[llvm-dev] PointerIntPair

[Paul C. Anagnostopoulos via llvm-dev]

Yes, sorry, I meant they are not orthogonal. I'll get a better night's sleep tonight, I promise. It's not like I'm going to a party or anything.

I found three places in LLVM that rely on 3 bits being available, and plenty of places that rely on 2. So I'll be fine with a 2-bit enum. It's a simple change.


At 12/31/2020 03:33 PM, David Blaikie wrote:
>On Thu, Dec 31, 2020 at 11:13 AM Paul C. Anagnostopoulos via llvm-dev <<mailto:llvm-dev at lists.llvm.org>llvm-dev at lists.llvm.org> wrote:
>The bits are orthogonal, so I think an enum makes more sense.
>
>I'm not quite following this - the enum would be used if these two properties are mutually exclusive, right? (ie: you can have one or the other, but not both) - that's not what I'd describe as "orthogonal" - orthogonal things vary independently of one another. (ie: you can have one feature, or the other, or neither, or both) If these properties can't both be enabled at the same time, then they're not orthogonal - they're mutually exclusive.
>Â 
>I take it from your response that you think a PointerIntPair is a fine thing to use.
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>There are no alignment requirements on an X86, for example. I presume that compilers impose alignments anyway, but are they consistent enough to rely on?
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>Depends what you're pointing to - if you were pointing to a char it'll have alignment 1 (necessary - if you had an array of characters you can't have padding between them, so you'll have pointers with no alignment/no spare low bits).
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>If you're only pointing to malloc/new'd blocks, you've got some wiggle room imposed by the allocator. In some places LLVM code explicitly overaligns certain types to provide more spare low bits in their pointers.
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>At 12/31/2020 01:51 PM, Craig Topper wrote:
>>A PointerIntPair can be placed in the pointer part of another PointerIntPair. So you can nest them to have 2 booleans without creating an enum. This works because PointerIntPair always puts the bit in the highest bit possible leaving the low bits free to be used by another int.
>>
>>The number of bits available in a pointer depends on the alignment requirement of the type the pointer points to. If it’s as a class/struct it depends on the largest alignment required by its fields. The alignof operator is used to check the alignment.