Re: Crash when cross compiling for ARM with GCC-8-2-0 and -ftree-loop-distribute-patterns

["Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>]

On 18/10/2019 13:41, Josef Wolf wrote:
> On Fri, Oct 18, 2019 at 11:25:53AM +0100, Richard Earnshaw (lists) wrote:
>>
>> void *
>> __attribute__ ((__optimize__ ("-fno-tree-loop-distribute-patterns")))
>> memset (void *s, int c, size_t n)
>> {
>>    int i;
>>    for (i=0; i<n; i++)
>>      ((char *)s)[i] = c;
>>
>>    return s;
>> }
> 
> Wouldn't
> 
>     void *memset (void *s, int c, size_t n)
>     {
>             return __builtin_memset (s, c, n);
>     }
> 
> be a cleaner solution to this?
> 
> Unfortunately, this compiles to a jump to itself. No matter whether I use the
> -fno-builtin-memset flag or not.
> 

On most targets __builtin_memset will only compile to in-lined code if 
the size is known (and sufficiently small), it's intended for cases 
where you probably don't want a loop, but do want to make use of known 
size and alignment.  It's not expected to be an all-singing all-dancing 
memset for this specific CPU.

Writing a good memset can be hard (writing memcpy is even harder) and 
compilers rarely do as well as the best assembly code when trying to 
handle all the important cases; but they can do better in the limited 
conditions where the size and alignment are statically known since many 
hard-to-predict branches can be entirely eliminated.

So in most cases, you *want* the compiler to call memset if the 
operation cannot really be optimized.

R.