Fix another -freorder-blocks-and-partition glitch with Windows SEH

Since GCC 8, the -freorder-blocks-and-partition pass can split a function
into hot and cold parts, thus generating 2 FDEs for a single function in
DWARF for exception purposes and doing an equivalent trick for Windows SEH.

Now the Windows system unwinder does not support arbitrarily large frames
and there is even a hard limit on the encoding of the CFI, which changes
the stack allocation strategy when it is topped and which must be reflected
everywhere.

gcc/
	* config/i386/winnt.c (i386_pe_seh_cold_init): Properly deal with
	frames larger than the SEH maximum frame size.
gcc/testsuite/
	* gnat.dg/opt92.adb: New test.

gcc/config/i386/winnt.c

@@ -921,15 +921,17 @@ i386_pe_seh_cold_init (FILE *f, const char *name)
 
   /* In the normal case, the frame pointer is near the bottom of the frame
      so we can do the full stack allocation and set it afterwards.  There
-     is an exception when the function accesses prior frames so, in this
-     case, we need to pre-allocate a small chunk before setting it.  */
-  if (crtl->accesses_prior_frames)
-    alloc_offset = seh->cfa_offset;
-  else
+     is an exception if the function overflows the SEH maximum frame size
+     or accesses prior frames so, in this case, we need to pre-allocate a
+     small chunk of stack before setting it.  */
+  offset = seh->sp_offset - INCOMING_FRAME_SP_OFFSET;
+  if (offset < SEH_MAX_FRAME_SIZE && !crtl->accesses_prior_frames)
     alloc_offset = seh->sp_offset;
+  else
+    alloc_offset = MIN (seh->cfa_offset + 240, seh->sp_offset);
 
   offset = alloc_offset - INCOMING_FRAME_SP_OFFSET;
-  if (offset > 0 && offset < SEH_MAX_FRAME_SIZE)
+  if (offset > 0)
     fprintf (f, "\t.seh_stackalloc\t" HOST_WIDE_INT_PRINT_DEC "\n", offset);
 
   for (int regno = 0; regno < FIRST_PSEUDO_REGISTER; regno++)
@@ -958,7 +960,7 @@ i386_pe_seh_cold_init (FILE *f, const char *name)
       fprintf (f, ", " HOST_WIDE_INT_PRINT_DEC "\n", offset);
     }
 
-  if (crtl->accesses_prior_frames)
+  if (alloc_offset != seh->sp_offset)
     {
       offset = seh->sp_offset - alloc_offset;
       if (offset > 0 && offset < SEH_MAX_FRAME_SIZE)

gcc/testsuite/gnat.dg/opt92.adb

@@ -0,0 +1,38 @@
+-- { dg-do compile { target { lp64 || llp64 } } }
+-- { dg-options "-O2 -gnatws" }
+
+procedure Main is
+
+   subtype Int64 is Long_Long_Integer;
+
+   type Arr is array (Int64 range <>) of Boolean;
+
+   Pow : constant := 10;
+
+   procedure Compute (B : in out Arr) is
+      Factor : Int64 := 3;
+      Num    : Int64;
+   begin
+      while Factor <= 10 ** (Pow / 2) loop
+         Num := Factor;
+         while Num < 10 ** Pow loop
+            if B (Num) then
+               Factor := Num;
+               exit;
+            end if;
+            Num := Num + 2;
+         end loop;
+         Num := Factor * Factor;
+         while Num < 10 ** Pow loop
+            B (Num) := False;
+            Num        := Num + Factor * 2;
+         end loop;
+         Factor := Factor + 2;
+      end loop;
+   end;
+
+   B : Arr (1 .. 10 ** Pow) := (others => True);
+
+begin
+   Compute (B);   
+end;