[Bug gas/12848] New: ARM: Thumb-2: Range check on b.w is off by a factor of 2

[dave.martin at linaro dot org]

http://sourceware.org/bugzilla/show_bug.cgi?id=12848

           Summary: ARM: Thumb-2: Range check on b.w is off by a factor of
                    2
           Product: binutils
           Version: 2.22 (HEAD)
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gas
        AssignedTo: address@hidden
        ReportedBy: address@hidden


The 32-bit Thumb long branch encoding has a range of +/-16MiB, but gas checks
+/-32MiB instead (which is, perhaps coincidentally, the range of the B
instruction in ARM state).

This can result in bogus branch offsets in the assembly output.

Having checked the ARM ARM, this looks like a big in gas, not objdump.

Since this behaviour only affects long distance branches within a single
section (i.e., branches fixed up directly by the assembler) it is unlikely to
be seen often in the wild.

(*) indicates the incorrect branch instructions.


$ binutils/gas/as-new -mthumb -o tst.o tst.s && arm-linux-gnueabi-objdump -d
tst.o

tst.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <e>:
       0:       f3ff 97ff       b.w     1000002 <f+0xffffee>     (*)
       4:       f400 9000       b.w     ff000008 <g+0xfd000000>
       8:       f3ff 97ff       b.w     100000a <f+0xfffff6>
       c:       f400 9000       b.w     ff000010 <g+0xfd000008>  (*)
      10:       f7ff bfff       b.w     12 <e+0x12>              (*)

00000014 <f>:
      14:       f7ff bff8       b.w     8 <e+0x8>                (*)
        ...

02000008 <g>:
 2000008:       f000 b804       b.w     2000014 <g+0xc>          (*)

$ cat tst.s
.syntax unified

.type f, %function
e:
        b       . - 0xfffffe   @ gas mis-assembles as a forward branch
        b       . - 0xfffffc
        b       . + 0x1000002
        b       . + 0x1000004  @ gas mis-assembles as a backward branch
        b       . + 0x2000002  @ gas mis-assembles as a backward branch
@       b       . + 0x2000004  @ gas correctly detects this as out of range

f:      b       g              @ gas mis-assembles as a backward branch

        .space 0x1fffff0

g:      b       f              @ gas mis-assembles as a forward branch

@ If "b.w" is used instead of "b", the results are exactly the same.

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