gimple-fold: fix location of loads for memory ops [PR104308]

gimple-fold: fix location of loads for memory ops [PR104308]

PR analyzer/104308 reports that when -Wanalyzer-use-of-uninitialized-value
complains about certain memmove operations where the source is
uninitialized, the diagnostic uses UNKNOWN_LOCATION:

In function 'main':
cc1: warning: use of uninitialized value '*(short unsigned int *)&s + 1' [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
  'main': event 1
    |
    |pr104308.c:5:8:
    |    5 |   char s[5]; /* { dg-message "region created on stack here" } */
    |      |        ^
    |      |        |
    |      |        (1) region created on stack here
    |
  'main': event 2
    |
    |cc1:
    | (2): use of uninitialized value '*(short unsigned int *)&s + 1' here
    |

The issue is that gimple_fold_builtin_memory_op converts a memmove to:

  _3 = MEM <unsigned short> [(char * {ref-all})_1];
  MEM <unsigned short> [(char * {ref-all})&s] = _3;

but only sets the location of the 2nd stmt, not the 1st.

Fixed thusly, giving:

pr104308.c: In function 'main':
pr104308.c:6:3: warning: use of uninitialized value '*(short unsigned int *)&s + 1' [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
    6 |   memmove(s, s + 1, 2); /* { dg-warning "use of uninitialized value" } */
      |   ^~~~~~~~~~~~~~~~~~~~
  'main': events 1-2
    |
    |    5 |   char s[5]; /* { dg-message "region created on stack here" } */
    |      |        ^
    |      |        |
    |      |        (1) region created on stack here
    |    6 |   memmove(s, s + 1, 2); /* { dg-warning "use of uninitialized value" } */
    |      |   ~~~~~~~~~~~~~~~~~~~~
    |      |   |
    |      |   (2) use of uninitialized value '*(short unsigned int *)&s + 1' here
    |

One side-effect of this change is a change in part of the output of
gcc.dg/uninit-40.c from:

  uninit-40.c:47:3: warning: ‘*(long unsigned int *)(&u[1][0][0])’ is used uninitialized [-Wuninitialized]
     47 |   __builtin_memcpy (&v[1], &u[1], sizeof (V));
        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  uninit-40.c:45:5: note: ‘*(long unsigned int *)(&u[1][0][0])’ was declared here
     45 |   V u[2], v[2];
        |     ^

to:

  uninit-40.c:47:3: warning: ‘u’ is used uninitialized [-Wuninitialized]
     47 |   __builtin_memcpy (&v[1], &u[1], sizeof (V));
        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  uninit-40.c:45:5: note: ‘u’ declared here
     45 |   V u[2], v[2];
        |     ^

What's happening is that pass "early_uninit"(29)'s call to
maybe_warn_operand is guarded by this condition:
  1051            else if (gimple_assign_load_p (stmt)
  1052                     && gimple_has_location (stmt))

Before the patch, the stmt:
  _3 = MEM <unsigned long> [(char * {ref-all})&u + 8B];
has no location, and so early_uninit skips this operand at line
1052 above.  Later, pass "uninit"(217) tests the var_decl "u$8", and
emits a warning for it.

With the patch, the stmt has a location, and so early_uninit emits a
warning for "u" and sets a NW_UNINIT warning suppression at that
location.  Later, pass "uninit"(217)'s test of "u$8" is rejected
due to that per-location suppression of uninit warnings, from the
earlier warning.

gcc/ChangeLog:
	PR analyzer/104308
	* gimple-fold.cc (gimple_fold_builtin_memory_op): When optimizing
	to loads then stores, set the location of the new load stmt.

gcc/testsuite/ChangeLog:
	PR analyzer/104308
	* gcc.dg/analyzer/pr104308.c: New test.
	* gcc.dg/uninit-40.c (foo): Update expression in expected message.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>