-Warray-bounds interprets int *var as int var[0] ?

[Andrew Pinski]

On Wed, Nov 23, 2022 at 9:15 AM Georg-Johann Lay <avr@gjlay.de> wrote:
>
> The following code throws a warning which I do not understand.
>
> Purpose is to save and restore SREG, which is a special function
> register (SFR) defined by its hardware address as:
>
> #define SREG (*(volatile uint8_t*) (0x3F + __AVR_SFR_OFFSET__))
>
> which is the common C idiom to define such an SFR. The C code is:
>
> <code>
> typedef __UINT8_TYPE__ uint8_t;
>
> #define SREG (*(volatile uint8_t*) (0x3F + __AVR_SFR_OFFSET__))
>
> static __inline__ uint8_t __iCliRetVal (void)
> {
>      __asm__ __volatile__ ("cli" ::: "memory");
>      return 1;
> }
>
> static __inline__ void __iRestore (const uint8_t *__s)
> {
>      SREG = *__s;
>      __asm__ volatile ("" ::: "memory");
> }
>
> void foo (void)
> {
>
>     for (uint8_t sreg_save __attribute__((__cleanup__(__iRestore))) = SREG,
>              __ToDo = __iCliRetVal();
>           __ToDo ;
>           __ToDo = 0 )
>     {
>         __asm ("nop");
>     }
> }
> </code>
>
> The documentation of attribute cleanup says that the function provided
> to cleanup (__iRestore) must take a pointer type that is compatible with
> the attributed variable, which is the case.  The warning is:
>
>   avr-gcc-13 -c foo-i.c -mmcu=atmega8 -Os -Wall -save-temps -dumpbase ""
> foo-i.c: In function 'foo':
> foo-i.c:20:71: warning: array subscript 0 is outside array bounds of
> 'volatile uint8_t[0]' {aka 'volatile unsigned char[]'} [-Warray-bounds]
>     20 |    for (uint8_t sreg_save
> __attribute__((__cleanup__(__iRestore))) = SREG,
>        |
>       ~^~~~
> In function '__iRestore',
>      inlined from 'foo' at foo-i.c:20:17:
> foo-i.c:13:42: warning: array subscript 0 is outside array bounds of
> 'volatile uint8_t[0]' {aka 'volatile unsigned char[]'} [-Warray-bounds]
>     13 |     SREG = *__s;
>        |     ~~~~~~~~~~~~                         ^
>
> To me this looks like a GCC problem, and older versions of the compiler
> don't complain.  Or is there actually an issue with that code? Purpose
> of the code is to save / restore SREG around a block of code, "nop" in
> the example.
>
> The warning complains about the places that are using SREG, so it that
> macro definition wrong?

Either you need to use --param=min-pagesize=0 as an option or you need
to modify the avr backend to set that by default.

Thanks,
Andrew Pinski

>
> Thanks in advance,
>
> Johann
>
>
>  > avr-gcc-13 -v
> Using built-in specs.
> Reading specs from
> /home/DATA/gnu/install/gcc-master-avr/bin/../lib/gcc/avr/13.0.0/device-specs/specs-avr2
> COLLECT_GCC=avr-gcc-13
> COLLECT_LTO_WRAPPER=/home/DATA/gnu/install/gcc-master-avr/bin/../libexec/gcc/avr/13.0.0/lto-wrapper
> Target: avr
> Configured with: ../../source/gcc-master/configure --target=avr
> --disable-nls --with-dwarf2 --enable-languages=c,c++ --with-gnu-as
> --with-gnu-ld --disable-shared --with-fixed-point=no
> --prefix=/home/john/gnu/install/gcc-master-avr --enable-checking=release
> Thread model: single
> Supported LTO compression algorithms: zlib
> gcc version 13.0.0 20221103 (experimental) (GCC)
>