On Wed, Jun 12, 2019 at 5:22 AM Li Jia He <heli...@linux.ibm.com> wrote:
>
> Hi,
>
> I recently did some analysis on the automatic vectorization of gcc, I
> found that singed char can not be vectorized in the following code.
>
> ---
> #define ITERATIONS 1000000
>
> #if defined(do_reduce_signed_char)
> #define TYPE signed char
> #elif defined(do_reduce_unsigned_char)
> #define TYPE unsigned char
> #else
> #error bad define
> #endif
>
> #define SIZE (16384/sizeof(TYPE))
>
> static TYPE x[SIZE] __attribute__ ((aligned (16)));
>
> void obfuscate(void *a, ...);
>
> static void __attribute__((noinline)) do_one(void)
> {
> unsigned long i;
> TYPE a = 0;
>
> obfuscate(x);
>
> for (i = 0; i < SIZE; i++)
> a += x[i];
>
> obfuscate(x, a);
> }
>
> int main(void)
> {
> unsigned long i;
>
> for (i = 0; i < ITERATIONS; i++)
> do_one();
>
> return 0;
> }
> ---
> If we use the following command line
>
> gcc reduce.c -Ddo_reduce_unsigned_char -Ofast -c -S -fdump-tree-vect-details
>
> We can see that this code can be vectorized under the unsigned char data
> type.
> If we use the following command
>
> gcc reduce.c -Ddo_reduce_signed_char -Ofast -c -S -fdump-tree-vect-details
>
> We can see that this code cannot be vectorized under the singed char
> data type.
> I found in the below code for singed char
> ---
> a += x[i];
> ---
> Will do something like the following conversion.
> ---
> a = (signed char) ((unsigned char) x[i] + (unsigned char) a);
> ---
> As a result, the reduction in the code cannot be effectively identified.
> Can we vectorize the code like the above when the data type is signed char ?
Probably another case of https://gcc.gnu.org/PR65930
Richard.
> Thanks,
> Lijia He
>
