Date: Sat, 7 Oct 2017 23:21:53 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: How to handle attempts to combine ARM Thumb with frame pointers? On Fri, Oct 06, 2017 at 05:53:38PM -0700, 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? At the core, I think this is a bug in GCC and clang, in the sense that they shouldn't be enforcing fixed registers in a way that conflicts with asm constraints. IIRC this was fixed on x86 for ebx and ebp a while back. But indeed if it's the state of things, that's how it is. If you do want to test for broken configurations, rather than hard-coding an assumption that some configuration is broken, you should test for it. This would look something like, if ARCH is arm, try compiling a trivial function with inline asm using r7 and see if it fails. If so, exit with an error or perhaps try adding -fomit-frame-pointer and retrying. I haven't tried any of this yet so I don't know how ugly/hackish it would be and whether it would be appropriate to include but it sounds like it could be. If clang generates broken code silently, though, I don't know any good way to test for that. Rich
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