On January 2, 2017 8:36:34 PM GMT+01:00, Jakub Jelinek <ja...@redhat.com> wrote:
>Hi!
>
>Even when we know the exact return value of a snprintf call with 0
>length
>(i.e. one that doesn't write anything into a buffer), there can be %n
>directives in the format string that have side-effects that shouldn't
>be
>discarded.
>
>The following patch avoids the replacement of the snprintf call
>with just the constant if there is any %n directive.
>
>Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
Richard.
>2017-01-02 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/78965
> * gimple-ssa-sprintf.c (pass_sprintf_length::compute_format_length):
> Change first argument from const call_info & to call_info &. For %n
> set info.nowrite to false.
>
> * gcc.dg/pr78965.c: New test.
>
>--- gcc/gimple-ssa-sprintf.c.jj 2017-01-01 12:45:35.000000000 +0100
>+++ gcc/gimple-ssa-sprintf.c 2017-01-02 12:09:29.902945255 +0100
>@@ -131,7 +131,7 @@ public:
> void handle_gimple_call (gimple_stmt_iterator*);
>
> struct call_info;
>- bool compute_format_length (const call_info &, format_result *);
>+ bool compute_format_length (call_info &, format_result *);
> };
>
> bool
>@@ -2357,7 +2357,7 @@ add_bytes (const pass_sprintf_length::ca
> that caused the processing to be terminated early). */
>
> bool
>-pass_sprintf_length::compute_format_length (const call_info &info,
>+pass_sprintf_length::compute_format_length (call_info &info,
> format_result *res)
> {
> /* The variadic argument counter. */
>@@ -2624,6 +2624,9 @@ pass_sprintf_length::compute_format_leng
> return false;
>
> case 'n':
>+ /* %n has side-effects even when nothing is actually printed to
>+ any buffer. */
>+ info.nowrite = false;
> break;
>
> case 'c':
>--- gcc/testsuite/gcc.dg/pr78965.c.jj 2017-01-02 12:12:15.959803429
>+0100
>+++ gcc/testsuite/gcc.dg/pr78965.c 2017-01-02 12:11:06.000000000 +0100
>@@ -0,0 +1,14 @@
>+/* PR tree-optimization/78965 */
>+/* { dg-do run { target c99_runtime } } */
>+/* { dg-options "-O2" } */
>+/* { dg-add-options c99_runtime } */
>+
>+int
>+main ()
>+{
>+ int a = 5, b = 6;
>+ int c = __builtin_snprintf (0, 0, "a%nb%nc", &a, &b);
>+ if (a + b + c != 6)
>+ __builtin_abort ();
>+ return 0;
>+}
>
> Jakub
