[Bug gas/15043] New: [avr] Support new command line options -mdes and -matomics

[gjl at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=15043

             Bug #: 15043
           Summary: [avr] Support new command line options -mdes and
                    -matomics
           Product: binutils
           Version: 2.24 (HEAD)
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: gas
        AssignedTo: address@hidden
        ReportedBy: address@hidden
                CC: address@hidden
    Classification: Unclassified
            Target: avr


The avr tools currently support around 200 devices and there are more,
unsupported devices.

It makes hardly any sense to add a -mmcu=device switch for each and every
target:  For binutils, it's enough to know the instruction set architecture
(ISA) to assemble for.

Thus, command line switches like -mdes and -matomic could greatly reduce the
time until new devices are supported, because the compiler need not to wait
until respective support in binutils is available:

The compiler could just call avr-as with -mmcu=core -mdes if it knows that the
device supports the DES instruction and with -mmcu=core -mno-des, otherwise.

The options shall work as follows:

With -mdes, the assembler will accept and assemble the DES instruction.
With -mno-des, the assembler will issue an "unknown instruction" error.

If a device is specified with -mmcu= then the assembler knows the right setting
for -m[no-]des and uses it, provided it is not explicitly overridden by
-m[no-]des. Alternatively, -m[no-]des could be ignored in that case.

If a core like avr4 definitely does not support DES instruction, the option is
ignored and the behavior the same as it would be with -mno-des.

Similar for -m[no-]atomics and the instructions XCH, LAC, LAS, LAT.

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