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

2 files changed, 13 insertions, 0 deletions

diff --git a/llvm/test/Assembler/dllimport-dsolocal-diag.ll b/llvm/test/Assembler/dllimport-dsolocal-diag.ll
new file mode 100644
index 00000000000..e68e800fdca
--- /dev/null
+++ b/llvm/test/Assembler/dllimport-dsolocal-diag.ll
@@ -0,0 +1,4 @@
+; RUN: not llvm-as < %s -o /dev/null 2>&1 | FileCheck %s
+
+declare dso_local dllimport void @fun()
+; CHECK: error: dso_location and DLL-StorageClass mismatch
diff --git a/llvm/test/Assembler/ifunc-dsolocal-daig.ll b/llvm/test/Assembler/ifunc-dsolocal-daig.ll
new file mode 100644
index 00000000000..86e941d6cac
--- /dev/null
+++ b/llvm/test/Assembler/ifunc-dsolocal-daig.ll
@@ -0,0 +1,9 @@
+; RUN: not llvm-as < %s -o /dev/null 2>&1 | FileCheck %s
+
+@foo = dso_local ifunc i32 (i32), i64 ()* @foo_ifunc
+; CHECK: error: dso_local is invalid on ifunc
+
+define internal i64 @foo_ifunc() {
+entry:
+  ret i64 0
+}