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Date: Fri, 6 Oct 2017 18:41:27 -0700
From: Andre McCurdy <>
Subject: Re: How to handle attempts to combine ARM Thumb with frame pointers?

On Fri, Oct 6, 2017 at 5:53 PM, Andre McCurdy <> 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?

Sorry to reply to my own thread, but it looks like this long standing
clang bug may actually be due to this same issue:

The only real difference being that where gcc aborts with an error
about reusing r7, clang silently generates broken code :-(

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