[Bug middle-end/95556] Not replacing __builtin___memcpy_chk() as documented

[pg...@j-davis.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95556

--- Comment #9 from Jeff Davis  ---
I still feel like the documentation is misleading on this point.

Regardless, it doesn't seem like you think there is any bug here, so go ahead
and close.

[Bug middle-end/95556] Not replacing __builtin___memcpy_chk() as documented

[pg...@j-davis.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95556

--- Comment #7 from Jeff Davis  ---
"...built-in functions are optimized into the normal string functions like
memcpy if the last argument is (size_t) -1..."

My reading of the document lead me to believe that a last argument of -1
*would* be a normal library call. And certainly should be with
-fno-builtin-memcpy, right?

If that's not what's happening, should the document be clarified?

[Bug c/95556] Not replacing __builtin___memcpy_chk() as documented

[pg...@j-davis.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95556

--- Comment #3 from Jeff Davis  ---
Original larger case was discovered in PostgreSQL:

https://www.postgresql.org/message-id/99b2eab335c1592c925d8143979c8e9e81e1575f.ca...@j-davis.com

[Bug middle-end/95556] Not replacing __builtin___memcpy_chk() as documented

[pg...@j-davis.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95556

--- Comment #2 from Jeff Davis  ---
Created attachment 48688
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48688&action=edit
Example 3

Another example that works (i.e. builtin is properly replaced by memcpy as
described in the document).

The only difference between this working example and the failing example2.c is
that I replaced the sizeof() with a constant.

[Bug c/95556] Not replacing __builtin___memcpy_chk() as documented

[pg...@j-davis.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95556

--- Comment #1 from Jeff Davis  ---
Created attachment 48687
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48687&action=edit
Example 1

[Bug c/95556] New: Not replacing __builtin___memcpy_chk() as documented

[pg...@j-davis.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95556

Bug ID: 95556
   Summary: Not replacing __builtin___memcpy_chk() as documented
   Product: gcc
   Version: 7.5.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
  Assignee: unassigned at gcc dot gnu.org
  Reporter: pg...@j-davis.com
  Target Milestone: ---

Created attachment 48686
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48686&action=edit
Example 2

GCC's Object Size Checking doc says:

  "There are built-in functions added for many common
   string operation functions, e.g., for memcpy 
   __builtin___memcpy_chk built-in is provided. This
   built-in has an additional last argument, which is
   the number of bytes remaining in the object the dest
   argument points to or (size_t) -1 if the size is not
   known. The built-in functions are optimized into the
   normal string functions like memcpy if the last
   argument is (size_t) -1 or if it is known at compile
   time that the destination object will not be
   overflowed..."

https://gcc.gnu.org/onlinedocs/gcc/Object-Size-Checking.html

In the attached example1.c, __builtin___memcpy_chk() is optimized into the
normal memcpy(), as expected.

But in a slightly different example2.c, it is not, despite an object size of
-1.

When the checked version is left in place (like example2.c), it causes a
significant regression in my case.

This is important because Ubuntu 18.04 uses _FORTIFY_SOURCE, which ends up
using __builtin___memcpy_chk() for memcpy. If gcc is arbitrarily leaving it in
place when it should be (according to the docs) optimized away, that affects a
lot of code.

I'm seeing this on Ubuntu 18.04 with both:

  gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
  gcc-9 (Ubuntu 9.2.1-19ubuntu1~18.04.york0) 9.2.1 20191109

It happens with or without -fno-builtin-memcpy (which is not a surprise, since
I am directly calling the builtin version anyway).

Compiled using:
  gcc-9 -O2 -c -S -o example1.S example1.c
  gcc-9 -O2 -c -S -o example2.S example2.c

example1.S:50:
callmemcpy@PLT

example2.S:75:
rep movsq