[Bug c/71769] New: Invalid warning from -Wunsafe-loop-optimizations for a finite loop

[nightstrike at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71769

            Bug ID: 71769
           Summary: Invalid warning from -Wunsafe-loop-optimizations for a
                    finite loop
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nightstrike at gmail dot com
  Target Milestone: ---

Created attachment 38834
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38834&action=edit
Testcase

In the attached testcase, function f compiles without warning for -O[023], but
function g warns for -O[0123].  The warning is "warning: cannot optimize
possibly infinite loops [-Wunsafe-loop-optimizations]".  The functions are very
similar, and I don't think it should warn in either case, as the conditionals
verify that the loop isn't infinite.

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/opt/gcc6/bin/../libexec/gcc/x86_64-pc-linux-gnu/6.1.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-6.1.0/configure --prefix=/gcc6
--enable-libstdcxx-time=rt --disable-multilib
--enable-languages=all,ada,obj-c++ --enable-plugins --with-gnu-as --with-gnu-ld
Thread model: posix
gcc version 6.1.0 (GCC)

$ gcc -c -std=gnu99 a.c -Wunsafe-loop-optimizations -O3

a.c: In function ‘g’:
a.c:18:2: warning: cannot optimize possibly infinite loops
[-Wunsafe-loop-optimizations]
  for(int i = a; i <= b; ++i)
  ^~~

$ cat a.c
// Warning with -Wunsafe-loop-optimizations and -O1 only
void f(int * x, int a, int b) {
        if (b < a) b = a;
        if (a < 1) return;

        for (int i = a; i <= b; ++i)
                x[i] += 1;
}

struct S {
        unsigned int * data;
        unsigned int * d2;
};

// Warning with -Wunsafe-loop-optimizations and -O1 or above
void g(struct S * x, int a, int b) {
        if (b < a) b = a;
        if (a < 1) return;

        for(int i = a; i <= b; ++i)
                __builtin_memcpy(x->data, x->d2, 5);
}