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0 pointslelanthran5d ago0 comments

> You can have a universal variable length field, for example 2 bytes for strings < 32768, then four bytes, 8 bytes etc.

To hold the length of a string, I'd do something similar to unicode:

7-bits for size + 1-bit for continuation, then 15 bits for size + 1 bit for continuation, then 23-bits for size + 1 bit for continuation, etc.

Or maybe even do it exactly the same as unicode:

    0XXX XXXX -> length of string is in those 7 bits
    1XXX XXXX  XXXX XXXX -> length of string is in those 7+8 bits
    11XX XXXX  XXXX XXXX  XXXX XXXX-> length of string is in those 6+8+8 bits
    ...

> On the critical short string path, it costs just a single bit test.

A few more clock cycles compared to NULL-termination, although my alternatives above require even more clock cycles.

If the hardware had instructions for sentinel values, things would be easier (Like how DOS calls used '$' termination for strings) and safer.

Load a sentinel byte into a register and have dedicated copy and compare instructions that take each two addresses (src and dst) and copies (or compares) src/dst until the terminator is reached (with copy copying the sentinel as well).

Considering that sentinel values are needed so often, and are so useful, it's surprising that this is not in any ISA. What we have now is kludgy workarounds in the HLL for this. It's hard to blame the HLL, because some workaround has to be implemented.

0XXX XXXX -> length of string is in those 7 bits 1XXX XXXX XXXX XXXX -> length of string is in those 7+8 bits 11XX XXXX XXXX XXXX XXXX XXXX-> length of string is in those 6+8+8 bits ...

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5 comments · 2 top-level

rswail5d ago· 3 in thread

So what exactly is the NUL/0 in the code below other than a sentinel value?

    while (*d++ = *s++)
        ;

lelanthranOP5d ago

I am not sure what this is in response to. Can you explain which point of mine you are responding to?

rswail5d ago

> If the hardware had instructions for sentinel values, things would be easier (Like how DOS calls used '$' termination for strings) and safer.

A zero is a sentinel value and is catered to by all ISAs.

Why would using a "$" be any easier/safer than a NUL?

lelanthranOP5d ago

> A zero is a sentinel value and is catered to by all ISAs.

> Why would using a "$" be any easier/safer than a NUL?

I didn't say it had to be '$'; I specifically said that the sentinel would be loaded into a register. In that case it could be anything, including zero (for the snippet you posted), or INT_MAX if the code iterated across an array of integers, etc.

By having rep/mov variants that use sentinels, a lot of the HLL problems go away - Java, C#, Python, etc would all look very different today if the ISAs from the 80s included sentinal variant of memory instructions.

1 more reply

cornholio5d ago

Personally, I would avoid UTF-8 levels of complexity because you only pay the size cost once per string. A simple 2-bytes + optional 4 bytes continuation scheme handles strings up to 140TB and increases the size of the average string by just 2 bytes (compared to 1 byte for nul termination).

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