[lld] [llvm] [WebAssembly] Add segment NO_STRIP flag to support private retained data (PR #81539)
Yuta Saito via llvm-commits
llvm-commits at lists.llvm.org
Tue Feb 13 01:39:02 PST 2024
https://github.com/kateinoigakukun updated https://github.com/llvm/llvm-project/pull/81539
>From a37d9c96d5ed7a0cba8e89694d91a0fa35eee273 Mon Sep 17 00:00:00 2001
From: Yuta Saito <kateinoigakukun at gmail.com>
Date: Thu, 1 Feb 2024 06:32:42 +0000
Subject: [PATCH] [WebAssembly] Add segment RETAIN flag to support private
retained data
In WebAssembly, we have `WASM_SYMBOL_RETAIN` symbol flag to mark the
referenced content as retained. However, the flag is not enough to
express retained data that is not referenced by any symbol.
This patch adds a new segment flag WASM_SEG_FLAG_RETAIN to support
"private" linkage data that is retained by llvm.used.
This kind of data that is not referenced but must be retained is usually
used with encapsulation symbols (__start/__stop). Swift runtime uses
this technique and depends on the fact "all metadata sections in live
objects are retained", which was not guaranteed with `--gc-sections`
before this patch.
---
lld/test/wasm/no-strip-segment.s | 62 +++++++++++++++
lld/wasm/InputChunks.h | 1 +
lld/wasm/MarkLive.cpp | 39 ++++++++--
llvm/include/llvm/BinaryFormat/Wasm.h | 1 +
.../CodeGen/TargetLoweringObjectFileImpl.h | 3 +
.../CodeGen/TargetLoweringObjectFileImpl.cpp | 29 +++++--
llvm/lib/MC/MCParser/WasmAsmParser.cpp | 3 +
llvm/lib/MC/MCSectionWasm.cpp | 2 +
llvm/lib/ObjectYAML/WasmYAML.cpp | 1 +
llvm/test/CodeGen/WebAssembly/no-strip.ll | 22 ++++++
llvm/test/MC/WebAssembly/no-dead-strip.ll | 76 +++++++++++++++----
11 files changed, 210 insertions(+), 29 deletions(-)
create mode 100644 lld/test/wasm/no-strip-segment.s
create mode 100644 llvm/test/CodeGen/WebAssembly/no-strip.ll
diff --git a/lld/test/wasm/no-strip-segment.s b/lld/test/wasm/no-strip-segment.s
new file mode 100644
index 00000000000000..57bbf0bb9f24fa
--- /dev/null
+++ b/lld/test/wasm/no-strip-segment.s
@@ -0,0 +1,62 @@
+# RUN: split-file %s %t
+# RUN: llvm-mc -filetype=obj --triple=wasm32-unknown-unknown -o %t/main.o %t/main.s
+# RUN: llvm-mc -filetype=obj --triple=wasm32-unknown-unknown -o %t/liba_x.o %t/liba_x.s
+# RUN: llvm-mc -filetype=obj --triple=wasm32-unknown-unknown -o %t/liba_y.o %t/liba_y.s
+# RUN: rm -f %t/liba.a
+# RUN: llvm-ar rcs %t/liba.a %t/liba_x.o %t/liba_y.o
+# RUN: wasm-ld %t/main.o %t/liba.a --gc-sections -o %t/main.wasm --print-gc-sections | FileCheck %s --check-prefix=GC
+# RUN: obj2yaml %t/main.wasm | FileCheck %s
+
+# --gc-sections should remove non-retained and unused "weathers" section from live object liba_x.o
+# GC: removing unused section {{.*}}/liba.a(liba_x.o):(weathers)
+# Should not remove retained "greetings" sections from live objects main.o and liba_x.o
+# GC-NOT: removing unused section %t/main.o:(greetings)
+# GC-NOT: removing unused section %t/liba_x.o:(greetings)
+
+# Note: All symbols are private so that they don't join the symbol table.
+
+#--- main.s
+ .functype grab_liba () -> ()
+ .globl _start
+_start:
+ .functype _start () -> ()
+ call grab_liba
+ end_function
+
+ .section greetings,"R",@
+ .asciz "hello"
+ .section weathers,"R",@
+ .asciz "cloudy"
+
+#--- liba_x.s
+ .globl grab_liba
+grab_liba:
+ .functype grab_liba () -> ()
+ end_function
+
+ .section greetings,"R",@
+ .asciz "world"
+ .section weathers,"",@
+ .asciz "rainy"
+
+#--- liba_y.s
+ .section greetings,"R",@
+ .asciz "bye"
+
+
+# "greetings" section
+# CHECK: - Type: DATA
+# CHECK: Segments:
+# CHECK: - SectionOffset: 7
+# CHECK: InitFlags: 0
+# CHECK: Offset:
+# CHECK: Opcode: I32_CONST
+# CHECK: Value: 1024
+# CHECK: Content: 68656C6C6F00776F726C6400
+# "weahters" section.
