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From: "wilco at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/94986] missing diagnostic on ARM thumb2 compilation with
 -pg when using r7 in inline asm
Date: Wed, 03 Jun 2020 16:37:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94986

Wilco <wilco at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |wilco at gcc dot gnu.org

--- Comment #1 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Arnd Bergmann from comment #0)
> I reported a bug against clang for a Linux kernel failure, but=20
>  it was suggested that the clang behavior is probably correct in this cor=
ner
> case while gcc gets it wrong, see https://bugs.llvm.org/show_bug.cgi?id=
=3D45826
>=20
> echo 'void f(void) { asm("mov r7, #0" ::: "r7"); }' | arm-linux-gnueabi-g=
cc
> -march=3Darmv7-a -O2  -mthumb -pg -S -xc -
>=20
> silently accepts an inline asm statement that clobbers the frame pointer,
> but gcc rejects the same code if any of '-O0', '-fomit-frame-pointer' or
> 'fno-omit-frame-pointer' are used:
>=20
> <stdin>: In function 'f':
> <stdin>:1:44: error: r7 cannot be used in 'asm' here
>=20
> If using r7 in this case is indeed invalid, we need to ensure the kernel
> does not do this, and having gcc reject it would be helpful.

GCC will reject it if you explicitly enable the frame pointer. The logic se=
ems
wrong in that it doesn't report an error if the frame pointer is implicitly
enabled via -pg. As a workaround for the kernel, just use -pg and
-fno-omit-frame-pointer together.

Corrupting a frame pointer loses the ability to follow the frame chain, sim=
ilar
to a function built with -fomit-frame-pointer which will use r7 as a general
purpose register.

However this always reports an error since this corruption of the frame poi=
nter
will cause a crash:

int *f(int x) { asm("mov r7, #0" ::: "r7"); return __builtin_alloca (x); }=