Hello Chirag,
Regarding a byte-swap operation, it seems that you have a reasonable use-case
on a bi-endian machine. Feel free to request a new operator on the "public
comments" page at http://dwarfstd.org/Comment.php
Note that a byte-swap operator would swap all bytes in the top-of-stack value,
which on your 64-bit machine would of course be a 64-bit value. As you want a
32-bit swapped value, you would still need to do a shift afterward, but even
so, "DW_OP_byte_swap DW_OP_const1u 32 DW_OP_shr" would be considerably shorter
than what you have to do now.
Of course a new operator would be introduced in a new DWARF revision, which is
likely to be years away. In the meantime let me suggest a shorter expression
for doing the byte-swap operation. The book "Hacker's Delight" shows a
straightforward 32-bit byte swap with masks no wider than 16 bits, as follows:
x = (x << 24) | ((x & 0xff00) << 8) | ((x >> 8) & 0xff00) | (x
>> 24);
Your 64-bit machine will of course use 64-bit values on the expression stack,
so to keep the result "32-bit clean" we want to do one additional mask:
x = ((x & 0xff) << 24) | ((x & 0xff00) << 8) | ((x >> 8) &
0xff00) | (x >> 24);
Translating this into a DWARF expression, I get the following:
DW_OP_dup, DW_OP_const1u 0xff, DW_OP_and, DW_OP_lit24,
DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_const2u 0xff00, DW_OP_and, DW_OP_lit8,
DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_lit8, DW_OP_shr, DW_OP_const2u 0xff00,
DW_OP_and, DW_OP_swap, DW_OP_lit24, DW_OP_shr, DW_OP_or, DW_OP_or, DW_OP_or
I hope this is helpful to you.
--paulr
From: Dwarf-Discuss <dwarf-discuss-boun...@lists.dwarfstd.org> On Behalf Of
Chirag Patel via Dwarf-Discuss
Sent: Monday, October 28, 2019 12:47 AM
To: dwarf-discuss@lists.dwarfstd.org
Subject: [Dwarf-Discuss] dwarf stack operator for byte swap.
Hello Dwarf experts.
I am currently working trying to encode dwarf of binaries with having bi-endian
variables marked with DW_AT_endianity attribute.
The location calculation for some variable depends on other variable with
different endianity, also the value of this other variable is known at runtime.
At the moment I am using location list to calculate the correct location of
first variable and list of dwarf operators to reverse the endianity of variable
"__gbloffset__" in below case (I only needed lower 32 bits on 64 bit machine).
0x000001e5: DW_TAG_base_type
DW_AT_byte_size (0x04)
DW_AT_encoding (DW_ATE_signed)
DW_AT_name ("int")
DW_AT_endianity (DW_END_big)
...
0x00000057: DW_TAG_variable
DW_AT_name ("__gbloffset__")
DW_AT_type (0x000001e5 "int")
DW_AT_external (true)
DW_AT_decl_file ("...")
DW_AT_decl_line (8)
DW_AT_location (DW_OP_addr 0) // pre linkage
DW_AT_linkage_name ("_gblsection__")
0x00000170: DW_TAG_variable
DW_AT_name ("VAR1")
DW_AT_type (0x0000010b "fixed.dec.display.72")
DW_AT_decl_file ("...")
DW_AT_decl_line (10)
DW_AT_location (DW_OP_addr 0x0, DW_OP_call4 0x57,
DW_OP_deref_size, 4,
DW_OP_dup, DW_OP_constu 0xff, DW_OP_lit0, DW_OP_shl, DW_OP_and, DW_OP_lit24,
DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_constu 0xff, DW_OP_lit8, DW_OP_shl,
DW_OP_and, DW_OP_lit8, DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_constu 0xff,
DW_OP_lit16, DW_OP_shl, DW_OP_and, DW_OP_lit8, DW_OP_shr, DW_OP_swap,
DW_OP_constu 0xff, DW_OP_lit24, DW_OP_shl, DW_OP_and, DW_OP_lit24, DW_OP_shr,
DW_OP_swap, DW_OP_or, DW_OP_or, DW_OP_or, DW_OP_plus)
DW_AT_linkage_name ("VAR1")
In above snippet of dwarf dump, I am using yellow highlighted list of operators
to swap the bytes.
I think there should be a support for DW_OP_byte_swap simple operator to
accomplice this simple task. Does this idea looks like it can be useful? Is
there any specific reason why dwarf spec does not have it or I am missing
something subtle.
I hope I conveyed the idea properly, apologies in advanced as English is not my
first language.
Thanks and regards,
Chirag Patel
Software Engineer | Raincode Labs India
Tel: (+91) 080 41159811
Mob: (+91) 9049336744
www.raincodelabs.com<http://www.raincodelabs.com/>
[linkedin-button]<https://in.linkedin.com/in/chirag-patel->
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