PointerType API Change

Christopher,

The API for getting PointerType objects has just changed to make
Embedded C address space information explicit. The old semantics of
PointerType::get() now apply to PointerType::getUnqual(), which
returns a pointer in the generic address space. PointerType::get() now
requires both a type and an address space.

What is the reason of such change?

Christopher,

The API for getting PointerType objects has just changed to make
Embedded C address space information explicit. The old semantics of
PointerType::get() now apply to PointerType::getUnqual(), which
returns a pointer in the generic address space. PointerType::get() now
requires both a type and an address space.

What is the reason of such change?

Sorry for not providing that. Here’s the conversation with Chris:

Hi Anton,

Would it be possible to keep get() unchanged, with a default behaviour, plus
a warning? Otherwise everybody (assuming everybody gets type void*) will
have to update their LLVM passes, and either maintain two versions of the
passes or require their clients to use a certain LLVM version. Then passes
could be "address-space-safe" or not. If the default parameter value for
get() could be a unique ID for "not specified" instead of "the default
address space", then one should even be able to continue to use get()
isntead of sth like getQual(...).

Torvald

Would it be possible to keep get() unchanged, with a default behaviour, plus
a warning? Otherwise everybody (assuming everybody gets type void*) will
have to update their LLVM passes, and either maintain two versions of the
passes or require their clients to use a certain LLVM version.

AFAIK API compatibility is not guaranteed across LLVM point releases, so I believe clients are tied to a specific version of LLVM in any case.

Then passes
could be “address-space-safe” or not. If the default parameter value for
get() could be a unique ID for “not specified” instead of “the default
address space”, then one should even be able to continue to use get()
isntead of sth like getQual(…).

The reason for the change was to make it absolutely clear in the source where address space qualifiers are preserved/added or stripped from the pointer type. Allowing clients to use get() and then dynamically track “undefined” address spaces under the hood is counter to this goal.

Informally, what I'd like to have is getUnqual() semantics as default for
get(), thus giving you the same safety properties but without having to
change all occurrences. If clients do handle address spaces, they could use
getQual(...) and getUnqual().
I don't see how this would be counter to your goals. If a module with
address spaces comes along, the pass could still abort and thell the user
that it doesn't know how to handle this, which would give users a complete
incremental upgrade path.

I know that this approach might not really encourage developers to consider
address space issues. Are they important and widespread enough that
everybody should (or is proper address space handling trivial enough)?

Torvald

I don’t have particularly strong feelings about this, however Chris did mention that he would like passes to take address spaces into account. Handling them properly is pretty trivial, I believe.

Take a look at the this exchange on LLVM commits for some background discussion on this:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20071210/056223.html

My opinion is that we don't want to have "second rate" features that are subtly broken. In fact, Christophers conversion from using PointerType::get to use PointerType::getUnqual did expose several bugs (which were easy to fix once found).

Unfortunately, we don't guarantee API stability across llvm releases, so I think that this breakage is reasonable. It is also really trivial to hack around this locally while in migration: just default the address space specifier in PointerType::get() to default to 0 in your local tree.

-Chris