Warning when passing const T * to __builtin_prefetch() wrt/ dropped qualifiers, expected 'void *'

[llvmbot]

Bugzilla Link	3912
Resolution	FIXED
Resolved on	Feb 22, 2010 12:50
Version	unspecified
OS	MacOS X
Reporter	LLVM Bugzilla Contributor

Extended Description

version: checker-0.181

clang prints a warning message when a const qualified pointer is passed to __builtin_prefetch(). It would appear that clang thinks prefetch is defined along the lines of 'void __builtin_prefetch(void *)'.

expected behavior: clang shouldn't warn about this. Unless, of course, there's a compelling reason that clang doesn't want to treat the pointed to memory as const / read-only.

Small test demonstrating issue (distilled down from other sources):

#include <stdio.h>

#define PREFETCH_BUFFER(ptr, off) { const char *p = ((const char *)(ptr)) + ((off)) + 64U; __builtin_prefetch(p); }

int main(int argc, char *argv[]) {
char buffer[4096];

PREFETCH_BUFFER(&buffer[128], 32);

return(0);
}

output from clang:

clang_pf.c:8:3: warning: passing 'char const *' discards qualifiers, expected 'void *'
PREFETCH_BUFFER(&buffer[128], 32);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/johne/rkl/clang_pf.c:3:113: note: instantiated from:
#define PREFETCH_BUFFER(ptr, off) { const char *p = ((const char *)(ptr)) + ((off)) + 64U; __builtin_prefetch(p); }

From gcc-4.2/gcc/builtins.def, where GCC defines it as a const qualified pointer:

DEF_GCC_BUILTIN (BUILT_IN_PREFETCH, "prefetch", BT_FN_VOID_CONST_PTR_VAR, ATTR_NOVOPS_LIST)

Also: http://gcc.gnu.org/onlinedocs/gcc-4.2.4/gcc/Other-Builtins.html#Other-Builtins =
Built-in Function: void __builtin_prefetch (const void *addr, ...)

[lattner]

Looks like a typo, nice catch! Fixed here:
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20090413/015779.html