Re: [PATCH, AArch64] Detect assembler vector mov parsing bug

[Richard Earnshaw <rearnsha at arm dot com>]

On 14/07/14 20:54, Richard Henderson wrote:
> The strchr implementation that was recently committed to mainline tickles an
> assembler bug fixed by
> 
> commit 8db49cc2de52033990ae5d4d6aacedc8f986e803
> Author: Will Newton <will.newton@linaro.org>
> Date:   Thu Oct 31 17:21:11 2013 -0700
> 
>     config/tc-aarch64.c: Avoid trying to parse a vector mov as immediate.
>     ...
> 
> Obviously not a problem with a relatively new binutils installed, but Carlos
> ran into this problem building mainline glibc on a system using binutils 2.23.
> 
> It's not a silent error (we die linking ld.so), but its somewhat cryptic.
> Carlos opined to me that we ought to error out at configure time with an
> understandable message.
> 
> Rather than check for version numbers, let's check for functionality.
> 
> Ok?
> 
> 

We could probably work around the problem by using "mov x<n>, d<m>",
since the two operations are architecturally equivalent.  But I think
the vector style more accurately reflects the intent of the code.  Since
we are only reading, rather than writing, part of the vector, I don't
think there are likely to be major performance issues from such a change.

I haven't tested the above, hence the 'probably'.

R.

> r~
> 
> 
> z
> 
> 
> 	* sysdeps/aarch64/configure.ac: Check for assembler mov bug.
> 	* sysdeps/aarch64/configure: Regenerate.
> 
> diff --git a/sysdeps/aarch64/configure.ac b/sysdeps/aarch64/configure.ac
> index 7851dd4..6654684 100644
> --- a/sysdeps/aarch64/configure.ac
> +++ b/sysdeps/aarch64/configure.ac
> @@ -20,3 +20,19 @@ if test $libc_cv_aarch64_be = yes; then
>  else
>    LIBC_CONFIG_VAR([default-abi], [lp64])
>  fi
> +
> +# Check to see if the assembler has fixed a parsing bug wrt vector "mov"
> +# instructions leading to undefined symbols.
> +AC_MSG_CHECKING([for assembler vector mov bug])
> +old_LDFLAGS="$LDFLAGS"
> +LDFLAGS="$LDFLAGS -nostdlib -nostartfiles"
> +AC_LINK_IFELSE([[void _start(void) { asm("mov x0,v0.2d[0]"); }]],
> +  [AC_MSG_RESULT([no])],
> +  [AC_MSG_RESULT([yes])
> +   AC_MSG_ERROR(
> +[*** Version of binutils is too old to support AArch64.
> +*** Your version of binutils contains a bug in the handling
> +*** of vector names. Please upgrade to a version of binutils
> +*** that contains commit:
> +*** 8db49cc2de52033990ae5d4d6aacedc8f986e803])])
> +LDFLAGS="$old_LDFLAGS"
>