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Represent runtime preemption in the IR.

Authored by sfertile on Oct 26 2017, 8:00 AM.

Description

Represent runtime preemption in the IR.

Currently we do not represent runtime preemption in the IR, which has several
drawbacks:

  1. The semantics of GlobalValues differ depending on the object file format you are targeting (as well as the relocation-model and -fPIE value).
  2. We have no way of disabling inlining of run time interposable functions, since in the IR we only know if a function is link-time interposable. Because of this llvm cannot support elf-interposition semantics.
  3. In LTO builds of executables we will have extra knowledge that a symbol resolved to a local definition and can't be preemptable, but have no way to propagate that knowledge through the compiler.

This patch adds preemptability specifiers to the IR with the following meaning:

dso_local --> means the compiler may assume the symbol will resolve to a
definition within the current linkage unit and the symbol may be accessed
directly even if the definition is not within this compilation unit.

dso_preemptable --> means that the compiler must assume the GlobalValue may be
replaced with a definition from outside the current linkage unit at runtime.

To ease transitioning dso_preemptable is treated as a 'default' in that
low-level codegen will still do the same checks it did previously to see if a
symbol should be accessed indirectly. Eventually when IR producers emit the
specifiers on all Globalvalues we can change dso_preemptable to mean 'always
access indirectly', and remove the current logic.

Differential Revision: https://reviews.llvm.org/D20217

llvm-svn: 316668

Details

Committed
sfertileOct 26 2017, 8:00 AM
Differential Revision
D20217: Represent runtime preemption in the IR.
Parents
rG2232243863bc: AMDGPU: Handle s_buffer_load_dword hazard on SI
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