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Re: MIPS: 64-bit DWARF


Hello!

This is about switching MIPS from 64-bit to 32-bit DWARF format (as other
architectures are doing), for the benefit of prelink.

On 2010-07-06 09:59, I wrote:
> On 2010-07-01 15:43, I wrote:
>> On 2010-05-07 13:23, Jan Kratochvil wrote:
>>> On Wed, 05 May 2010 16:59:36 +0200, Andrew Stubbs wrote:
>>>> > prelink includes code to manipulate the contents of the debug info.
>>> [...]
>>>> Maybe prelink has failed to relocate the debug info in this case?
>>>
>>> The attached patch should fix it.
>>>
>>> Although SHT_MIPS_DWARF value should be recognized only for e_machine as
>>> EM_MIPS or EM_MIPS_RS3_LE or EM_MIPS_X?  Or are there some other rules?
>>
>>> --- prelink/src/dso.c-orig	2010-04-13 16:41:15.000000000 +0200
>>> +++ prelink/src/dso.c	2010-05-07 15:16:25.000000000 +0200
>>> @@ -1381,6 +1381,7 @@ adjust_dso (DSO *dso, GElf_Addr start, G
>>>        switch (dso->shdr[i].sh_type)
>>>  	{
>>>  	case SHT_PROGBITS:
>>> +	case SHT_MIPS_DWARF:
>>>  	  name = strptr (dso, dso->ehdr.e_shstrndx, dso->shdr[i].sh_name);
>>>  	  if (strcmp (name, ".stab") == 0
>>>  	      && adjust_stabs (dso, i, start, adjust))
>>
>> Unfortunately, this patch causes the following regression for about every
>> test of the prelink testsuite, in -mabi=64 configurations (only):
>>
>>     mips-wrs-linux-gnu-prelink -c ./prelink.conf -C ./prelink.cache --ld-library-path=. --dynamic-linker=./ld.so.1 -vm ./reloc1
>>     Laying out 4 libraries in virtual address space 0000005800000000-0000009800000000
>>     Assigned virtual address space slots for libraries:
>>     ./ld.so.1                                                    0000005800000000-0000005800032328
>>     ./libc.so.6                                                  0000005800040000-00000058001cbdb0
>>     ./reloc1lib1.so                                              00000058001d0000-00000058001e0b10
>>     ./reloc1lib2.so                                              00000058001f0000-00000058002009d0
>>     Prelinking /scratch/thomas/issue8927/obj/test-4.3a-294-mips-wrs-linux-gnu/host-i686-pc-linux-gnu/mabi_64/prelink.d/ld-2.8.so
>>     mips-wrs-linux-gnu-prelink: /scratch/thomas/issue8927/obj/test-4.3a-294-mips-wrs-linux-gnu/host-i686-pc-linux-gnu/mabi_64/prelink.d/ld-2.8.so: 64-bit DWARF not supported
>
> This happens when processing .debug_lines, which is generated by GAS.
>
> prelink does not support the 64-bit DWARF format.  From a quick glance,
> GDB does, see dwarf2read.c:read_initial_length.
>
> MIPS is, per gas/config/tc-mips, the only architecture to override
> DWARF2_FORMAT (which defaults to 32-bit), and thus possibly can emit
> 64-bit DWARF code, which it actually does for -mabi=64, see
> tc-mips.c:mips_dwarf2_format.
>
> In <http://sourceware.org/ml/binutils/2002-11/msg00429.html> this 64-bit
> DWARF format support has originally been added.
>
> What's our way forward?  Can we switch MIPS back to only using the 32-bit
> DWARF format (save the IRIX case)?  Do we need to extend prelink?
>
> As I understand the GCC code, it always uses the 32-bit DWARF format,
> even for MIPS 64-bit ABIs.

Here is the patch that I tested for (a non-HEAD version of) binutils.
This makes prelink happy, and there are no regressions in the GDB
testsuite.  (Anything else to test?)  I left in the 64-bit stuff for
TE_IRIX, as I have no idea about it.

Index: gas/config/tc-mips.c
===================================================================
--- gas/config/tc-mips.c	(revision 291480)
+++ gas/config/tc-mips.c	(working copy)
@@ -15499,20 +15499,16 @@
 #endif
 }
 
+#ifdef TE_IRIX
 enum dwarf2_format
 mips_dwarf2_format (void)
 {
   if (HAVE_64BIT_SYMBOLS)
-    {
-#ifdef TE_IRIX
-      return dwarf2_format_64bit_irix;
-#else
-      return dwarf2_format_64bit;
-#endif
-    }
+    return dwarf2_format_64bit_irix;
   else
     return dwarf2_format_32bit;
 }
+#endif
 
 int
 mips_dwarf2_addr_size (void)
Index: gas/config/tc-mips.h
===================================================================
--- gas/config/tc-mips.h	(revision 291480)
+++ gas/config/tc-mips.h	(working copy)
@@ -165,9 +165,13 @@
 extern void mips_enable_auto_align (void);
 #define md_elf_section_change_hook()	mips_enable_auto_align()
 
+#ifdef TE_IRIX
 enum dwarf2_format;
 extern enum dwarf2_format mips_dwarf2_format (void);
-#define DWARF2_FORMAT() mips_dwarf2_format ()
+# define DWARF2_FORMAT() mips_dwarf2_format ()
+#else
+/* Use GAS' defaults.  */
+#endif
 
 extern int mips_dwarf2_addr_size (void);
 #define DWARF2_ADDR_SIZE(bfd) mips_dwarf2_addr_size ()

Is this approach OK, or is there a real reason to have 64-bit DWARF
format on MIPS?  If it's OK, then I'll port the patch to binutils HEAD
((unused) asection * added to DWARF2_FORMAT).


Regards,
 Thomas

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