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

[Georg-Johann Lay]

Am 23.11.22 um 18:18 schrieb Andrew Pinski:
> On Wed, Nov 23, 2022 at 9:15 AM Georg-Johann Lay <arv@jglay.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

Ok, thanks. I filed it as PR107842.

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)