PATCH: S/390: document frame link state machine

[Jim Blandy <jimb at zwingli dot cygnus dot com>]

2001-12-04  Jim Blandy  <jimb@redhat.com>

	* s390-tdep.c (s390_get_frame_info): Doc fixes.

Index: gdb/s390-tdep.c
===================================================================
RCS file: /cvs/cvsfiles/devo/gdb/s390-tdep.c,v
retrieving revision 2.24
diff -c -r2.24 s390-tdep.c
*** gdb/s390-tdep.c	2001/12/03 18:50:57	2.24
--- gdb/s390-tdep.c	2001/12/04 23:48:44
***************
*** 208,215 ****
    int gprs_saved[S390_NUM_GPRS];
    int fprs_saved[S390_NUM_FPRS];
    int regidx, instrlen;
!   int save_link_regidx, subtract_sp_regidx;
!   int const_pool_state, save_link_state;
    int frame_pointer_found, varargs_state;
    int loop_cnt, gdb_gpr_store, gdb_fpr_store;
    int frame_pointer_regidx = 0xf;
--- 208,214 ----
    int gprs_saved[S390_NUM_GPRS];
    int fprs_saved[S390_NUM_FPRS];
    int regidx, instrlen;
!   int const_pool_state;
    int frame_pointer_found, varargs_state;
    int loop_cnt, gdb_gpr_store, gdb_fpr_store;
    int frame_pointer_regidx = 0xf;
***************
*** 217,222 ****
--- 216,244 ----
    int err = 0;
    disassemble_info info;
  
+   /* What we've seen so far regarding saving the back chain link:
+      0 -- nothing yet; sp still has the same value it had at the entry
+           point.  Since not all functions allocate frames, this is a
+           valid state for the prologue to finish in.
+      1 -- We've saved the original sp in some register other than the
+           frame pointer (hard-coded to be %r11, yuck).
+           save_link_regidx is the register we saved it in.
+      2 -- We've seen the initial `bras' instruction of the sequence for
+           reserving more than 32k of stack:
+                 bras %rX, .+8
+                 .long N
+                 s %r15, 0(%rX)
+           where %rX is not the constant pool register.
+           subtract_sp_regidx is %rX, and fextra_info->stack_bought is N.
+      3 -- We've reserved space for a new stack frame.  This means we
+           either saw a simple `ahi %r15,-N' in state 1, or the final
+           `s %r15, ...' in state 2.
+      4 -- The frame and link are now fully initialized.  We've
+           reserved space for the new stack frame, and stored the old
+           stack pointer captured in the back chain pointer field.  */
+   int save_link_state;
+   int save_link_regidx, subtract_sp_regidx;
+ 
    /* What we've seen so far regarding r12 --- the GOT (Global Offset
       Table) pointer.  We expect to see `l %r12, N(%r13)', which loads
       r12 with the offset from the constant pool to the GOT, and then
***************
*** 493,499 ****
        /* Alternatively check for the complex construction for
           buying more than 32k of stack
           BRAS gprx,.+8
!          long vals    %r15,0(%gprx)  gprx currently r1 */
        if ((save_link_state == 1) && (instr[0] == 0xa7)
  	  && ((instr[1] & 0xf) == 0x5) && (instr[2] == 0)
  	  && (instr[3] == 0x4) && ((instr[1] >> 4) != CONST_POOL_REGIDX))
--- 515,522 ----
        /* Alternatively check for the complex construction for
           buying more than 32k of stack
           BRAS gprx,.+8
!          long val
!          s    %r15,0(%gprx)  gprx currently r1 */
        if ((save_link_state == 1) && (instr[0] == 0xa7)
  	  && ((instr[1] & 0xf) == 0x5) && (instr[2] == 0)
  	  && (instr[3] == 0x4) && ((instr[1] >> 4) != CONST_POOL_REGIDX))