3 ms·

Access to CPU-specific flags like the carry flag. When doing a multiple-precision addition, if the CPU carry bit is inaccessible, then it must instead be compar

Access to CPU-specific flags like the carry flag. When doing a multiple-precision addition, if the CPU carry bit is inaccessible, then it must instead be compared to the result to see if it overflowed, which takes 3-5 more instructions per limb or worse on modern superscalar processors that are serial in terms of data accesses.

nec4b 5y ago
Portable by definition means it cannot capture every specific detail of every architecture out there.
- pjmlp 5y ago
  By definition there isn't such thing as portable Assembly.
  - deleted 5y ago
    [deleted]
  - potiuper 5y ago
    Not by definition, but by practice as generating assembly is usually done to optimize performance. Languages restricted to generate a common feature set between isomorphic ISAs and bijectively associated assembly languages are at a performance disadvantage compared to the full ISA specific assembly.
- potiuper 5y ago
  The carry flag is not a specific architectural detail; it is part of the output from the prime example of a basic electronic circuit that is the digital adder.
  - nec4b 5y ago
    It is an architectural detail. MIPS for example doesn't have a carry flag. You have to do the exact same procedure in assembly on MIPS you have described in your previous comment when doing multiple word addition.
    - potiuper 5y ago
      MIPS along with DEC alpha do not have adder circuits as a design goal was not to have flags. This along with the scenario of multi-precision arithmetic resulted in the implementation of multi-precision arithmetic on MIPS and DEC alpha using a form of branch like instruction with an ALU stage instead of the unavailable instructions dependent on the alternative adder circuits. The choice of having or not having a carry flag alone is not an architectural detail as the choices of [micro]architecture involve how operations are implemented in circuits/hardware. The carry flag specifically has to be considered in the context of its [the adder] circuit when making architecture decisions. The lack of control over flags leading to the inability to compile the same C program to the same assembly operations with same behavior on different hardware demonstrates that a strictly C program is unable to express a consistent portable assembly program across all hardware.
      - nec4b 5y ago
        You are basically arguing CF == !CF. On MIPS you literally have to compare your result with an operand in order to establish if there was an overflow. On Arm you just look at the status register. When I was doing embedded development, I used C as portable assembly in practice and I know I wasn't the only one. Can you do everything an architecture enables you in a portable way? No, of course not.

nec4b 5y ago

Portable by definition means it cannot capture every specific detail of every architecture out there.

pjmlp 5y ago
By definition there isn't such thing as portable Assembly.
- deleted 5y ago
  [deleted]
- potiuper 5y ago
  Not by definition, but by practice as generating assembly is usually done to optimize performance. Languages restricted to generate a common feature set between isomorphic ISAs and bijectively associated assembly languages are at a performance disadvantage compared to the full ISA specific assembly.
potiuper 5y ago
The carry flag is not a specific architectural detail; it is part of the output from the prime example of a basic electronic circuit that is the digital adder.
- nec4b 5y ago
  It is an architectural detail. MIPS for example doesn't have a carry flag. You have to do the exact same procedure in assembly on MIPS you have described in your previous comment when doing multiple word addition.
  - potiuper 5y ago
    MIPS along with DEC alpha do not have adder circuits as a design goal was not to have flags. This along with the scenario of multi-precision arithmetic resulted in the implementation of multi-precision arithmetic on MIPS and DEC alpha using a form of branch like instruction with an ALU stage instead of the unavailable instructions dependent on the alternative adder circuits. The choice of having or not having a carry flag alone is not an architectural detail as the choices of [micro]architecture involve how operations are implemented in circuits/hardware. The carry flag specifically has to be considered in the context of its [the adder] circuit when making architecture decisions. The lack of control over flags leading to the inability to compile the same C program to the same assembly operations with same behavior on different hardware demonstrates that a strictly C program is unable to express a consistent portable assembly program across all hardware.
    - nec4b 5y ago
      You are basically arguing CF == !CF. On MIPS you literally have to compare your result with an operand in order to establish if there was an overflow. On Arm you just look at the status register. When I was doing embedded development, I used C as portable assembly in practice and I know I wasn't the only one. Can you do everything an architecture enables you in a portable way? No, of course not.