Re: [patch] Backtrace prints wrong argument value

[Luis Machado <luisgpm at linux dot vnet dot ibm dot com>]

:ADDPATCH PowerPC-64:

Hi,

Bringing back this topic, i've written a patch to address this issue on
ppc's side, providing a function to specify call-clobbered registers
based on the ABI, similar to the S390's.

Looking forward to suggestions/corrections.

Best regards


> There is simply not enough information in the debug info to handle
> this correctly.  Let me give you another example:
> 
>      move var to r3
>      test something
>      if true, branch to Lfoo
>      call abort, which clobbers r3
> Lfoo:
>      do something with r3
> 
> At every instruction after the move, the debug info should say that
> var is in r3.  Right?  No matter which location we pick here, while
> backtracing from abort, we'll print the wrong value for var and
> there's no point where the debug info will say it is undefined.
> 
> If you want us to get this right using DWARF info, I believe your only
> choice is to approach the DWARF working group about it.
> 
> Now, in GDB we may have other options.  We might be able to get the
> list of call clobbered registers based on the ABI.  Compare with s390,
> which already does this (dwarf2_frame_set_init_reg).  Does adding
> this to PowerPC help your example any?
> 
-- 
Luis Machado
IBM Linux Technology Center
e-mail: luisgpm@linux.vnet.ibm.com

2007-09-13  Luis Machado  <luisgpm@br.ibm.com>

    * rs6000-tdep.c (ppc_dwarf2_frame_init_reg): New function.
    (rs6000_gdbarch_init): Install ppc_dwarf2_frame_init_reg as 
    default dwarf2_frame_set_init_reg function.

Index: gdb/rs6000-tdep.c
===================================================================
--- gdb.orig/rs6000-tdep.c	2007-09-13 07:26:12.000000000 -0700
+++ gdb/rs6000-tdep.c	2007-09-13 07:41:10.000000000 -0700
@@ -3468,6 +3468,68 @@
   return &rs6000_frame_base;
 }
 
+/* DWARF-2 frame support. Used to handle the detection of
+   clobbered registers during function calls.  */
+
+ static void
+ppc_dwarf2_frame_init_reg (struct gdbarch *gdbarch, int regnum,
+                            struct dwarf2_frame_state_reg *reg,
+          struct frame_info *next_frame)
+{
+
+  struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
+
+  /* PPC32 and PPC64 ABI's are the same regarding volatile and
+     non-volatile registers. We will use the same code for both.  */
+
+  /* Call-saved GP registers.  */
+  if ((regnum >= (tdep->ppc_gp0_regnum + 14)
+    && regnum <= (tdep->ppc_gp0_regnum + 31))
+    || (regnum == (tdep->ppc_gp0_regnum + 1)))
+    reg->how = DWARF2_FRAME_REG_SAME_VALUE;
+
+  /* Call-clobbered GP registers.  */
+  if ((regnum >= (tdep->ppc_gp0_regnum + 3)
+    && regnum <= (tdep->ppc_gp0_regnum + 12))
+    || (regnum == tdep->ppc_gp0_regnum))
+    reg->how = DWARF2_FRAME_REG_UNDEFINED;
+
+  /* Deal with FP registers, if supported.  */
+  if (tdep->ppc_fp0_regnum >= 0)
+    {
+      /* Call-saved FP registers.  */
+      if ((regnum >= (tdep->ppc_fp0_regnum + 14)
+        && regnum <= (tdep->ppc_fp0_regnum + 31)))
+        reg->how = DWARF2_FRAME_REG_SAME_VALUE;
+
+      /* Call-clobbered FP registers.  */
+      if ((regnum >= (tdep->ppc_fp0_regnum)
+        && regnum <= (tdep->ppc_fp0_regnum + 13)))
+        reg->how = DWARF2_FRAME_REG_UNDEFINED;
+    }
+
+  /* Deal with ALTIVEC registers, if supported.  */
+  if (tdep->ppc_vr0_regnum > 0 && tdep->ppc_vrsave_regnum > 0)
+    {
+      /* Call-saved Altivec registers.  */
+      if ((regnum >= (tdep->ppc_vr0_regnum + 20)
+        && regnum <= (tdep->ppc_vr0_regnum + 31))
+        || regnum == tdep->ppc_vrsave_regnum)
+        reg->how = DWARF2_FRAME_REG_SAME_VALUE;
+
+      /* Call-clobbered Altivec registers.  */
+      if ((regnum >= (tdep->ppc_vr0_regnum)
+        && regnum <= (tdep->ppc_vr0_regnum + 19)))
+        reg->how = DWARF2_FRAME_REG_UNDEFINED;
+    }
+
+  if (regnum == gdbarch_pc_regnum (current_gdbarch))
+    reg->how = DWARF2_FRAME_REG_RA;
+  else if (regnum == gdbarch_sp_regnum (current_gdbarch))
+    reg->how = DWARF2_FRAME_REG_CFA;
+}
+
+
 /* Initialize the current architecture based on INFO.  If possible, re-use an
    architecture from ARCHES, which is a list of architectures already created
    during this debugging session.
@@ -3790,6 +3852,10 @@
           tdep->ppc_vr0_regnum = 71;
           tdep->ppc_vrsave_regnum = 104;
         }
+
+      /* Frame handling.  */
+      dwarf2_frame_set_init_reg (gdbarch, ppc_dwarf2_frame_init_reg);
+
       /* Fall Thru */
     case GDB_OSABI_NETBSD_AOUT:
     case GDB_OSABI_NETBSD_ELF: