c6692f819e
Generalize D99629 for ELF. A default visibility non-local symbol is preemptible
in a -shared link. `isInterposable` is an insufficient condition.
Moreover, a non-preemptible alias may be referenced in a sub constant expression
which intends to lower to a PC-relative relocation. Replacing the alias with a
preemptible aliasee may introduce a linker error.
Respect dso_preemptable and suppress optimization to fix the abose issues. With
the change, `alias = 345` will not be rewritten to use aliasee in a `-fpic`
compile.
```
int aliasee;
extern int alias __attribute__((alias("aliasee"), visibility("hidden")));
void foo() { alias = 345; } // intended to access the local copy
```
While here, refine the condition for the alias as well.
For some binary formats like COFF, `isInterposable` is a sufficient condition.
But I think canonicalization for the changed case has little advantage, so I
don't bother to add the `Triple(M.getTargetTriple()).isOSBinFormatELF()` or
`getPICLevel/getPIELevel` complexity.
For instrumentations, it's recommended not to create aliases that refer to
globals that have a weak linkage or is preemptible. However, the following is
supported and the IR needs to handle such cases.
```
int aliasee __attribute__((weak));
extern int alias __attribute__((alias("aliasee")));
```
There are other places where GlobalAlias isInterposable usage may need to be
fixed.
Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D107249
25 lines
377 B
LLVM
25 lines
377 B
LLVM
; RUN: opt < %s -passes=globalopt -S | FileCheck %s
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define internal void @f() {
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; CHECK-NOT: @f(
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; CHECK: define dso_local void @a
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ret void
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}
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@a = dso_local alias void (), void ()* @f
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define hidden void @g() {
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call void() @a()
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ret void
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}
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@b = internal alias void (), void ()* @g
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; CHECK-NOT: @b
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define void @h() {
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call void() @b()
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; CHECK: call void @g
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ret void
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}
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