On Wed, Dec 21, 2016 at 10:40:32AM +0800, Ben Coman wrote:

On Wed, Dec 21, 2016 at 9:03 AM, commits@squeakvm.org wrote:

...

Revision: 3753 Author: lewis Date: 2016-12-20 17:03:04 -0800 (Tue, 20 Dec 2016) Log Message:

---

Do not use -O3 optimization, -O2 is safer and works well.

gcc 4.9.2 gives crashes and heisenbugs with OSPP compiled with -O3.

Symptoms: (1 to: 10) collect: [ :e | (CommandShell new pipeline: 'ps > /dev/null | cat') output] ==> segfaults in some of the spawned child processes

This appears to be an actual compiler bug, although it goes away when print statements are added, so I cannot say for sure (and it is difficult to attach gdb to the newly spawned child process before it crashes). The bad behavior happens only with -O3 and there are no real performance benefits compared to -O2 (bytecodes faster, sends slower).

I'd be interested to know the reason sends get slower (if known). cheers -ben

No clue, I was just sanity checking to make sure that -O2 was not horribly worse. It was not. But I suspect all of this is likely to vary depending on gcc compiler version and phase of the moon.

Dave

On Wed, Dec 21, 2016 at 10:11:39AM +0000, Jan Vrany wrote:

On Tue, 2016-12-20 at 21:54 -0500, David T. Lewis wrote:

...

?? On Wed, Dec 21, 2016 at 10:40:32AM +0800, Ben Coman wrote:

...

?? I'd be interested to know the reason sends get slower (if known). cheers -ben

No clue, I was just sanity checking to make sure that -O2 was not horribly worse. It was not. But I suspect all of this is likely to vary depending on gcc compiler version and phase of the moon.

This post:

http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html

explains the dependency on moon phases (and compiler versions) :-)

Thanks. Indeed there is probably some undefined C behavior in there, although I was not able to spot it. If anyone is interested in lending their eyes to the problem, I was able to localize the crash to intermittent segfaults that occurred in OSProcessPlugin>>fixPointersInArrayOfStrings: which is generated in C (for a V3 image, not Spur) as:

/* Use the address offsets in offsetArray to fix up the pointers in cStringArray. The result is a C array of pointers to char, used for argv and env vectors. */

static sqInt fixPointersInArrayOfStringswithOffsetscount(char *flattenedArrayOfStrings, sqInt *offsetArray, sqInt count) { sqInt idx; char **ptr;

ptr = ((char **) flattenedArrayOfStrings); idx = 0; while (idx < count) { ptr[idx] = (flattenedArrayOfStrings + (((offsetArray[idx]) >> 1))); idx += 1; } return null; }

Sorry, ignore that last one. It fired off incomplete.

On Wed, Dec 21, 2016 at 10:35 PM, David T. Lewis lewis@mail.msen.com wrote:

On Wed, Dec 21, 2016 at 10:11:39AM +0000, Jan Vrany wrote:

...

On Tue, 2016-12-20 at 21:54 -0500, David T. Lewis wrote:

...

?? On Wed, Dec 21, 2016 at 10:40:32AM +0800, Ben Coman wrote:

...

?? I'd be interested to know the reason sends get slower (if known). cheers -ben

No clue, I was just sanity checking to make sure that -O2 was not horribly worse. It was not. But I suspect all of this is likely to vary depending on gcc compiler version and phase of the moon.

This post:

http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html

explains the dependency on moon phases (and compiler versions) :-)

Thanks. Indeed there is probably some undefined C behavior in there,

although

I was not able to spot it. If anyone is interested in lending their eyes

to

the problem, I was able to localize the crash to intermittent segfaults

that

occurred in OSProcessPlugin>>fixPointersInArrayOfStrings: which is

generated

in C (for a V3 image, not Spur) as:

/* Use the address offsets in offsetArray to fix up the pointers in

cStringArray.

    The result is a C array of pointers to char, used for argv and

env vectors. */

static sqInt fixPointersInArrayOfStringswithOffsetscount(char

*flattenedArrayOfStrings, sqInt *offsetArray, sqInt count) {

sqInt idx;
char **ptr;

    ptr = ((char **) flattenedArrayOfStrings);
    idx = 0;
    while (idx < count) {
            ptr[idx] = (flattenedArrayOfStrings +

(((offsetArray[idx]) >> 1)));

            idx += 1;
    }
    return null;

}

I've taken a guess at its usage and turned it into an executable test case. Could you confirm this...

#include <stdio.h> typedef int sqInt; int null = 0; static sqInt fixPointersInArrayOfStringswithOffsetscount(char *flattenedArrayOfStrings, sqInt *offsetArray, sqInt count) { sqInt idx; char **ptr; ptr = ((char **) flattenedArrayOfStrings); idx = 0; while (idx < count) { ptr[idx] = (flattenedArrayOfStrings + (((offsetArray[idx])

...

