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by pseudohadamard 103 days ago 

  I think that any ISA designer who believes that omitting from the ISA the means for detecting integer overflow is a good idea deserves the death penalty
Given that the C standard (C99 ยง3.4.3/1) declares integer overflow to be UB which means the compiler can and often will do anything it damn well pleases with your code, I can understand why the RISC-V designers, under the influence of the stupidity of the C standard, could leave out overflow detection. I'm not saying it's a good idea, in fact it's complete and utter braindamage, but I can see where they got it from.
1 comments
snvzz 103 days ago
The premise that the ISA has no means for detecting integer overflow is false.

This is explicitly documented in the spec, even.

e.g.

    add   t2, t0, t1
    bltu  t2, t0, overflow
The implication that the ISA is not designed by competent engineers does not pass basic scrutiny, either.
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rep_lodsb 103 days ago
That only works for unsigned integers.
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camel-cdr 103 days ago
Signend 64-bit is the worst case. When I tried to enable overflow checking thr overhead of RISC-V and Arm was comparable: https://news.ycombinator.com/item?id=46588159#46668916
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snvzz 103 days ago
Refer to the spec for the official idioms to handle every case.
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rep_lodsb 103 days ago
Yes, you can detect signed overflow that way, but it's a lot more instructions so it won't be used in practice.

The designers of RISC-V included the bare minimum needed to compile C, everything else was deemed irrelevant.

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snvzz 103 days ago
>but it's a lot more instructions so it won't be used in practice.

It will be used when it needs to be handled. e.g. where elsewhere, an exception would actually handle it. Which is seldom the case.

More instructions doesn't mean slower, either. Superscalar machines have a hard time keeping themselves busy, and this is an easily parallelizable task.

>The designers of RISC-V included the bare minimum needed to compile C, everything else was deemed irrelevant.

Refer to "Computer Architecture: A Quantitative Approach" by by John L. Hennessy and David A. Patterson, for the actual methodology followed.

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