[llvm-bugs] [Bug 50763] New: [MC][Thumb2] #-0 assembled as #INT_MIN instead

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https://bugs.llvm.org/show_bug.cgi?id=50763

            Bug ID: 50763
           Summary: [MC][Thumb2] #-0 assembled as #INT_MIN instead
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: ARM
          Assignee: unassignedbugs at nondot.org
          Reporter: rprichard at google.com
                CC: llvm-bugs at lists.llvm.org, smithp352 at googlemail.com,
                    Ties.Stuij at arm.com

The LLVM and GNU ARM assemblers recognize a special "negative zero" immediate
operand. The intent seems to have been to select an instruction encoding that
uses non-positive address offsets instead of non-negative offsets.

AFAICT they both use the same syntax: an immediate of value 0 whose first token
is a '-'. So #-(0) and #-(1-1) are treated specially, but #(-0) is not, because
its first token is '(' instead of '-'.

LLVM represents these offsets internally using INT_MIN. It uses the same
immediate parsing for non-address-offset immediates, and in that case, a #-0
operand is sometimes converted into an #INT_MIN operand. This seems wrong and
doesn't match the GNU assembler:

cat >test.s <<EOF
  .syntax unified
  .arch armv7-a
  .text
  .thumb
func:
  mov r2, #-0
  mov r2, #0x80000000
  orn r2, r2, #-0
  orn r2, r2, #0x80000000
  add r2, #-0
  add r2, #0x80000000
  mvn r0, #-0
  mvn r0, #0x80000000
EOF
llvm-mc --triple armv7a-linux test.s --filetype=obj -o test.o
llvm-objdump -d test.o
arm-linux-gnueabi-as test.s -c -o test.o
llvm-objdump -d test.o

LLVM output:

       0: 4f f0 00 42   mov.w r2, #2147483648
       4: 4f f0 00 42   mov.w r2, #2147483648
       8: 62 f0 00 42   orn r2, r2, #2147483648
       c: 62 f0 00 42   orn r2, r2, #2147483648
      10: 02 f1 00 42   add.w r2, r2, #2147483648
      14: 02 f1 00 42   add.w r2, r2, #2147483648
      18: 6f f0 00 40   mvn r0, #2147483648
      1c: 6f f0 00 40   mvn r0, #2147483648

GAS output:

       0: 4f f0 00 02   mov.w r2, #0
       4: 4f f0 00 42   mov.w r2, #2147483648
       8: 62 f0 00 02   orn r2, r2, #0
       c: 62 f0 00 42   orn r2, r2, #2147483648
      10: 02 f1 00 02   add.w r2, r2, #0
      14: 02 f1 00 42   add.w r2, r2, #2147483648
      18: 6f f0 00 00   mvn r0, #0
      1c: 6f f0 00 40   mvn r0, #2147483648

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