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tdep/2561: MMX register number to register value mapping is incorrect
- From: tom dot honermann at oracle dot com
- To: gdb-gnats at sources dot redhat dot com
- Date: 8 Dec 2008 05:48:54 -0000
- Subject: tdep/2561: MMX register number to register value mapping is incorrect
- Reply-to: tom dot honermann at oracle dot com
>Number: 2561
>Category: tdep
>Synopsis: MMX register number to register value mapping is incorrect
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: change-request
>Submitter-Id: net
>Arrival-Date: Mon Dec 08 05:58:01 UTC 2008
>Closed-Date:
>Last-Modified:
>Originator: Tom Honermann
>Release: gdb-6.8
>Organization:
>Environment:
Linux 2.6.9 - Oracle Enterprise Linux 4u4 (Equiv to RedHat 4u4)
>Description:
Intel MMX registers (mm0-mm7) are aliased to the x87 floating point registers (st0-st7). The x87 floating point registers comprise a register stack that revolves as values are pushed onto it. The MMX registers however are directly mapped to the backing store for the floating point registers (r0-r7). Thus, mm0==r0, mm1==r1, ...mm7=r7 but st0 is not necessarily equal to r0. When displaying MMX registers, gdb computes the association between the MMX and x87 data registers incorrectly.
>How-To-Repeat:
Compile the attached program as a 32bit x86 Linux program (Makefile included). Run it and it will display what the MMX register values should be. The program will then generate a SIGSEGV to break into gdb at a good point for analyzing the x87 and MMX registers. Run the program in GDB and when the SIGSEGV is received, run 'info all-reg' to see all registers. Compare the GDB mm0-mm7 register values to those displayed by the program. The values to register number mapping will be off.
>Fix:
The problem is in 'i386_mmx_regnum_to_fp_regnum' in gdb-6.8/gdb/i386-tdep.c. The current code for computing the association is:
fpreg = (mmxreg + tos) % 8;
This should be:
fpreg = ((8 + mmxreg) - tos) % 8;
>Release-Note:
>Audit-Trail:
>Unformatted:
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