Re^7: Module for 128-bit integer math?

in reply to Re^6: Module for 128-bit integer math?
in thread Module for 128-bit integer math?

Does int128() and uint128() croak for "0400000000000000000000000000000000000000"?

It should now ;-)

Does $x + undef and the like warn?

yes

(Note that «undef + $x» cannot warn without making «my $x; $x += int128(...)» warn.)

Operations with undefs always generate warnings. Emulating the Perl behavior for built-ins up to this level is not one of my targets.

Does signed to unsigned promotion occur when required? (e.g. when adding two a large number overflows.)
Does unsigned to signed promotion occur when required? (e.g. when subtraction overflows to something negative.)

No, automatic promotions would just complicate the semantics of the module too much. Currently, the module provides modulo 2**128 signed and unsigned arithmetic and the semantics are pretty simple:

Mixing Math::UInt128 with Perl native types always generates Math::UInt128 values.
Mixing Math::Int128 with Perl native types always generates Math::Int128 values.
If Math::UInt128 and Math::Int128 types are mixed in some operation, the result is cast to the type of the left operator.

Are the string buffers always suitably aligned for a int128_t? (You use memcpy in some spots, but casts in others.)

Internally 128bit values are stored on the stack or on the PV slot of an SV allocated with newSV(16). As GCC seems to use 64bit instructions to load 128 bits integers from memory, alignment should be right.

Comment on Re^7: Module for 128-bit integer math?

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Re^8: Module for 128-bit integer math? by ikegami (Patriarch) on Feb 09, 2011 at 16:07 UTC
So you say that newSV(16) always allocates the bytes on a suitable memory boundary. I didn't know of any such guarantee, thus my question. Well, then you can replace those calls to `Copy` with casts and assignments Update: Fixed bad grammar. Also, the second paragraph should be ignored because it makes sense to support passing OOK strings to native_to_int128 functions.	[reply] [d/l]
Re^9: Module for 128-bit integer math? by salva (Canon) on Feb 09, 2011 at 17:07 UTC
So you say that newSV(16) always allocates the bytes on a 128-bit aligned No, what I say is that the code generated by GCC uses 64bit operations to move data between the memory and the processor registers so a 128bits alignment is not really required. Any sane allocator will return memory aligned for 64bits or better. Perl custom allocator (used when set from Configure) guarantees that alignment would be suitable for doubles (so, 64bits)... or at least that's what a comment says in the place where `MEM_ALIGNBYTES` macro is introduced. update: Regarding the use of `Copy`, yes, it is because any string allocated outside the module can be unaligned. That includes SVs using the OOK hack and probably others.	[reply] [d/l] [select]
Re^10: Module for 128-bit integer math? by ikegami (Patriarch) on Feb 09, 2011 at 17:08 UTC
Thanks! Update: I guess the issue will need to be revisited when systems support native 128-bit operations.	[reply]

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