Cygwin: testsuite: Add x86_64 code to "dynamically load cygwin" test

This still needs some more voudou to actually work.

Also update how-cygtls-works.txt a bit

winsup/cygwin/DevDocs/how-cygtls-works.txt

@@ -4,8 +4,8 @@ All cygwin threads have separate context in an object of class _cygtls.  The
 storage for this object is kept on the stack in the bottom __CYGTLS_PADSIZE__
 bytes.  Each thread references the storage via the Thread Environment Block
 (aka Thread Information Block), which Windows maintains for each user thread
-in the system, with the address in the FS segment register.  The memory
-is laid out as in the NT_TIB structure from <w32api/winnt.h>:
+in the system, with the address in a segment register (FS on x86, GS on x86_64).
+The memory is laid out as in the NT_TIB structure from <w32api/winnt.h>:
 
 typedef struct _NT_TIB {
 	struct _EXCEPTION_REGISTRATION_RECORD *ExceptionList;
@@ -20,16 +20,10 @@ typedef struct _NT_TIB {
 	struct _NT_TIB *Self;
 } NT_TIB,*PNT_TIB;
 
-Cygwin sees it like this:
-
-extern exception_list *_except_list asm ("%fs:0");      // exceptions.cc
-extern char *_tlsbase __asm__ ("%fs:4");                // cygtls.h
-extern char *_tlstop __asm__ ("%fs:8");                 // cygtls.h
-
-And accesses cygtls like this:
-
-#define _my_tls (((_cygtls *) _tlsbase)[-1])            // cygtls.h
+Cygwin accesses cygtls like this (see cygtls.h):
 
+#define _my_tls (*((_cygtls *) ((PBYTE) NtCurrentTeb()->Tib.StackBase \
+		                - __CYGTLS_PADSIZE__)))
 
 Initialization always goes through _cygtls::init_thread().  It works
 in the following ways:
@@ -65,11 +59,4 @@ __CYGTLS_PADSIZE__ bytes down from there are overwritten.
 
 Debugging
 
-You can examine the segment registers in gdb via "info w32 selector $fs"
-(which is using GetThreadSelectorEntry()) to get results like this:
-
-    Selector $fs
-    0x03b: base=0x7ffdd000 limit=0x00000fff 32-bit Data (Read/Write, Exp-up)
-    Priviledge level = 3. Byte granular.
-
-"x/3x 0x7ffdd000" will give you _except_list, _tlsbase, and _tlstop.
+You can examine the TIB in gdb via "info w32 tib"

winsup/testsuite/winsup.api/cygload.cc

@@ -43,7 +43,10 @@ using std::string;
 cygwin::padding *cygwin::padding::_main = NULL;
 DWORD cygwin::padding::_mainTID = 0;
 
-// A few cygwin constants.
+// Cygwin signal constants
+#undef SIGINT
+#undef SIGTERM
+
 static const int SIGHUP = 1;
 static const int SIGINT = 2;
 static const int SIGTERM = 15;  // Cygwin won't deliver this one to us;
@@ -68,17 +71,30 @@ cygwin::padding::padding ()
 
   _end = _padding + sizeof (_padding);
   char *stackbase;
-#ifdef __GNUC__
+#ifdef __GNUC__ /* GCC */
+# ifdef __x86_64__
+    __asm__ (
+    "mov %%gs:8, %0"
+    :"=r"(stackbase)
+    );
+# elif __X86__
   __asm__ (
     "movl %%fs:4, %0"
     :"=r"(stackbase)
     );
-#else
+# else
+#  error Unknown architecture
+# endif
+#else /* !GCC assumed to be MSVC */
+# ifdef __X86__
   __asm
       {
         mov eax, fs:[4];
         mov stackbase, eax;
       }
+#else
+#  error Unknown architecture
+# endif
 #endif
   _stackbase = stackbase;
 

winsup/testsuite/winsup.api/cygload.exp

@@ -14,7 +14,13 @@ proc ws_spawn {cmd args} {
     verbose send "catchCode = $rv\n"
 }
 
-ws_spawn "$MINGW_CXX $srcdir/$subdir/cygload.cc -o mingw-cygload.exe -lstdc++ -Wl,-e,_cygloadCRTStartup@0"
+if { [string match "i686" $target_alias] } {
+    set entrypoint "_cygloadCRTStartup@0"
+} else {
+    set entrypoint "cygloadCRTStartup"
+}
+
+ws_spawn "$MINGW_CXX $srcdir/$subdir/cygload.cc -o mingw-cygload.exe -static -Wl,-e,$entrypoint"
 
 if { $rv != {0 {}} } {
     verbose -log "$rv"