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Date: Fri, 6 Oct 2017 17:53:38 -0700
From: Andre McCurdy <>
Subject: How to handle attempts to combine ARM Thumb with frame pointers?

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?

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