[Bug target/63762] New: [ARM]GCC generates UNPREDICTABLE STR with Rn = Rt when hard-float abi is used

[renlin.li at arm dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63762

            Bug ID: 63762
           Summary: [ARM]GCC generates UNPREDICTABLE STR with Rn = Rt when
                    hard-float abi is used
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: renlin.li at arm dot com
                CC: marcus.shawcroft at arm dot com, ramana.radhakrishnan at 
arm dot com
            Target: arm*

Created attachment 33910
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33910&action=edit
test case

This was reported on a public mailing list on 2014-10-25:
http://lists.debian.org/debian-arm/2014/10/msg00057.html

The problem is reproducible with a recent FSF compiler:

$arm-none-eabi-gcc -mfloat-abi=hard -O2 -c unpreditable-str.c

or

$arm-none-linux-gnueabihf-gcc -O2 -c unpreditable-str.c 


/tmp/ccWQsFwY.s: Assembler messages:
/tmp/ccWQsFwY.s:176: Warning: source register same as write-back base

In this case the assembler is giving a warning rather than an error but the
instruction ("str r2, [r2], #4" in this case) is UNPREDICTABLE.