[Bug lto/78768] New: -Walloca-larger-than and -Wformat-length warnings disabled by -flto

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78768

            Bug ID: 78768
           Summary: -Walloca-larger-than and -Wformat-length warnings
                    disabled by -flto
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The -Walloca-larger-than and -Wformat-length options don't work with -flto. 
When compiling without -flto, GCC issues the two warnings for the following
program as expected.  When -flto is used, no warnings are issued.

$ (set -x && cat a.c && for o in -fno-lto -flto; do /build/gcc-svn/gcc/xgcc -B
/build/gcc-svn/gcc -O2 -Walloca-larger-than=100 -Wformat-length $o a.c; done)
+ cat a.c
char d[1];

int main (int argc, char *argv[])
{
  void *p = __builtin_alloca (1234);

  __builtin_sprintf (d, "%lx", (long)p);
}
+ for o in -fno-lto -flto
+ /build/gcc-svn/gcc/xgcc -B /build/gcc-svn/gcc -O2 -Walloca-larger-than=100
-Wformat-length -fno-lto a.c
a.c: In function ‘main’:
a.c:5:9: warning: argument to ‘alloca’ is too large [-Walloca-larger-than=]
   void *p = __builtin_alloca (1234);
         ^
a.c:5:9: note: limit is 100 bytes, but argument is 1234
a.c:7:29: warning: writing a terminating nul past the end of the destination
[-Wformat-length=]
   __builtin_sprintf (d, "%lx", (long)p);
                          ~~~^
a.c:7:3: note: format output between 2 and 17 bytes into a destination of size
1
   __builtin_sprintf (d, "%lx", (long)p);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ for o in -fno-lto -flto
+ /build/gcc-svn/gcc/xgcc -B /build/gcc-svn/gcc -O2 -Walloca-larger-than=100
-Wformat-length -flto a.c