Why does arm-*-as issue a warning for "strb r3, [r3], #1"?

[Kazu Hirata]

Hi,

When I feed "strb r3, [r3], #1" to arm-*-as, the assembler issues a
warning

  "source register same as write-back base"

While I like helpful warnings in general, I am wondering if the
assembler should issue a warning for something a little strange but
otherwise correct because a compiler, a major user of the assembler,
tries to get most out of the instruction set (modulo "discouraged"
instructions).  Note that gcc's testsuite treats a warning from the
assembler as FAIL.  (As far as I can tell from the instruction set
manual, I don't see anything that bans "strb r3, [r3], #1".)

I am tempted to submit a patch to remove this warning from tc-arm.c,
but I am no expert on ARM.

In case you are wondering, this instruction comes from
gcc.c-torture/compile/930210-1.c.

Thoughts?

Kazu Hirata