[PATCH] D74361: [Clang] Uninitialize attribute on global variables

[Jon Chesterfield via Phabricator via cfe-commits]

JonChesterfield created this revision.
JonChesterfield added reviewers: kcc, rjmccall, rsmith, glider, vitalybuka, pcc, eugenis, vlad.tsyrklevich, jdoerfert, gregrodgers.
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[Clang] Uninitialize attribute on global variables

Extends D54604 <https://reviews.llvm.org/D54604> to permit [[uninitialized]] on global and static variables

Initializing global variables is very cheap on hosted implementations. The
C semantics of zero initializing globals work very well there. It is not
necessarily cheap on freestanding implementations. Where there is no loader
available, code must be emitted near the start point to write the appropriate
values into memory.

At present, external variables can be declared in C++ and definitions provided
in assembly (or IR) to achive this effect. This patch removes a restriction on
the existing attribute in order to remove this reason for writing assembly for
performance sensitive freestanding implementations.

A close analogue in tree is LDS memory for amdgcn, where the kernel is
responsible for initializing the memory after it starts executing on the gpu.
Uninitalized variables in LDS are observably cheaper than zero initialized.

Patch follows the cuda __shared__ variable implementation which also produces
undef global variables, and reuses the [[uninitialized]] attribute from auto
variable initialisation. I think the existing docs are still appropriate.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D74361

Files:
  clang/include/clang/Basic/Attr.td
  clang/lib/CodeGen/CGDecl.cpp
  clang/lib/CodeGen/CodeGenModule.cpp
  clang/lib/Sema/SemaDeclAttr.cpp
  clang/test/CodeGenCXX/attribute_uninitialized.cpp
  clang/test/Misc/pragma-attribute-supported-attributes-list.test
  clang/test/Sema/attr-uninitialized.c

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