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

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Mar 25, 2022 at 10:27 PM David Malcolm via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> 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.
>
> Successfully bootstrapped & regrtested on x86_64-pc-linux-gnu.
>
> OK for stage 4?  or for next stage 1?

OK for stage4.

Thanks,
Richard.

> 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>
> ---
>  gcc/gimple-fold.cc                       | 1 +
>  gcc/testsuite/gcc.dg/analyzer/pr104308.c | 8 ++++++++
>  gcc/testsuite/gcc.dg/uninit-40.c         | 2 +-
>  3 files changed, 10 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.dg/analyzer/pr104308.c
>
> diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
> index 5eff7d68ac1..e73bc6a7137 100644
> --- a/gcc/gimple-fold.cc
> +++ b/gcc/gimple-fold.cc
> @@ -1039,6 +1039,7 @@ gimple_fold_builtin_memory_op (gimple_stmt_iterator *gsi,
>                                                           new_stmt);
>                           gimple_assign_set_lhs (new_stmt, srcmem);
>                           gimple_set_vuse (new_stmt, gimple_vuse (stmt));
> +                         gimple_set_location (new_stmt, loc);
>                           gsi_insert_before (gsi, new_stmt, GSI_SAME_STMT);
>                         }
>                       if (dest_align < GET_MODE_ALIGNMENT (mode))
> diff --git a/gcc/testsuite/gcc.dg/analyzer/pr104308.c b/gcc/testsuite/gcc.dg/analyzer/pr104308.c
> new file mode 100644
> index 00000000000..9cd5ee6feee
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/analyzer/pr104308.c
> @@ -0,0 +1,8 @@
> +#include <string.h>
> +
> +int main()
> +{
> +  char s[5]; /* { dg-message "region created on stack here" } */
> +  memmove(s, s + 1, 2); /* { dg-warning "use of uninitialized value" } */
> +  return 0;
> +}
> diff --git a/gcc/testsuite/gcc.dg/uninit-40.c b/gcc/testsuite/gcc.dg/uninit-40.c
> index 8708079d397..567707a885e 100644
> --- a/gcc/testsuite/gcc.dg/uninit-40.c
> +++ b/gcc/testsuite/gcc.dg/uninit-40.c
> @@ -44,7 +44,7 @@ foo (int *q)
>    /* memcpy folding is too target dependent to test it everywhere.  */
>    V u[2], v[2];
>    u[0][0][0] = 1;
> -  __builtin_memcpy (&v[1], &u[1], sizeof (V));         /* { dg-warning "'\\*\\(\(long \)?long unsigned int \\*\\)\\(&u\\\[1\\\]\\\[0\\\]\\\[0\\\]\\)' is used uninitialized" "" { target i?86-*-* x86_64-*-* } } */
> +  __builtin_memcpy (&v[1], &u[1], sizeof (V));         /* { dg-warning "'u' is used uninitialized" "" { target i?86-*-* x86_64-*-* } } */
>    baz (&v[1]);
>  #endif
>  }
> --
> 2.26.3
>