[Bug tree-optimization/104595] New: unvectorized loop due to bool condition loaded from memory

[linkw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104595

            Bug ID: 104595
           Summary: unvectorized loop due to bool condition loaded from
                    memory
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: linkw at gcc dot gnu.org
  Target Milestone: ---

For the case:

#include "stdbool.h"
#define N 256
typedef char T;
extern T a[N];
extern T b[N];
extern T c[N];
extern bool pb[N];
extern char pc[N];

void predicate_by_bool() {
  for (int i = 0; i < N; i++)
    c[i] = pb[i] ? a[i] : b[i];
}

void predicate_by_char() {
  for (int i = 0; i < N; i++)
    c[i] = pc[i] ? a[i] : b[i];
}

Simply compiled with -Ofast -mcpu=power10, vectorizer can vectorize the 2nd
function predicate_by_char but can't vectorize the first. It seems currently
GCC just supports very limited case with bool types such as some patterns in
vect_recog_bool_pattern.

I guess here the size of bool seems to be a problem, for the size of bool, C
says "An object declared as type _Bool is large enough to store the values 0
and 1.", C++ says "The value of sizeof(bool) is implementation defined and
might differ from 1.". But the "implementation defined" looks to be compiler
defined? then compiler should be aware of it when compiling. If so, we can use
the equivalent size type for the load instead and make it compare with zero to
get the predicate just like the char variant, I think the expectation to see
both these loops vectorized is reasonable then?