[PATCH] D117262: [NFC] Store Address's alignment into PointerIntPairs

David Blaikie via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Tue Jan 18 23:02:57 PST 2022

dblaikie added inline comments.

Comment at: clang/lib/CodeGen/Address.h:30
+  // Int portion stores lower 3 bits of the log of the alignment.
+  llvm::PointerIntPair<llvm::Type *, 3, unsigned> ElementType;
aeubanks wrote:
> nikic wrote:
> > Are we guaranteed 3 bits even on 32-bit architectures?
> Apparently not, a static assert fires for a 32-bit build of clang. I was assuming 3 was fine based on the PointerIntPair comments.
> ```
> /// PointerIntPair - This class implements a pair of a pointer and small
> /// integer.  It is designed to represent this in the space required by one
> /// pointer by bitmangling the integer into the low part of the pointer.  This
> /// can only be done for small integers: typically up to 3 bits, but it depends
> /// on the number of bits available according to PointerLikeTypeTraits for the
> /// type.
> ```
> I suppose we could have a separate 32-bit vs 64-bit implementation of `Address`, although that's not very nice.
You can get more spare bits by overaligning the type that you're pointing to, if that's suitable/workable. We do that in a few places in LLVM for this sort of reason.

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