[Bug c/57627] New: -Wsizeof-pointer-memaccess should make an exception for character types

[Bug c/57627] New: -Wsizeof-pointer-memaccess should make an exception for character types

[harald at gigawatt dot nl]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57627

            Bug ID: 57627
           Summary: -Wsizeof-pointer-memaccess should make an exception
                    for character types
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: harald at gigawatt dot nl

$ cat test.c
void f1(int *dst, int *src)
{ __builtin_memcpy(dst, src, sizeof(int *)); }

void f2(char *dst, char *src)
{ __builtin_memcpy(dst, src, sizeof(char *)); }

void f3(unsigned char *dst, unsigned char *src)
{ __builtin_memcpy(dst, src, sizeof(unsigned char *)); }
$ gcc -c test.c -Wall
test.c: In function ‘f1’:
test.c:2:36: warning: argument to ‘sizeof’ in ‘__builtin_memcpy’ call is the
same pointer type ‘int *’ as the destination; expected ‘int’ or an explicit
length [-Wsizeof-pointer-memaccess]
 { __builtin_memcpy(dst, src, sizeof(int *)); }
                                    ^
test.c: In function ‘f2’:
test.c:5:36: warning: argument to ‘sizeof’ in ‘__builtin_memcpy’ call is the
same pointer type ‘char *’ as the destination; expected ‘char’ or an explicit
length [-Wsizeof-pointer-memaccess]
 { __builtin_memcpy(dst, src, sizeof(char *)); }
                                    ^
test.c: In function ‘f3’:
test.c:8:36: warning: argument to ‘sizeof’ in ‘__builtin_memcpy’ call is the
same pointer type ‘unsigned char *’ as the destination; expected ‘unsigned
char’ or an explicit length [-Wsizeof-pointer-memaccess]
 { __builtin_memcpy(dst, src, sizeof(unsigned char *)); }
                                    ^
$ gcc --version
gcc (GCC) 4.8.1 20130603 (Red Hat 4.8.1-1)
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The warning makes sense for most types (f1), but for character types, I don't
think GCC should give any warning (f2 and f3). I'm seeing this warning for
correct code that stores pointers in raw byte buffers, and I would be surprised
if that is less common than code that really means to memcpy a single byte.
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From: "amylaar at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/57425] [4.8/4.9 regression] RTL alias analysis unprepared to handle stack slot sharing
Date: Sat, 15 Jun 2013 19:25:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW425

--- Comment #7 from Jorn Wolfgang Rennecke <amylaar at gcc dot gnu.org> ---
Created attachment 30310
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id0310&actioníit
patch for alias problem during cse pass

(In reply to Richard Biener from comment #4)
> Note that I only recently fixed a wrong alias query from using
> true_dependence
> to anti_dependence.  Similar issues may exist elsewhere.

Indeed, looking at the .172r.cse1 dump more closely, I saw it
actually contained actually two dumps, first looking OK, the
second with the deleterious changes in place.

First, cse is applied to the asms.  I'm not sure if this should
be called a bug or a feature.
At any rate, cse does have an alias problem as you suspected.
The attached patch addresses this issue, but doesn't fix the testcase;
something goes again wrong between the reload and postreload dump.

[Bug c/57627] -Wsizeof-pointer-memaccess should make an exception for character types

[harald at gigawatt dot nl]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57627

--- Comment #1 from Harald van Dijk <harald at gigawatt dot nl> ---
I just realised the very similar example

void f4(char *dst, char *src)
{ __builtin_memcpy(dst, src, sizeof(src)); }

void f5(unsigned char *dst, unsigned char *src)
{ __builtin_memcpy(dst, src, sizeof(src)); }

also gets a warning, but in that case, I fully agree with the warning GCC
currently gives. So, to clarify, my request/suggestion is *only* to suppress
the warning for f2 and f3 in my report, and leave the behaviour for these f4
and f5 as it is.