[lld] 2f65166 - [lld:elf] Weaken the requirement for a computed binding to be STB_LOCAL
Nathan Lanza via llvm-commits
llvm-commits at lists.llvm.org
Mon May 24 20:33:12 PDT 2021
Author: Nathan Lanza
Date: 2021-05-24T23:32:21-04:00
New Revision: 2f6516605615e99221181ec2a1947f9ad55aa9d3
URL: https://github.com/llvm/llvm-project/commit/2f6516605615e99221181ec2a1947f9ad55aa9d3
DIFF: https://github.com/llvm/llvm-project/commit/2f6516605615e99221181ec2a1947f9ad55aa9d3.diff
LOG: [lld:elf] Weaken the requirement for a computed binding to be STB_LOCAL
Given the following scenario:
```
// Cat.cpp
struct Animal { virtual void makeNoise() const = 0; };
struct Cat : Animal { void makeNoise() const override; };
extern "C" int puts(char const *);
void Cat::makeNoise() const { puts("Meow"); }
void doThingWithCat(Animal *a) { static_cast<Cat *>(a)->makeNoise(); }
// CatUser.cpp
struct Animal { virtual void makeNoise() const = 0; };
struct Cat : Animal { void makeNoise() const override; };
void doThingWithCat(Animal *a);
void useDoThingWithCat() {
Cat *d = new Cat;
doThingWithCat(d);
}
// cat.ver
{
global: _Z17useDoThingWithCatv;
local: *;
};
$ clang++ Cat.cpp CatUser.cpp -fpic -flto=thin -fwhole-program-vtables
-shared -O3 -fuse-ld=lld -Wl,--lto-whole-program-visibility
-Wl,--version-script,cat.ver
```
We cannot devirtualize `Cat::makeNoise`. The issue is complex:
Due to `-fsplit-lto-unit` and usage of type metadata, we place the Cat
vtable declaration into module 0 and the Cat vtable definition with type
metadata into module 1, causing duplicate entries (Undefined followed by
Defined) in the `lto::InputFile::symbols()` output.
In `BitcodeFile::parse`, after processing the `Undefined` then the
`Defined`, the final state is `Defined`.
In `BitcodeCompiler::add`, for the first symbol, `computeBinding`
returns `STB_LOCAL`, then we reset it to `Undefined` because it is
prevailing (`versionId` is `preserved`). For the second symbol, because
the state is now `Undefined`, `computeBinding` returns `STB_GLOBAL`,
causing `ExportDynamic` to be true and suppressing devirtualization.
In D77280, the `computeBinding` change used a stricter `isDefined()`
condition to make weak``Lazy` symbol work.
This patch relaxes the condition to weaker `!isLazy()` to keep it
working while making the devirtualization work as well.
Differential Revision: https://reviews.llvm.org/D98686
Added:
lld/test/ELF/lto/devirt_split_unit_localize.ll
Modified:
lld/ELF/Symbols.cpp
Removed:
################################################################################
diff --git a/lld/ELF/Symbols.cpp b/lld/ELF/Symbols.cpp
index 39ff36af86663..1039be369d9e4 100644
--- a/lld/ELF/Symbols.cpp
+++ b/lld/ELF/Symbols.cpp
@@ -279,7 +279,7 @@ uint8_t Symbol::computeBinding() const {
if (config->relocatable)
return binding;
if ((visibility != STV_DEFAULT && visibility != STV_PROTECTED) ||
- (versionId == VER_NDX_LOCAL && isDefined()))
+ (versionId == VER_NDX_LOCAL && !isLazy()))
return STB_LOCAL;
if (!config->gnuUnique && binding == STB_GNU_UNIQUE)
return STB_GLOBAL;
diff --git a/lld/test/ELF/lto/devirt_split_unit_localize.ll b/lld/test/ELF/lto/devirt_split_unit_localize.ll
new file mode 100644
index 0000000000000..d11d659a4caa3
--- /dev/null
+++ b/lld/test/ELF/lto/devirt_split_unit_localize.ll
@@ -0,0 +1,114 @@
+; REQUIRES: x86
+;; Test that that a vtable defined locally in one module but external in another
+;; does not prevent devirtualization.
