __test_select_win_while_queue();
 }
+
+// verify select behaviour with _INPLACE_DATA cases.
+void _test_select_inplace() {
+    auto ch = makechan<int>();
+    int i;
+
+    // inplace tx
+    go([ch]() {
+        _selcase sel[1];
+        sel[0] = ch.sends(NULL);
+        *(int *)&sel[0].itxrx = 12345;
+        sel[0].flags = _INPLACE_DATA;
+        int _ = select(sel);
+        ASSERT(_ == 0);
+    });
+
+    i = ch.recv();
+    ASSERT(i == 12345);
+
+    // inplace rx - currently forbidden to keep casev const.
+    _selcase sel[1];
+    sel[0] = ch.recvs();
+    sel[0].flags = _INPLACE_DATA;
+    const char *err = NULL;
+    try {
+        select(sel);
+    } catch (...) {
+        err = recover();
+    }
+    ASSERT(err != NULL);
+    ASSERT(!strcmp(err, "select: recv into inplace data"));
+
+    // _selcase ptx/prx
+    _selcase cas = _default;
+
+    err = NULL;
+    try {
+        cas.ptx();
+    } catch (...) {
+        err = recover();
+    }
+    ASSERT(err != NULL);
+    ASSERT(!strcmp(err, "_selcase: ptx: op != send"));
+
+    err = NULL;
+    try {
+        cas.prx();
+    } catch (...) {
+        err = recover();
+    }
+    ASSERT(err != NULL);
+    ASSERT(!strcmp(err, "_selcase: prx: op != recv"));
+
+    cas = ch.sends(&i);
+    ASSERT(cas.ptx() == &i);
+    cas.flags = _INPLACE_DATA;
+    ASSERT(cas.ptx() == &cas.itxrx);
+
+    cas = ch.recvs(&i);
+    ASSERT(cas.prx() == &i);
+    cas.flags = _INPLACE_DATA;
+    err = NULL;
+    try {
+        cas.prx();
+    } catch (...) {
+        err = recover();
+    }
+    ASSERT(err != NULL);
+    ASSERT(!strcmp(err, "_selcase: prx: recv with inplace data"));
+}