[llvm] 30e8f83 - [GlobalOpt] Don't replace alias with aliasee if either alias/aliasee may be preemptible
Fangrui Song via llvm-commits
llvm-commits at lists.llvm.org
Tue Feb 1 10:41:24 PST 2022
Author: Fangrui Song
Date: 2022-02-01T10:41:16-08:00
New Revision: 30e8f83c84c5a302a559722fc0d2973dc3f425ee
URL: https://github.com/llvm/llvm-project/commit/30e8f83c84c5a302a559722fc0d2973dc3f425ee
DIFF: https://github.com/llvm/llvm-project/commit/30e8f83c84c5a302a559722fc0d2973dc3f425ee.diff
LOG: [GlobalOpt] Don't replace alias with aliasee if either alias/aliasee may be preemptible
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
Added:
Modified:
llvm/docs/LangRef.rst
llvm/lib/Transforms/IPO/GlobalOpt.cpp
llvm/test/Transforms/GlobalOpt/2009-02-15-ResolveAlias.ll
llvm/test/Transforms/GlobalOpt/alias-resolve.ll
llvm/test/Transforms/GlobalOpt/alias-used-address-space.ll
llvm/test/Transforms/GlobalOpt/alias-used.ll
Removed:
################################################################################
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 40f23205e531e..920834b10897b 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -894,6 +894,11 @@ some can only be checked when producing an object file:
* No global value in the expression can be a declaration, since that
would require a relocation, which is not possible.
+* If either the alias or the aliasee may be replaced by a symbol outside the
+ module at link time or runtime, any optimization cannot replace the alias with
+ the aliasee, since the behavior may be
diff erent. The alias may be used as a
+ name guaranteed to point to the content in the current module.
+
.. _langref_ifunc:
IFuncs
diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index a29d4f60a653e..1cb32e32c895a 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -2225,6 +2225,13 @@ OptimizeGlobalAliases(Module &M,
for (GlobalValue *GV : Used.used())
Used.compilerUsedErase(GV);
+ // Return whether GV is explicitly or implicitly dso_local and not replaceable
+ // by another definition in the current linkage unit.
+ auto IsModuleLocal = [](GlobalValue &GV) {
+ return !GlobalValue::isInterposableLinkage(GV.getLinkage()) &&
+ (GV.isDSOLocal() || GV.isImplicitDSOLocal());
+ };
+
for (GlobalAlias &J : llvm::make_early_inc_range(M.aliases())) {
// Aliases without names cannot be referenced outside this module.
if (!J.hasName() && !J.isDeclaration() && !J.hasLocalLinkage())
@@ -2236,18 +2243,20 @@ OptimizeGlobalAliases(Module &M,
}
// If the alias can change at link time, nothing can be done - bail out.
- if (J.isInterposable())
+ if (!IsModuleLocal(J))
continue;
Constant *Aliasee = J.getAliasee();
GlobalValue *Target = dyn_cast<GlobalValue>(Aliasee->stripPointerCasts());
// We can't trivially replace the alias with the aliasee if the aliasee is
// non-trivial in some way. We also can't replace the alias with the aliasee
- // if the aliasee is interposable because aliases point to the local
- // definition.
+ // if the aliasee may be preemptible at runtime. On ELF, a non-preemptible
+ // alias can be used to access the definition as if preemption did not
+ // happen.
// TODO: Try to handle non-zero GEPs of local aliasees.
- if (!Target || Target->isInterposable())
+ if (!Target || !IsModuleLocal(*Target))
continue;
+
Target->removeDeadConstantUsers();
// Make all users of the alias use the aliasee instead.
diff --git a/llvm/test/Transforms/GlobalOpt/2009-02-15-ResolveAlias.ll b/llvm/test/Transforms/GlobalOpt/2009-02-15-ResolveAlias.ll
index c29753385a59c..ce7d4fcb8e253 100644
--- a/llvm/test/Transforms/GlobalOpt/2009-02-15-ResolveAlias.ll
+++ b/llvm/test/Transforms/GlobalOpt/2009-02-15-ResolveAlias.ll
@@ -2,13 +2,13 @@
define internal void @f() {
; CHECK-NOT: @f(
-; CHECK: define void @a
+; CHECK: define dso_local void @a
ret void
}
- at a = alias void (), void ()* @f
+ at a = dso_local alias void (), void ()* @f
-define void @g() {
+define hidden void @g() {
call void() @a()
ret void
}
diff --git a/llvm/test/Transforms/GlobalOpt/alias-resolve.ll b/llvm/test/Transforms/GlobalOpt/alias-resolve.ll
index b4bee769e93d2..efac61200a3ef 100644
--- a/llvm/test/Transforms/GlobalOpt/alias-resolve.ll
+++ b/llvm/test/Transforms/GlobalOpt/alias-resolve.ll
@@ -1,16 +1,19 @@
; RUN: opt < %s -passes=globalopt -S | FileCheck %s
@foo1 = alias void (), void ()* @foo2
+;; foo2 is dso_local and non-weak. Resolved.
