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rurban (7989)

  {rurban} {at} {}

cygwin maintainer for perl, parrot, clisp, postgresql, ... and some perl modules (perl-libwin32, perl-Win32-GUI). Has also some hairy CPAN packages: B::C, B::Generate, C::DynaLib, B::Debugger ...

Journal of rurban (7989)

Saturday February 21, 2009
03:14 PM

parrot packfile alignment to keep cross-platform portability

[ #38522 ]
I thought that having portable parrot bytecode (pbc) is a good idea, similar to portable perl5 bytecode (B::Bytecode). And parrot was designed for bytecode portability. So it should be better than perl5 plc. And it is.

Anyway, it does not work since a while. Some random notes for debugging this:

We support 32-bit and 64-bit architectures, that makes a wordsize (integer) and ptrsize of 4 and 8, and the floats can be 8-byte (double), 12-byte (long double on i386) and 16-byte (long double elsewhere).

The trouble with portability is besides the required bitshuffling when reading foreign pbc files, the trouble of what to do with deprecated features (new parrot reading old file), and new features (old parrot reading new file).

And interestingly better architectures require a ptr alignment > 1. Sparc 32-bit requires 4-byte ptr alignment, Sparc 64-bit requires 8-byte ptr alignment. You can reset this with a compiler flag but it is not recommended for performance reasons.

This hits us when writing code on our simple i386 platform with a ptr alignment of 1 and a wordsize of 4.
The 64-bit sparc just does cursor++ (a opcode_t ptr) running through the file. cursor is 8-byte there, 4 byte here. So we have to take care in the writer also to properly align code, because it must be easily readable on the foreign machine also.

The section I'm tempted to adding to the pdd31_bytecode.pod is:

=head4 8-byte ptr_alignment

We should be aware that some systems such as a Sparc/PPC 64-bit use strict 8-byte ptr_alignment per default, and all (opcode_t*)cursor++ or (opcode_t*)cursor += advances must ensure that the cursor ptr is 8-byte aligned. We enforce 16-byte alignment at the start and end of all segments and for all strings, but not in-between, esp. with 4-byte integers and 4-byte opcode_t pointers.

Which means that for a 32-bit (4-byte) pbc on a 8-byte ptr_alignment machine the pmc designer should take care that integers and opcode_t pointers appear pairwise in the frozen format, so that the 16-byte padding at the end of a segment already happens on an already 8-byte aligned pointer (xxx0 or xxx8), and not on a 4-byte ptr (xxx4 or xxxc) alignment. Operations on aligned pointers are much faster than on un-aligned pointers.

With #define TRACE_PACKFILE 1 in include/parrot/packfile.h you can enable debugging output for the packfile reader, the parrot utils pbc_dump accept a --debug flag to use this. But you can also use the debugger to check alignment problems.

There's nice table in the PPC manual which I need often, because when staring at the ALIGN'ed ptr's you have to see errors, which will happen if start reading at the wrong point. Un-aligned.

Operand Length Addr if aligned (in bits, 0b)
Byte 8 bits xxxx any
Halfword 2 byte xxx0 even
Word 4 byte xx00 0 4 8 c
Doubleword 8 byte x000 0 8
Quadword 16 byte 0000 0
So look for any ending hexbyte but 0 and 8 for your ptr's to see 8-byte misalignment, and 0 for proper 16-byte alignment.

The interesting point, which I first got wrong, is that the ptr alignment (we guarantee 16-byte ptr alignment) is for the cursor stepper cursor++ to advance the ptr in memory not to the next char, but the to next word = opcode_t.

This is the macro (WRONG), which automates the cursor stepper. To makes things complicates we already copied the contents into memory, so the base address is not 0 but pf->src and the alignment must be guaranteed for all three ptrs involved: the base pf->src, the cursor (current position) and the offset, the relative position in the file, cursor - pf->src.

#define ALIGN_16(st, cursor) \
(cursor) += ROUND_16((const char *)(cursor) - (const char *)(st))/sizeof (opcode_t)

cursor += advances the pointer by the alignment calculation in the macro. But this does not do ptr alignment! The ptr must already be properly aligned in terms of the native ptrsize. So on 32-bit (4-byte) there must be 0 4 8 or c as last hexdigit in the cursor and the offset. The padding can only 0, 1, 2 or 3.
On 64-bit (8-byte) there must be 0 or 8 as last hexdigit in the cursor and the offset. The padding can only 0 or 1.

My padding tables for ALIGN_16:

ptrsize=8 pad
0 0
8 1

ptrsize=4 pad
0 0
4 1
8 2
c 3

The other problems are simply looking innocent steppers, like this in PF_fetch_string() (WRONG):

size = ROUND_UP_B(size, wordsize);
*((const unsigned char **) (cursor)) += size;
or this from PF_fetch_op() (CORRECT):

o = (pf->fetch_op)(*((const unsigned char **)stream));
*((const unsigned char **) (stream)) += pf->header->wordsize;

Here we cast the ptrsize stream down to 1 to be able to advance misaligned pointers, but upwards in the directory segment we should be aligned again. And PF_fetch_op() and PF_fetch_integer() only guarantee 4-byte alignment, not 8-byte as needed on 64-bit strict PowerPC.

Strings are the most potential problem to destroy alignment, because they may be mod 1, mod 2 or mod 3 byte long, any size. But strings are safer because we ensure proper ptrsize alignment for them. 4-byte integers and pointers are more problematic because on 8-byte machine we easily get misalignment in uneven integer or pointer pairs.

So look for uneven ptrs, also when thaw'ing pmc data. This must also be aligned!

Did you see catch the error above? No? Me neither, but there is one, and it is hidden in the ROUND_UP_B macro, which is not 32/64-bit proof. It only works on the native platform.

And don't whine about this stupid machines. They are right, even if its just a 64-bit sparc which is that strict (ptr alignment 8, remember), unaligned code is shorter, but much slower.

So it's better to catch unaligned data while compiling or even running (as on the sparc), than running slow and misaligned.

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