[PATCH] D32248: CodeGen: Cast alloca to expected address space

[Yaxun Liu via Phabricator via cfe-commits]

yaxunl added inline comments.


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Comment at: lib/CodeGen/TargetInfo.cpp:7296
+  unsigned getASTAllocaAddressSpace() const override {
+    return LangAS::Count + getABIInfo().getDataLayout().getAllocaAddrSpace();
+  }
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rjmccall wrote:
> yaxunl wrote:
> > rjmccall wrote:
> > > Can we rename LangAS::Count to something more meaningful like LangAS::FirstTargetAddressSpace?  I think this would clarify a lot of code.
> > > 
> > > Is the OpenCL special case in ASTContext::getTargetAddressSpace still correct with this patch?  A pointer in LangAS::Default should be lowered as a generic pointer instead of a pointer into the alloca address space, right?
> > Will do.
> > 
> > The OpenCL special case in ASTContext::getTargetAddressSpace is still correct with this patch. In OpenCL private address space is still represented in AST by LangAS::Default, so a pointer in LangAS::Default should be lowered as a pointer to alloca address space.
> Then I don't understand.  If __private is always the alloca address space, why does IRGen have to add addrspace casts after allocating local variables in it?  IRGen's invariant is that the value stored in LocalDeclMap for a variable of AST type T is a value of type [[T]] addrspace([[AS]]) *, where [[T]] is the lowered IR type for T and [[AS]] is the target address space of the variable.  For auto variables, AS is always LangAS::Default, which according to you means that [[AS]] is always LLVM's notion of the alloca address space, which is exactly the type produced by alloca.  So why is there ever a cast?
> 
> My understanding was that, on your target, __private is (at least sometimes) a 64-bit extension of the alloca address space, which is 32-bit.  But that makes __private a different address space from the alloca address space, so the special case in getTargetAddressSpace is still wrong, because that special case means that a pointer-to-__private type will be lowered to be a 32-bit pointer type.
> 
> Also, I'm not sure why the normal AddrSpaceMap mechanism isn't sufficient for mapping LangAS::Default.
This work is mostly for C++.

For OpenCL, the default address space is mapped to alloca address space (5), there is no address space cast inserted.

For C++, the default address space is mapped to generic address space (0), therefore the address space cast is needed.


https://reviews.llvm.org/D32248