[llvm-bugs] [Bug 28583] New: Clang warnings for optimizations in GCC string.h

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28583

            Bug ID: 28583
           Summary: Clang warnings for optimizations in GCC string.h
           Product: clang
           Version: 3.8
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jibz-llvmbugs at stdip.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16751
  --> https://llvm.org/bugs/attachment.cgi?id=16751&action=edit
Output of clang -E -frewrite-includes -O2 -c foo.c

Compiling foo.c:

#include <string.h>
int main() { (void) strspn("foo", "f"); }

on Linux with `clang -O2 -c foo.c' results in:

[jibsen at localhost build]$ clang -O2 -c foo.c
foo.c:2:498: warning: array index 2 is past the end of the array (which
contains 2 elements) [-Warray-bounds]
int main() { (void) __extension__ ({ char __a0, __a1, __a2;
(__builtin_constant_p ("f") && ((size_t)(const void *)(("f") + 1) -
(size_t)(const void *)("f") == 1) ? ((__builtin_constant_p ("foo") &&
((size_t)(const void *)(("foo") + 1) - (size_t)(const void *)("foo") == 1)) ?
__builtin_strspn ("foo", "f") : ((__a0 = ((const char *) ("f"))[0], __a0 ==
'\0') ? ((void) ("foo"), (size_t) 0) : ((__a1 = ((const char *) ("f"))[1], __a1
== '\0') ? __strspn_c1 ("foo", __a0) : ((__a2 = ((const char *) ("f"))[2], __a2
== '\0') ? __strspn_c2 ("foo", __a0, __a1) : (((const char *) ("f"))[3] == '\0'
? __strspn_c3 ("foo", __a0, __a1, __a2) : __builtin_strspn ("foo", "f")))))) :
__builtin_strspn ("foo", "f")); }); }
                                                                               
                                                                               
                                                                               
                                                                               
                                                                               
                                                                               
                 ^     ~
foo.c:2:576: warning: array index 3 is past the end of the array (which
contains 2 elements) [-Warray-bounds]
int main() { (void) __extension__ ({ char __a0, __a1, __a2;
(__builtin_constant_p ("f") && ((size_t)(const void *)(("f") + 1) -
(size_t)(const void *)("f") == 1) ? ((__builtin_constant_p ("foo") &&
((size_t)(const void *)(("foo") + 1) - (size_t)(const void *)("foo") == 1)) ?
__builtin_strspn ("foo", "f") : ((__a0 = ((const char *) ("f"))[0], __a0 ==
'\0') ? ((void) ("foo"), (size_t) 0) : ((__a1 = ((const char *) ("f"))[1], __a1
== '\0') ? __strspn_c1 ("foo", __a0) : ((__a2 = ((const char *) ("f"))[2], __a2
== '\0') ? __strspn_c2 ("foo", __a0, __a1) : (((const char *) ("f"))[3] == '\0'
? __strspn_c3 ("foo", __a0, __a1, __a2) : __builtin_strspn ("foo", "f")))))) :
__builtin_strspn ("foo", "f")); }); }
                                                                               
                                                                               
                                                                               
                                                                               
                                                                               
                                                                               
                                                                               
               ^     ~
2 warnings generated.

I haven'd dug through the macros, but I assume Clang is not supposed to
generate warnings for standard functions, at least without any warning flags.

Attached is output of clang -E -frewrite-includes.

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