+
+;; Hybrid WPD
+; RUN: split-file %s %t
+; RUN: opt --thinlto-bc --thinlto-split-lto-unit -o %t/Cat.o %t/Cat.ll
+; RUN: opt --thinlto-bc --thinlto-split-lto-unit -o %t/User.o %t/User.ll
+; RUN: echo '{ global: _Z17useDoThingWithCatv; local: *; };' > %t/version.exp
+
+; RUN: ld.lld %t/Cat.o %t/User.o -shared -o %t/libA.so -save-temps --lto-whole-program-visibility \
+; RUN: -mllvm -pass-remarks=. --version-script %t/version.exp 2>&1 | \
+; RUN: FileCheck %s --check-prefix=REMARK
+
+; REMARK-DAG: <unknown>:0:0: single-impl: devirtualized a call to _ZNK3Cat9makeNoiseEv
+; REMARK-DAG: <unknown>:0:0: single-impl: devirtualized a call to _ZNK3Cat9makeNoiseEv
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+;--- Cat.ll
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.Cat = type { %struct.Animal }
+%struct.Animal = type { i32 (...)** }
+
+$_ZTS6Animal = comdat any
+
+$_ZTI6Animal = comdat any
+
+ at .str = private unnamed_addr constant [5 x i8] c"Meow\00", align 1
+ at _ZTV3Cat = dso_local unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* bitcast ({ i8*, i8*, i8* }* @_ZTI3Cat to i8*), i8* bitcast (void (%struct.Cat*)* @_ZNK3Cat9makeNoiseEv to i8*)] }, align 8, !type !0, !type !1, !type !2, !type !3
+ at _ZTVN10__cxxabiv120__si_class_type_infoE = external dso_local global i8*
+ at _ZTS3Cat = dso_local constant [5 x i8] c"3Cat\00", align 1
+ at _ZTVN10__cxxabiv117__class_type_infoE = external dso_local global i8*
+ at _ZTS6Animal = linkonce_odr dso_local constant [8 x i8] c"6Animal\00", comdat, align 1
+ at _ZTI6Animal = linkonce_odr dso_local constant { i8*, i8* } { i8* bitcast (i8** getelementptr inbounds (i8*, i8** @_ZTVN10__cxxabiv117__class_type_infoE, i64 2) to i8*), i8* getelementptr inbounds ([8 x i8], [8 x i8]* @_ZTS6Animal, i32 0, i32 0) }, comdat, align 8
+ at _ZTI3Cat = dso_local constant { i8*, i8*, i8* } { i8* bitcast (i8** getelementptr inbounds (i8*, i8** @_ZTVN10__cxxabiv120__si_class_type_infoE, i64 2) to i8*), i8* getelementptr inbounds ([5 x i8], [5 x i8]* @_ZTS3Cat, i32 0, i32 0), i8* bitcast ({ i8*, i8* }* @_ZTI6Animal to i8*) }, align 8
+
+define dso_local void @_ZNK3Cat9makeNoiseEv(%struct.Cat* nocapture nonnull readnone dereferenceable(8) %this) unnamed_addr align 2 {
+entry:
+ %call = tail call i32 @puts(i8* nonnull dereferenceable(1) getelementptr inbounds ([5 x i8], [5 x i8]* @.str, i64 0, i64 0))
+ ret void
+}
+
+declare dso_local noundef i32 @puts(i8* nocapture noundef readonly) local_unnamed_addr
+
+define dso_local void @_Z14doThingWithCatP6Animal(%struct.Animal* %a) local_unnamed_addr {
+entry:
+ %tobool.not = icmp eq %struct.Animal* %a, null
