[Linuxppc-users] OpenMPI deviations & Power hardware bounds checking

[Ian McIntosh]

I posted an answer to the second question, for XL compilers.  Someone else
might want to post a GCC or an LLVM answer?



The XL compilers available on the IBM POWER processors on the Little Endian
Linux, Big Endian Linux or AIX operating systems have a different
implementation of array bounds checking.
Using the -qcheck or its synonym -C option turns on various kinds of
checking. -qcheck=bounds checks array bounds. When this is used, the
compilers check that every array reference has a valid subscript.
The hardware instruction used is a conditional trap, comparing the
subscript to the upper limit and trapping if the subscript is too large or
too small. In C and C++ the lower limit is 0. In Fortran it defaults to 1
but can be any integer. When it is not zero, the lower limit is subtracted
from the subscript being checked, and the check compares that to the upper
limit minus the lower limit.
When the limit is known at compile time and small enough, a conditional
trap immediate instruction is enough. When the limit is calculated at
execution time or is greater than 65535, a conditional trap instruction
comparing two registers is needed.
The performance impact is small for several reasons:
1. The conditional trap instructions are fast.
2. They are executed in a standard integer pipeline. Since most POWER CPUs
have 2 or 4 integer pipelines, there is usually an otherwise empty slot to
put the trap in, so it is often essentially zero cost.
3. When it can the compiler optimizer moves the conditional trap out of
loops so it is executed only once, checking all loop iterations at once.
4. When it can prove the actual subscript cannot exceed the limit, the
optimizer discards the instruction.
5. Also when it can prove the subscript will also be invalid, the optimizer
uses an unconditional trap.
6. If necessary -qcheck can be used during testing and skipped for
production builds, but the overhead is small enough that's not usually
necessary.
If my memory is correct, one long ago paper reported a 2% slowdown in one
case and 0% in another. Since that CPU had only one integer pipeline, the
slowdown should be significantly less with modern CPUs.
Other checking using the same mechanism is available to detect
dereferencing NULL pointers, dividing an integer by zero, using an
uninitialized auto variable, specially written asserts, etc.
This doesn't include all kinds of invalid memory usage, but it does handle
the most common kind, does it very efficiently, and is very easy to use.

- Ian McIntosh          IBM Canada Lab         Compiler Back End Support
and Development




From:	"Nicole Trudeau" <nitrud at ca.ibm.com>
To:	linuxppc-users at lists.ozlabs.org
Date:	2017-06-09 04:23 PM
Subject:	[Linuxppc-users] OpenMPI deviations & Power hardware bounds
            checking
Sent by:	"Linuxppc-users" <linuxppc-users-bounces
            +ianm=ca.ibm.com at lists.ozlabs.org>



Hi everyone,

I monitor user questions that come in about IBM XL compilers for Linux on
Power and 2 things came up on my radar that are not directly related to XL
that the broader Linux on Power team may be able to help answer - feel free
to reply directly on the Stack Overflow or developerWorks links - really
appreciate the help in advance!
   1.	developerWorks Forum: Thanks to Bill Buros for commenting today.
      Here's a summary of the problem "I'm having a problem with openmpi
      2.1.0 and Abinit. When i run abinit task with np (number of
      processes) = 1 task executes fine. With np = 2 small deviations
      appear. With larger np task ends with an error. I compiled abinit
      using GNU C/C++/Fortran 4.8.5 compilers and NVIDIA Cuda 8.0.61"
   2.	Stack Overflow: "Do any CPUs have hardware support for bounds
      checking?" A reply has been given about x86, but no reply about Power
      yet. Does Power support it?

Thanks,
Nicole Trudeau
zSystems Software > zBuild > Compilers
Digital Marketing
C2-818, Toronto, Canada Office

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