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Date: Fri, 6 Oct 2017 18:24:18 -0700
From: Khem Raj <raj.khem@...il.com>
To: musl@...ts.openwall.com
Subject: Re: How to handle attempts to combine ARM Thumb with frame pointers?

On Fri, Oct 6, 2017 at 5:53 PM, Andre McCurdy <armccurdy@...il.com> wrote:
> When compiling for ARM Thumb or Thumb2 with frame pointers enabled (ie
> -O0 or with -fno-omit-frame-pointer in CFLAGS) the frame pointer is
> stored in r7, which leads to build errors ("error: r7 cannot be used
> in asm here") whenever a syscall macro is included in a C function.
> It's certainly a corner case, but one which I've run into recently.
>
> Would it be worth trying to catch this combination earlier and failing
> from the configure script? It's not trivial to do reliably since I
> think detecting whether or not frame pointers are going to be used by
> examining CFLAGS means determining the effective optimisation level if
> multiple -O0, -Os, etc options are given, together with the effective
> outcome of potentially multiple -fno-omit-frame-pointer and
> -fomit-frame-pointer options.
>
> I can work on a patch for the configure script but first wanted to
> check what the philosophy is - should the configure script be trying
> to catch every possible misconfiguration?

I think these files assume that frame pointer is not stored, quick fix is to
compile the specific file with -fomit-frame-pointer something like
CFLAGS-<filenname> = -fomit-frame-pointer

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