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Re: ELF octets_per_byte
- From: Nick Clifton <nickc at redhat dot com>
- To: Pedro Alves <palves at redhat dot com>, dgisselq at ieee dot org, binutils at sourceware dot org, gdb at sourceware dot org
- Date: Wed, 24 Feb 2016 11:49:01 +0000
- Subject: Re: ELF octets_per_byte
- Authentication-results: sourceware.org; auth=none
- References: <1456242622 dot 30661 dot 448 dot camel at jericho> <56CD8B33 dot 7050608 at redhat dot com>
Hi Pedro, and GDB-land...
> GDB 7.11 gained support for this,
As a matter of interest, can I bring up the issue of DWARF and LEB128 encoding ?
According to the DWARF 4 specification:
7.6 Variable Length Data
Unsigned LEB128 (ULEB128) numbers are encoded as follows: start at
the low order end of an unsigned integer and chop it into 7-bit chunks.
Place each chunk into the low order 7 bits of a byte. Typically,
several of the high order bytes will be zero; discard them. Emit the
remaining bytes in a stream, starting with the low order byte; set
the high order bit on each byte except the last emitted byte. The
high bit of zero on the last byte indicates to the decoder that it has
encountered the last byte.
Note the use of the word "byte" here. I could not find any reference to octets
in the document, but I think that the implication is that a byte is the smallest
addressable storage unit available on the target architecture, and not necessarily
always an 8-bit quantity. This does mean however that for targets with 32-bit
bytes for example, LEB128 encoding is very wasteful of space...
If the specification intends that a "byte" is an 8-bit quantity then it needs to
specify how these 8-bit values are stored into a target storage unit when the
storage unit is larger than 8 bits, (ie little endian vs big endian). Plus it
should state whether a LEB128 value is padded out to fill a whole number of
storage units, or if they are packed in as tightly as possible. Plus the
algorithm in Appendix C ought to be extended to reference packing octets into