[llvm-bugs] [Bug 43382] New: [ARMv7] ldrb.w syntax issue

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43382

            Bug ID: 43382
           Summary: [ARMv7] ldrb.w syntax issue
           Product: clang
           Version: 9.0
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Tooling
          Assignee: unassignedclangbugs at nondot.org
          Reporter: eblot.ml at gmail.com
                CC: llvm-bugs at lists.llvm.org

[foreword: I did not find the proper component for clang's integrated-as
component. I picked up "C" which is likely not the right sub-component, please
reassign to the proper component]

Using clang/llvm 9.0.0 final with an ARMv7-M (ELF) target, the following syntax
is not accepted by clang assembler:

    ldrb.w  r2, [r3], #1

Full example:

   .text
   .arch   armv7
   .thumb
   .thumb_func
   .align 1
   .syntax unified

   mov     r3, r0
1: ldrb.w  r2, [r3], #1
   cmp     r2, #0
   bne   1b
   subs    r0, r3, r0
   subs    r0, #1
   bx      lr

Using "clang -target armv7em-none-eabi -mcpu=cortex-m4 -mfloat-abi=hard
-mfpu=fpv4-sp-d16 -mthumb -mabi=aapcs -c strlen.S":

    strlen.S:7:21: error: too many operands for instruction
    1: ldrb.w r2, [r3], #1


>From ARM®v7-M Architecture Reference Manual [ARM DDI 0403E.b], Section A7.7.45
LDRB (immediate), Encoding T2 ARMv7-M, 

    LDRB<c>.W <Rt>,[<Rn>{,#<imm12>}]

seems to be a valid syntax for the selected architecture.


If the opcode is changed to

   ldrb  r2, [r3], #1

clang does not complain anymore, and generate the .o output file

Now reading back this generated object file with llvm's objdump:

    $ arm-none-eabi-objdump -S strlen.o

    strlen.o:     file format elf32-littlearm


    Disassembly of section .text:

    00000000 <.text>:
       0: 4603        mov   r3, r0
       2: f813 2b01   ldrb.w   r2, [r3], #1
       6: 2a00        cmp   r2, #0
       8: d1fb        bne.n 2 <.text+0x2>
       a: 1a18        subs  r0, r3, r0
       c: 3801        subs  r0, #1
       e: 4770        bx lr

objdump decodes the ldrb instruction as ldrb.w, which is the syntax clang
rejects.

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