Re: [PATCH] arm/strlen-thumb2-Os.S: Correct assembly syntax for ldrb instruction

On 12/05/2020 18:58, Keith Packard via Newlib wrote:
> We want to use a post-indexed addressing mode (which means use the
> original register contents as the address, then increment that
> register) which is only valid in Encoding T3 of the LDRB instruction.
> 
> According to the ARMv7-M Architecture Reference Manual, the assembly
> syntax for Encoding T3 does not include the '.W' width specifier as
> that is used to specify Encoding T2, presumably to provide a wider
> immediate field for possible relocations.
> 
> GAS allows the .W specifier for this addressing mode and generates
> identical output with and without it. clang does not allow the .W
> specifier for this addressing mode, so removing it offers wider
> portability and closer adherance to the ARM assembly syntax
> specification.
> 
> Signed-off-by: Keith Packard <[email protected]>
> ---
>  newlib/libc/machine/arm/strlen-thumb2-Os.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/newlib/libc/machine/arm/strlen-thumb2-Os.S b/newlib/libc/machine/arm/strlen-thumb2-Os.S
> index 961f41a0a..aed8adf33 100644
> --- a/newlib/libc/machine/arm/strlen-thumb2-Os.S
> +++ b/newlib/libc/machine/arm/strlen-thumb2-Os.S
> @@ -45,7 +45,7 @@
>  
>  def_fn	strlen p2align=1
>  	mov     r3, r0
> -1:	ldrb.w  r2, [r3], #1
> +1:	ldrb    r2, [r3], #1
>  	cmp     r2, #0
>  	bne	1b
>  	subs    r0, r3, r0
> 

IIRC the .w was deliberate to keep the alignment right for some
subsequent instructions.

LLVM's assembler needs fixing if it doesn't accept '.w'.

R.