[Bug middle-end/95673] missing -Wstring-compare for an impossible strncmp test
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95673 Martin Sebor changed: What|Removed |Added Status|ASSIGNED|RESOLVED Resolution|--- |FIXED --- Comment #9 from Martin Sebor --- Implemented for GCC 11.
[Bug middle-end/95673] missing -Wstring-compare for an impossible strncmp test
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95673 --- Comment #8 from CVS Commits --- The master branch has been updated by Martin Sebor : https://gcc.gnu.org/g:b1ecb86569f63f897f6a95049c4ccf400bddeaad commit r11-5072-gb1ecb86569f63f897f6a95049c4ccf400bddeaad Author: Martin Sebor Date: Mon Nov 16 19:47:39 2020 -0700 PR middle-end/95673 - missing -Wstring-compare for an impossible strncmp test gcc/ChangeLog: PR middle-end/95673 * tree-ssa-strlen.c (used_only_for_zero_equality): Rename... (use_in_zero_equality): ...to this. Add a default argument. (handle_builtin_memcmp): Adjust to the name change above. (handle_builtin_string_cmp): Same. (maybe_warn_pointless_strcmp): Same. Pass in an explicit argument. gcc/testsuite/ChangeLog: PR middle-end/95673 * gcc.dg/Wstring-compare-3.c: New test.
[Bug middle-end/95673] missing -Wstring-compare for an impossible strncmp test
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95673 Martin Sebor changed: What|Removed |Added Target Milestone|--- |11.0 Keywords||patch --- Comment #7 from Martin Sebor --- Patch to enhance the warning: https://gcc.gnu.org/pipermail/gcc-patches/2020-October/555225.html
[Bug middle-end/95673] missing -Wstring-compare for an impossible strncmp test
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95673
Martin Sebor changed:
What|Removed |Added
Blocks||83819
--- Comment #6 from Martin Sebor ---
There warning doesn't trigger for the first strcmp call (i.e., strcmp(file,
"file123.txt")) because it's folded into a conditional expression involving
just the lengths of the strings before it reaches the stage when it would
otherwise be issued.
Here's a small test case showing how it happens:
int f (int c)
{
const char *s = c ? "12345" : "12";
return 0 == __builtin_strcmp (s, "123");
}
and the output pf the PRE pass (-fdump-tree-pre-details) shows:
...
Found partial redundancy for expression
{call_expr<__builtin_strcmp>,iftmp.0_3,addr_expr<"123">}@.MEM_5(D) (0001)
Created phi prephitmp_7 = PHI <-1(3), 1(5)>
in block 4 (0001)
Starting insert iteration 2
Replaced 0 with pretmp_11 in all uses of pretmp_9 = 0;
Replaced 0 with pretmp_10 in all uses of pretmp_8 = 0;
Replaced __builtin_strcmp (iftmp.0_3, "123") with prephitmp_7 in all uses of _1
= __builtin_strcmp (iftmp.0_3, "123");
...
Removing dead stmt _1 = __builtin_strcmp (iftmp.0_3, "123");
...
The PRE pass runs much earlier than the strlen pass that issues the warning so
I don't think there's going to be anything to emit the warning in this case
(involving all string literals).
The PRE optimization isn't smart enough to handle the case when at least one of
the arguments is an ordinary array and so then warning does trigger:
extern char a[3];
int f (int c)
{
const char *s = c ? "12345" : a;
return 0 == __builtin_strcmp (s, "123");
}
The call is then folded away by the strlen pass (by the same code that issues
the warning).
In general, the warning code doesn't necessarily know the contents of the
arrays it's based on, only their lengths and/or sizes, so it can't very well
print them. (Printing strings is also tricky because of constraints on the
volume of output, or the question how to deal with non-printable characters.)
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83819
[Bug 83819] [meta-bug] missing strlen optimizations
[Bug middle-end/95673] missing -Wstring-compare for an impossible strncmp test
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95673
Franz Sirl changed:
What|Removed |Added
CC||sirl at gcc dot gnu.org
--- Comment #5 from Franz Sirl ---
Wstring-compare seems to depend a lot on the optimizations. In the following
testcase warnings are also inconsistent and fragile (slight changes make the
warning disappear):
int c, d, e;
void f(void) {
const char *file = (c & 7) ? "file1234.txt" : "x";
const char *file2 = (c & 7) ? "y" : "file12.txt";
if (c == 2)
if (d
|| __builtin_strcmp(file, "file123.txt")
|| __builtin_strcmp(file2, "file123.txt")
)
e = 2;
}
With "gcc -c -O2 -Wstring-compare testcase.c" current trunk shows:
testcase.c: In function 'f':
testcase.c:8:12: warning: '__builtin_strcmp' of a string of length 11 and an
array of size 11 evaluates to nonzero [-Wstring-compare]
8 | || __builtin_strcmp(file2, "file123.txt")
|^~
There is no warning for line 7 ('file').
Maybe the warning could also be a bit more verbose when the information is
available, like for example:
warning: '__builtin_strcmp' of a string ('file2' with content "y" ||
"file12.txt") and an array ("file123.txt") of size 11 always evaluates to
nonzero [-Wstring-compare]
[Bug middle-end/95673] missing -Wstring-compare for an impossible strncmp test
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95673 Martin Sebor changed: What|Removed |Added Status|NEW |ASSIGNED Assignee|unassigned at gcc dot gnu.org |msebor at gcc dot gnu.org
[Bug middle-end/95673] missing -Wstring-compare for an impossible strncmp test
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95673 Martin Sebor changed: What|Removed |Added Keywords||diagnostic Component|regression |middle-end Resolution|INVALID |--- Ever confirmed|0 |1 Last reconfirmed||2020-06-19 Summary|Inconsistent optimization |missing -Wstring-compare |behavior when there is a|for an impossible strncmp |buffer overflow |test Status|RESOLVED|NEW --- Comment #4 from Martin Sebor --- Let me repurpose this bug to track the missing warning then.