+ br i1 %tobool.not, label %if.end, label %if.then
+
+if.then: ; preds = %entry
+ %0 = bitcast %struct.Animal* %a to %struct.Cat*
+ %1 = bitcast %struct.Animal* %a to void (%struct.Cat*)***
+ %vtable = load void (%struct.Cat*)**, void (%struct.Cat*)*** %1, align 8, !tbaa !4
+ %2 = bitcast void (%struct.Cat*)** %vtable to i8*
+ %3 = tail call i1 @llvm.type.test(i8* %2, metadata !"_ZTS3Cat")
+ tail call void @llvm.assume(i1 %3)
+ %4 = load void (%struct.Cat*)*, void (%struct.Cat*)** %vtable, align 8
+ tail call void %4(%struct.Cat* nonnull dereferenceable(8) %0)
+ br label %if.end
+
+if.end: ; preds = %if.then, %entry
+ ret void
+}
+
+declare i1 @llvm.type.test(i8*, metadata)
+
+declare void @llvm.assume(i1 noundef)
+
+!0 = !{i64 16, !"_ZTS3Cat"}
+!1 = !{i64 16, !"_ZTSM3CatKFvvE.virtual"}
+!2 = !{i64 16, !"_ZTS6Animal"}
+!3 = !{i64 16, !"_ZTSM6AnimalKFvvE.virtual"}
+!4 = !{!5, !5, i64 0}
+!5 = !{!"vtable pointer", !6, i64 0}
+!6 = !{!"Simple C++ TBAA"}
+
+;--- User.ll
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.Animal = type { i32 (...)** }
+%struct.Cat = type { %struct.Animal }
+
+ at _ZTV3Cat = available_externally dso_local unnamed_addr constant { [3 x i8*] } { [3 x i8*] [i8* null, i8* bitcast (i8** @_ZTI3Cat to i8*), i8* bitcast (void (%struct.Cat*)* @_ZNK3Cat9makeNoiseEv to i8*)] }, align 8, !type !0, !type !1, !type !2, !type !3
+ at _ZTI3Cat = external dso_local constant i8*
+ at llvm.compiler.used = appending global [1 x i8*] [i8* bitcast ({ [3 x i8*] }* @_ZTV3Cat to i8*)], section "llvm.metadata"
+
+declare dso_local void @_ZNK3Cat9makeNoiseEv(%struct.Cat* nonnull dereferenceable(8)) unnamed_addr
+
+define dso_local void @_Z17useDoThingWithCatv() local_unnamed_addr {
+entry:
+ %call = tail call noalias nonnull dereferenceable(8) i8* @_Znwm(i64 8)
+ %0 = bitcast i8* %call to i32 (...)***
+ store i32 (...)** bitcast (i8** getelementptr inbounds ({ [3 x i8*] }, { [3 x i8*] }* @_ZTV3Cat, i64 0, inrange i32 0, i64 2) to i32 (...)**), i32 (...)*** %0, align 8, !tbaa !4
+ %1 = bitcast i8* %call to %struct.Animal*
+ tail call void @_Z14doThingWithCatP6Animal(%struct.Animal* nonnull %1)
+ ret void
+}
+
+declare dso_local nonnull i8* @_Znwm(i64) local_unnamed_addr
+
+declare dso_local void @_Z14doThingWithCatP6Animal(%struct.Animal*) local_unnamed_addr
+
+!0 = !{i64 16, !"_ZTS3Cat"}
+!1 = !{i64 16, !"_ZTSM3CatKFvvE.virtual"}
+!2 = !{i64 16, !"_ZTS6Animal"}
+!3 = !{i64 16, !"_ZTSM6AnimalKFvvE.virtual"}
+!4 = !{!5, !5, i64 0}
+!5 = !{!"vtable pointer", !6, i64 0}
+!6 = !{!"Simple C++ TBAA"}
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