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[LangRef] Add elementtype attribute

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This adds an elementtype(<ty>) attribute, which can be used to
attach an element type to a pointer typed argument. It is similar
to byval/byref in purpose, but unlike those does not carry any
specific semantics by itself. However, certain intrinsics may
require it and interpret it in specific ways.

The in-tree use cases for this that I'm currently aware of are:

    call ptr @llvm.preserve.array.access.index.p0.p0(ptr elementtype(%ty) %base, i32 %dim, i32 %index)
    call ptr @llvm.preserve.struct.access.index.p0.p0(ptr elementtype(%ty) %base, i32 %gep_index, i32 %di_index)
    call token @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(void ()) @foo, i32 0, i32 0, i32 0, i32 0, ptr addrspace(1) %obj)

Notably, the gc.statepoint case needs a function as element type,
in which case the workaround of adding a separate %ty undef
argument would not work, as arguments cannot be unsized.

Differential Revision: https://reviews.llvm.org/D105407
This commit is contained in:
Nikita Popov 2021-07-04 15:52:20 +02:00
parent edc13daf17
commit 159ef87203
1 changed files with 16 additions and 0 deletions
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docs/LangRef.rst
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@ -1183,6 +1183,22 @@ Currently, only the following parameter attributes are defined:
The sret type argument specifies the in memory type, which must be
the same as the pointee type of the argument.
``elementtype(<ty>)``
The ``elementtype`` argument attribute can be used to specify a pointer
element type in a way that is compatible with `opaque pointers
<OpaquePointers.html>`.
The ``elementtype`` attribute by itself does not carry any specific
semantics. However, certain intrinsics may require this attribute to be
present and assign it particular semantics. This will be documented on
individual intrinsics.
The attribute may only be applied to pointer typed arguments of intrinsic
calls. It cannot be applied to non-intrinsic calls, and cannot be applied
to parameters on function declarations. For non-opaque pointers, the type
passed to ``elementtype`` must match the pointer element type.
.. _attr_align:
``align <n>`` or ``align(<n>)``