1)));

idx += 1; } return null; } int main() { char *flattenedArrayOfStrings = "abcd\0efgh\0ijkl\0"; sqInt offsetArray[] = {0, 5, 10}; printf("%s\n", flattenedArrayOfStrings); printf("%d %d %d\n", offsetArray[0], offsetArray[1], offsetArray[2]);

fixPointersInArrayOfStringswithOffsetscount( flattenedArrayOfStrings, offsetArray, 2 ); printf("%s\n", flattenedArrayOfStrings); }

$ cc test.c ; ./a.out abcd 0 5 10 Segmentation fault

ptr is being defined as a pointer to "one" string, but its being accessed as a consecutive list of strings. Even the first assignment to ptr seems wrong. flattenedArrayOfStrings is a string. The first dereference is a char, and then when it assigns to ptr[idx], it tries to dereferenced the char and boom!

If the first ptr assignment is changed as follows, it doesn't crash... ptr = &flattenedArrayOfStrings;

But I'm not sure it does what its supposed to ?? $ cc test.c ; ./a.out abcd 0 5 10 abcd cd abcd

Now if I do this... static sqInt fixPointersInArrayOfStringswithOffsetscount(char *flattenedArrayOfStrings, sqInt offsetArray[], sqInt count) { sqInt idx; char *ptr[count+1]; idx = 0; while (idx < count) { ptr[idx] = flattenedArrayOfStrings + offsetArray[idx]; idx += 1; } idx = 0; while (idx < count) { printf("%s\n", ptr[idx]); idx += 1; } return null; }

I can call it with count=3 and get this... $ cc test.c ; ./a.out abcd 0 5 10 abcd efgh ijkl abcd

but the original flattenedArrayOfStrings is unmodified, so I'm not sure if that is how its meant to behave?

cheers -ben

Huh, I couldn't find GCC examples quickly. However, here's something I actually ran into. The manual for the IBM XL C 10.x compiler for AIX on POWER explicitly states -O3 causes comparatively common functions to stop setting errno. Critically, however, this statement is *not* in every reference to the -O3 switch. Rather, it was in a manual set that was much more detailed than these kinds of links:

http://www.ibm.com/support/knowledgecenter/en/SSGH2K_13.1.3/com.ibm.xlc1313....

http://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.3/com.ibm.xlc1313.aix...

Notice how the above pages do not say anything about errno? IIRC the man page didn't say anything about errno with -O3 either. However, this one other page does:

http://www.ibm.com/support/knowledgecenter/en/SSGH2K_11.1.0/com.ibm.xlc111.a...

And in there, there are other statements such as "with -O3 some instructions are placed in code paths where they always execute, while with -O2 that's not the case". Looks like asking for undefined behavior in the general case to me.

This particular example was the breaking point for me, and from then on I started assuming -O3 is not behavior preserving, even if the manual doesn't immediately say there are problems. Also, manuals are not necessarily complete. That's not to say that -O3 isn't useful without the necessary verification steps. And while I couldn't quickly find similar examples for other compilers, I do remember reading them. From a VM engineering perspective however, and given all the gray area semi-undefined behavior things one is basically forced to do, my opinion is -O2 is the way to go.

Then again, even with -O2 one always has to read the manual and be aware of what's going on. For instance, in the latest GCC with -O2, the optimization -fdelete-null-pointer-checks is enabled. However, the manual also says:

======================== Assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null.

Note however that in some environments this assumption is not true. ========================

And yes, there are realistic places where NULL can be dereferenced. Again on the AIX versions I worked on, the memory at 0x0 was mapped. Due to some bugs at the time, HPS overwrote that memory. And yet, the system continued to appear to work. Thus, some null pointer segfaults for some platforms did not happen on AIX, even though the code and the wrong behavior were exactly the same.

Andres.

On 12/23/16 13:52 , Holger Freyther wrote:

...

On 23 Dec 2016, at 22:30, Andres Valloud avalloud@smalltalk.comcastbiz.net wrote:

Compiler manuals usually state -O3 and higher do not preserve language semantics. It's going to be hard to prove "bug" in the non-default presence of a switch known to potentially produce undefined behavior.

Where did you get that from? I tried reading it up but don't see anything.

Clang:

-O3 Like -O2, except that it enables optimizations that take longer to perform or that may generate larger code (in an attempt to make the program run faster).

=> so longer compilation time, bigger binary

gcc:

-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-loop-vectorize, -ftree-loop-distribute-patterns, -fsplit-paths -ftree-slp-vectorize, -fvect-cost-model, -ftree-partial-pre, -fpeel-loops and -fipa-cp-clone options.

=> None says that the language standard is broken

Intel ICC

-O3

Performs O2 optimizations and enables more aggressive loop transformations such as Fusion, Block-Unroll-and-Jam, and collapsing IF statements. The O3 optimizations may not cause higher performance unless loop and memory access transformations take place. The optimizations may slow down code in some cases compared to O2 optimizations. The O3 option is recommended for applications that have loops that heavily use floating-point calculations and process large data sets.

So both Intel ICC an GCC start doing more auto vectorization with -O3. Nothing of that is breaking the language semantic. So in most cases if -O3 breaks things.. the code has undefined behavior...

holger