[Bug gas/30746] New: [RISCV] upper immediate range should be signed

[andrew.jones at linux dot dev]

https://sourceware.org/bugzilla/show_bug.cgi?id=30746

            Bug ID: 30746
           Summary: [RISCV] upper immediate range should be signed
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: andrew.jones at linux dot dev
  Target Milestone: ---

Instructions which have upper immediates (U-immediate) operands (LUI and AUIPC)
should accept negative values, as upper immediates are signed. A U-immediate is
20 bits which will be shifted left 12 bits and the lower 12 bits will always be
zero. After shifting, bit 31 is the sign bit. This implies that when the
immediate is provided as an operand with the lower 12 bits removed, that it
should have the range -524288..524287. However, gas requires the operand to
have the range 0..1048575, even though, after shifting, 524288..1048575 will
represent -2147483648..-4096. This works, but it's inconvenient to developers.

Test 1:
-------
$ echo "lui a0, -1" | riscv64-linux-gnu-as
{standard input}: Assembler messages:
{standard input}:1: Error: lui expression not in range 0..1048575

Expected result:
----------------
Assembling generates 0xfffff537, which can currently be generated with 'lui a0,
0xfffff'

Test 2:
-------
$ echo "auipc a0, -1" | riscv64-linux-gnu-as
{standard input}: Assembler messages:
{standard input}:1: Error: lui expression not in range 0..1048575

Expected result:
----------------
Assembling generates 0xfffff517, which can currently be generated with 'auipc
a0, 0xfffff'

Proposed solution:
------------------
Support the range -524288..1048575, where 524288..1048575 aliases -524288..-1

Additional notes:
-----------------
Another issue, as can be seen in the output above for the auipc test, is that
gas always outputs 'lui' in the error message, even when the instruction was
not lui.

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