On Tue, Aug 15, 2017 at 05:27:37PM +0100, Wilco Dijkstra wrote:
>
> ping
This seems like a bit of a theoretical issue as we would normally build
libgcc with -fno-omit-frame-pointer anyway, but it can't hurt to guarantee
this, so OK.
Thanks,
James
> From: Wilco Dijkstra
> Sent: 31 July 2017 16:57
> To: GCC Patches; James Greenhalgh
> Cc: nd
> Subject: [PATCH][AArch64] PR71951: Fix unwinding with -fomit-frame-pointer
>
> As described in PR71951, if libgcc is built with -fomit-frame-pointer,
> unwinding crashes, for example while doing a backtrace. The underlying
> reason is the Dwarf unwinder does not setup the frame pointer register
> in the initialization code. When later unwinding a function that uses
> the frame pointer, it tries to read FP using _Unwind_GetGR, and this
> crashes if has never restored FP. To unwind correctly the first frame
> must save and restore FP (it is unwound in a special way so that it
> uses SP instead of FP). This is done by adding -fno-omit-frame-pointer.
>
> OK for commit and backport to GCC6/7?
>
> ChangeLog:
> 2017-07-31 Wilco Dijkstra <wdijk...@arm.com>
>
> PR target/71951
> * config/aarch64/aarch64.h (LIBGCC2_UNWIND_ATTRIBUTE): Define.
>
> --
> diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
> index
> 7f91edb5713d7e8eda2f0a024a0f97b4e111c4b0..03fd93046bdbdb03bd7d0c4573928f504640f7e1
> 100644
> --- a/gcc/config/aarch64/aarch64.h
> +++ b/gcc/config/aarch64/aarch64.h
> @@ -971,4 +971,12 @@ extern const char *host_detect_local_cpu (int argc,
> const char **argv);
> extern tree aarch64_fp16_type_node;
> extern tree aarch64_fp16_ptr_type_node;
>
> +/* The generic unwind code in libgcc does not initialize the frame pointer.
> + So in order to unwind a function using a frame pointer, the very first
> + function that is unwound must save the frame pointer. That way the frame
> + pointer is restored and its value is now valid - otherwise _Unwind_GetGR
> + crashes. Libgcc can now be safely built with -fomit-frame-pointer. */
> +#define LIBGCC2_UNWIND_ATTRIBUTE \
> + __attribute__((optimize ("no-omit-frame-pointer")))
> +
> #endif /* GCC_AARCH64_H */
>
