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

Nikita Popov via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Wed Jan 19 00:16:39 PST 2022

nikic 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;
dblaikie wrote:
> 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.
As we're storing pointers to core LLVM types here (Value and Type), I don't think it's possible to over-align them.

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