Instruction list length in bco->instrs redundant

[nomeata]

While reading through the BCO-related code I notice that in the first word of the instruction list of a BCO (bco->instrs->payload[0]), the length of the list (as the multiple of 16-bit-words) is stored. This is used in debugging code, e.g. in the disassembler.

However as far as I can tell this is duplicate information: bco->instrs->bytes already contains this information (as multiple of bytes).

Maybe the instruction list was not a proper StgArrWords? object before and this hack was required? If that is the case, I guess it can be removed now, saving a neglectable amount of memory and cleaning the code a bit.

[simonpj]

Thanks. Simon M is king in that area so I'll assign to him.

Simon

[simonmar]

Could you point me to the bit of the code you're talking about? I can't find it.

[nomeata]

Certainly:

Here, the first element of the instruction list is used to find nbcd:  http://hackage.haskell.org/trac/ghc/browser/rts/Disassembler.c#L276

The interpreter ignores the value (unless debugging is on):  http://hackage.haskell.org/trac/ghc/browser/rts/Interpreter.c#L789

And here, in assembleBCO, the number of instruction is prepended to the list of instructions:  http://hackage.haskell.org/trac/ghc/browser/compiler/ghci/ByteCodeAsm.lhs#L156 The assertion in line 161 supports the observation that the information is duplicated and the size of the ByteArray? is correct.

[simonmar]

Ah, I see. Yes, I think you're right, this field isn't necessary. It is probably there because the size of an StgArrWords used to be stored in units of words, but it was changed to be bytes.

I'll remove it.

[marlowsd@…]

commit 0c42e301337bdefa94d0c288bb6d689ac33baa4d

Author: Simon Marlow <marlowsd@gmail.com>
Date:   Wed Jan 9 11:51:58 2013 +0000

    remove unnecessary size field in BCO (#7518)

 compiler/ghci/ByteCodeAsm.lhs |    8 ++------
 rts/Interpreter.c             |    4 +---
 2 files changed, 3 insertions(+), 9 deletions(-)

[nomeata]

It seems you missed the code in Disassembler.c

[marlowsd@…]

commit 343548da7274cd15aaeabe72c6b37bce78e9af9c

Author: Simon Marlow <marlowsd@gmail.com>
Date:   Wed Jan 9 14:46:03 2013 +0000

    fix disassembler after removal of size field in bco->instrs  (#7518)

 rts/Disassembler.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)