[Bug tree-optimization/57156] miscompilation of call to _mm_cmpeq_epi8(a, a) or _mm_comtrue_epu8(a, a) with uninitialized a

["kretz at kde dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57156

--- Comment #8 from Matthias Kretz <kretz at kde dot org> ---
I just noticed the following in the Intel Optimization Reference Manual
(Version 028 from July 2013), section 2.2 "Sandy Bridge":
2.2.3.1 Renamer
[...]
There is another dependency breaking idiom - the "ones idiom".
• CMPEQ XMM1, XMM1; "ones idiom" set all elements to all "ones"
In this case, the micro-op must execute, however, since it is known that
regardless of the input data the output data is always "all ones" the micro-op
dependency upon its sources does not exist as with the zero idiom and it can
execute as soon as it finds a free execution port.

Therefore I think it makes sense if this feature is exposed somehow. Right now,
according to what you said, it is impossible to make use of this feature when
programming with intrinsics.
I see two solutions right now:
1. Let the cmpeq intrinsic reflect this special behavior: i.e. if the two
inputs are the same variable, this behaves like a function without input.
2. Provide a new intrinsic/builtin analogous to _mm_setzero_* /
_mm256_setzero_*
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From: "bardeau at iram dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug preprocessor/57881] New: Pre-processor pre-includes breaks preprocessing of non C/C++ code
Date: Thu, 11 Jul 2013 09:45:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW881

            Bug ID: 57881
           Summary: Pre-processor pre-includes breaks preprocessing of non
                    C/C++ code
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bardeau at iram dot fr

According to http://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/Standards.html the C
pre-precessor includes implicitly the file /usr/include/stdc-predef.h to any
pre-processed source.

For non C/C++ code, we are told here http://gcc.gnu.org/gcc-4.8/porting_to.html
to add the option -P. This just won't work in real use cases. People using the
C preprocessor on non C/C++ code combine the following options:
-P: no line markers added,
-C: do not discard comments. In non-C/C++ code, false-positive blocks of /* */
could be deleted out.
-traditional-cpp: preserve blank lines. In non C/C++ code, blank lines may be
relevant. At least, it preserves line numbering.

Obviously, pre-preprocessing of non-C/C++ code with "cpp -P -C -traditional"
adds now C comments and blank lines. Its certainly a feature, not a bug, but:

1) you should modify the comment in http://gcc.gnu.org/gcc-4.8/porting_to.html:
"For non C/C++ code, use the pre-processor flag -ffreestanding" for full
compatibility with possible options.

2) the option "-ffreestanding" has no meaning for non C/C++ code. You should
consider adding an option for preprocessing of non C/C++ sources, which gathers
the options "-P -C -traditional -ffreestanding", plus any future change needed.

Thanks for having a look into this!