; CHECK: @foo1 = alias void (), void ()* @bar2
- at foo2 = alias void(), void()* @bar1
-; CHECK: @foo2 = alias void (), void ()* @bar2
+ at foo2 = dso_local alias void(), void()* @bar1
+;; bar1 is dso_local and non-weak. Resolved.
+; CHECK: @foo2 = dso_local alias void (), void ()* @bar2
- at bar1 = alias void (), void ()* @bar2
-; CHECK: @bar1 = alias void (), void ()* @bar2
+ at bar1 = dso_local alias void (), void ()* @bar2
+; CHECK: @bar1 = dso_local alias void (), void ()* @bar2
- at weak1 = weak alias void (), void ()* @bar2
-; CHECK: @weak1 = weak alias void (), void ()* @bar2
+ at weak1 = weak dso_local alias void (), void ()* @bar2
+;; weak1 may be replaced with another definition in the linkage unit. Not resolved.
+; CHECK: @weak1 = weak dso_local alias void (), void ()* @bar2
@bar4 = private unnamed_addr constant [2 x i8*] zeroinitializer
@foo4 = weak_odr unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* @bar4, i32 0, i32 1)
@@ -19,10 +22,10 @@
@priva = private alias void (), void ()* @bar5
; CHECK: @priva = private alias void (), void ()* @bar5
-define void @bar2() {
+define dso_local void @bar2() {
ret void
}
-; CHECK: define void @bar2()
+; CHECK: define dso_local void @bar2()
define weak void @bar5() {
ret void
@@ -32,27 +35,32 @@ define weak void @bar5() {
define void @baz() {
entry:
call void @foo1()
-; CHECK: call void @bar2()
+;; foo1 is dso_preemptable. Not resolved.
+; CHECK: call void @foo1()
call void @foo2()
+;; foo2 is dso_local and non-weak. Resolved.
; CHECK: call void @bar2()
call void @bar1()
+;; bar1 is dso_local and non-weak. Resolved.
; CHECK: call void @bar2()
call void @weak1()
+;; weak1 is weak. Not resolved.
; CHECK: call void @weak1()
call void @priva()
+;; priva has a local linkage. Resolved.
; CHECK: call void @priva()
ret void
}
- at foo3 = alias void (), void ()* @bar3
+ at foo3 = dso_local alias void (), void ()* @bar3
; CHECK-NOT: bar3
define internal void @bar3() {
ret void
}
-;CHECK: define void @foo3
+;CHECK: define dso_local void @foo3
diff --git a/llvm/test/Transforms/GlobalOpt/alias-used-address-space.ll b/llvm/test/Transforms/GlobalOpt/alias-used-address-space.ll
index d119bfb305063..daf5acd8446c6 100644
--- a/llvm/test/Transforms/GlobalOpt/alias-used-address-space.ll
+++ b/llvm/test/Transforms/GlobalOpt/alias-used-address-space.ll
@@ -2,7 +2,7 @@
target datalayout = "p:32:32:32-p1:16:16:16"
- at c = addrspace(1) global i8 42
+ at c = hidden addrspace(1) global i8 42
@i = internal addrspace(1) global i8 42
diff --git a/llvm/test/Transforms/GlobalOpt/alias-used.ll b/llvm/test/Transforms/GlobalOpt/alias-used.ll
index 73dd604f2a71d..d8b7a765ed30f 100644
--- a/llvm/test/Transforms/GlobalOpt/alias-used.ll
+++ b/llvm/test/Transforms/GlobalOpt/alias-used.ll
@@ -1,6 +1,6 @@
; RUN: opt < %s -passes=globalopt -S | FileCheck %s
- at c = global i8 42
+ at c = dso_local global i8 42
@i = internal global i8 42
; CHECK: @ia = internal global i8 42
@@ -30,7 +30,7 @@
@ca = internal alias i8, i8* @c
; CHECK: @ca = internal alias i8, i8* @c
-define void @f() {
+define hidden void @f() {
ret void
}
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