[cfe-dev] Relational operators on pointers

[Stefan Marr]

Hi:

On 13 Oct 2011, at 01:11, Eli Friedman wrote:

> On Wed, Oct 12, 2011 at 3:13 PM, Stefan Marr
> <mailinglists at stefan-marr.de> wrote:
>> Hi:
>> 
>> On 12 Oct 2011, at 23:54, David Chisnall wrote:
>> 
>>> So, if you want to do a comparison between two unrelated pointers as unsigned values, the safest thing to do is cast them to uintptr_t and then do the comparison.
>> 
>> Right, done, and failed. In this particular case, as Eli pointed out, some optimization is applied.
> 
> We shouldn't be messing with integer comparisons...

I would guess, the transformation implied by assuming certain allocation behavior comes first, and then leaves nothing for the casts to do their thing.

I attached a smaller self-contained test-case.

Executed with:
$ /usr/bin/clang++ -m32 -Wextra -Wno-write-strings -fdiagnostics-show-option -Wno-unused-variable -Wno-unused-value -fno-rtti  -O4 bug-report.cpp -o bug-report.bin ; ./bug-report.bin

Results in the output:
beginning should be before the ends

Expected output:
done

The 'incorrect' output is consistently issued by binaries compiled by all compilers listed below, except of i686-apple-darwin10-g++-4.2.1.
And similar, -O0 does produce the correct output, too.

Adding casts like this:

if ((uintptr_t)read_write_memory_base >= (uintptr_t)read_write_memory_past_end) {
  printf("beginning should be before the end\n");
  exit(-1);
}

if ((uintptr_t)read_mostly_memory_base >= (uintptr_t)read_write_memory_past_end) { 

does not change the results: incorrect behavior for llvm-gcc, clang 2.9 and 3.0.
  

> 
>> All my casting to what ever type, and with whatever mechanism, did not help.
>> Not even using temp variables of 64bit types with which ever signage.
>> That all got just optimized away...
>> Leaving me with a failing test without apparent reason.
> 
> What version of clang are you using?

$ /usr/bin/clang++ --version
Apple clang version 3.0 (tags/Apple/clang-211.10.1) (based on LLVM 3.0svn)
Target: x86_64-apple-darwin10.8.0
Thread model: posix

$ /opt/local/bin/clang++ --version
clang version 2.9 (tags/RELEASE_29/final)
Target: x86_64-apple-darwin10
Thread model: posix

$ /usr/bin/llvm-g++-4.2 --version
i686-apple-darwin10-llvm-g++-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)
Copyright (C) 2007 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.

$ g++-4.2 --version
i686-apple-darwin10-g++-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5666) (dot 3)
Copyright (C) 2007 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.


Side note:
For llvm-g++-4.2 I had to use -O3 instead of -O4 because it aborted with the following error:

ld: lto: could not merge in /var/folders/In/InPYSzVvHvGU0ndnO3LOyU+++TI/-Tmp-//cc9rOCdg.o because Unknown instruction for architecture i386
collect2: ld returned 1 exit status
-bash: ./bug-report.bin: No such file or directory


Best regards
Stefan


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