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0 pointsTaniwha4y ago0 comments

Yeah but the code required for an overflow check is just one extra instruction (3 rather than 2)

0 comments

For generalised signed addition, the overhead is 3 instructions per addition. It can be one in specific contexts where more is known about the operands (e.g. addition of immediates).

It’s always 1 in x64/ARM64 as they have built-in support for overflow.

TaniwhaOP4y ago

you have to include the branch instruction too in any comparison

kccqzy4y ago

In x86 it is still one instruction: jc or jo after an addition.

brucehoult4y ago

And in RISC-V it's a "BLT sum,summand" after an unsigned addition, and a single branch instruction after a signed addition if you know the sign of one of them e.g. adding or subtracting a constant.

rurban4y ago

built-in support for overflow is not really builtin. There's only adc, but the C library has no such function, and compilers only got them added a few years ago. Almost nobody uses them, as they introduce a dependency. Most workaround that in much slower ways, worse than RISC-V

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