[Bug target/94986] missing diagnostic on ARM thumb2 compilation with -pg when using r7 in inline asm

["wilco at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94986

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

           What    |Removed                     |Added
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                 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 
>  it was suggested that the clang behavior is probably correct in this corner
> case while gcc gets it wrong, see https://bugs.llvm.org/show_bug.cgi?id=45826
> 
> echo 'void f(void) { asm("mov r7, #0" ::: "r7"); }' | arm-linux-gnueabi-gcc
> -march=armv7-a -O2  -mthumb -pg -S -xc -
> 
> 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:
> 
> <stdin>: In function 'f':
> <stdin>:1:44: error: r7 cannot be used in 'asm' here
> 
> 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 seems
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, similar
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 pointer
will cause a crash:

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