Add U128 type

[adlerjohn]

Fixes #1478

Blocked by

• Arithmetic ops don't set $of to overflow value, but overflow state fuel-vm#121
• Fix Shiftable trait to shift by u64. #1604

Soft-blocked by FuelLabs/fuel-tx#129, until which tests will quickly run out of gas. The current limit of 1M gas is hit after 4 multiplications or 2 divisions.

Implements https://en.wikipedia.org/wiki/Division_algorithm#Integer_division_(unsigned)_with_remainder for division.

[otrho]

Do these belong somewhere more general as pub definitions? Like num or somewhere?

[nfurfaro]

hmm. My understanding was that the flag op had to be used in the same asm block as the op you're trying to allow overflow for, otherwise, the bit is unset after the block returns. Maybe a misunderstanding on my part, perhaps I was confused with $of being cleared after an operation.
Is this working?

[nfurfaro]

Also, perhaps rather than movi r1 i3; here you could initialize a second register with the value as a named variable (or even better, as a const) to:
a.) help document the function better, and
b.) to follow the general guideline of "if it can be done outside of asm, do it outside of asm".

ie:

const WRAPPING_FLAG_VALUE = 3;
asm(r1: WRAPPING_FLAG_VALUE) {
 flag r1;
}

[nfurfaro]

And finally, it seems that the flag values for enable and disable are reversed.
An unset flag (ie: 0) should be the default state in which panic occurs on overflow.
Setting to a non-zero value should enable overflow.
Edit: I think it's actually just the names that seem reversed to me. like, disable_overflow should be disable_panic_on_overflow, and enable_overflow should be enable_panic_on_overflow.

[Braqzen]

Also, perhaps rather than movi r1 i3; here you could initialize a second register with the value as a named variable (or even better, as a const) to: a.) help document the function better, and b.) to follow the general guideline of "if it can be done outside of asm, do it outside of asm".

ie:

const WRAPPING_FLAG_VALUE = 3;
asm(r1: WRAPPING_FLAG_VALUE) {
 flag r1;
}

What about

asm(WRAPPING_FLAG_VALUE: 3) {
    flag WRAPPING_FLAG_VALUE;
}

[nfurfaro]

I think a combination of a descriptively-named register and a const value would be best, and in line with tx.sw and other modules where we use named const values to initialize registers with.

[nfurfaro]

Do these belong somewhere more general as pub definitions? Like num or somewhere?

We had talked about a stdlib module for wrapping VM opcodes, this would probably fit the bill. Will look for the open issue.
#1074
Perhaps I'll just add a task to that issue to create a new module and extract these in a separate PR.
Done.
cc @otrho

[adlerjohn]

Fixed in 36f5ef7. Filed #1664 to follow-up on cleaner constants.

[adlerjohn]

@otrho given that no other part of the stdlib needs these functions, can we defer it to a future PR?

[Braqzen]

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