+# CHECK: - SectionOffset: 25
+# CHECK: InitFlags: 0
+# CHECK: Offset:
+# CHECK: Opcode: I32_CONST
+# CHECK: Value: 1036
+# CHECK: Content: 636C6F75647900
diff --git a/lld/wasm/InputChunks.h b/lld/wasm/InputChunks.h
index ad1d45e335eac9..cf8a5249b19a00 100644
--- a/lld/wasm/InputChunks.h
+++ b/lld/wasm/InputChunks.h
@@ -81,6 +81,7 @@ class InputChunk {
void generateRelocationCode(raw_ostream &os) const;
bool isTLS() const { return flags & llvm::wasm::WASM_SEG_FLAG_TLS; }
+ bool isRetained() const { return flags & llvm::wasm::WASM_SEG_FLAG_RETAIN; }
ObjFile *file;
OutputSection *outputSec = nullptr;
diff --git a/lld/wasm/MarkLive.cpp b/lld/wasm/MarkLive.cpp
index b8ab7741ff1cb3..0f1c50854fdfd1 100644
--- a/lld/wasm/MarkLive.cpp
+++ b/lld/wasm/MarkLive.cpp
@@ -40,7 +40,9 @@ class MarkLive {
private:
void enqueue(Symbol *sym);
+ void enqueue(InputChunk *chunk);
void enqueueInitFunctions(const ObjFile *sym);
+ void enqueueRetainedSegments(const ObjFile *file);
void mark();
bool isCallCtorsLive();
@@ -56,21 +58,30 @@ void MarkLive::enqueue(Symbol *sym) {
LLVM_DEBUG(dbgs() << "markLive: " << sym->getName() << "\n");
InputFile *file = sym->getFile();
- bool needInitFunctions = file && !file->isLive() && sym->isDefined();
+ bool markImplicitDeps = file && !file->isLive() && sym->isDefined();
sym->markLive();
- // Mark ctor functions in the object that defines this symbol live.
- // The ctor functions are all referenced by the synthetic callCtors
- // function. However, this function does not contain relocations so we
- // have to manually mark the ctors as live.
- if (needInitFunctions)
+ if (markImplicitDeps) {
+ // Mark ctor functions in the object that defines this symbol live.
+ // The ctor functions are all referenced by the synthetic callCtors
+ // function. However, this function does not contain relocations so we
+ // have to manually mark the ctors as live.
enqueueInitFunctions(cast<ObjFile>(file));
+ // Mark retained segments in the object that defines this symbol live.
+ enqueueRetainedSegments(cast<ObjFile>(file));
+ }
if (InputChunk *chunk = sym->getChunk())
queue.push_back(chunk);
}
+void MarkLive::enqueue(InputChunk *chunk) {
+ LLVM_DEBUG(dbgs() << "markLive: " << toString(chunk) << "\n");
+ chunk->live = true;
+ queue.push_back(chunk);
+}
+
// The ctor functions are all referenced by the synthetic callCtors
// function. However, this function does not contain relocations so we
// have to manually mark the ctors as live.
@@ -83,6 +94,14 @@ void MarkLive::enqueueInitFunctions(const ObjFile *obj) {
}
}
+// Mark segments flagged by segment-level no-strip. Segment-level no-strip is
+// usually used to retain segments without having symbol table entry.
+void MarkLive::enqueueRetainedSegments(const ObjFile *file) {
+ for (InputChunk *chunk : file->segments)
+ if (chunk->isRetained())
+ enqueue(chunk);
+}
+
void MarkLive::run() {
// Add GC root symbols.
if (!config->entry.empty())
@@ -96,10 +115,14 @@ void MarkLive::run() {
if (WasmSym::callDtors)
enqueue(WasmSym::callDtors);
- // Enqueue constructors in objects explicitly live from the command-line.
for (const ObjFile *obj : ctx.objectFiles)
- if (obj->isLive())
+ if (obj->isLive()) {
+ // Enqueue constructors in objects explicitly live from the command-line.
enqueueInitFunctions(obj);
+ // Enqueue retained segments in objects explicitly live from the
+ // command-line.
+ enqueueRetainedSegments(obj);
+ }
mark();
diff --git a/llvm/include/llvm/BinaryFormat/Wasm.h b/llvm/include/llvm/BinaryFormat/Wasm.h
index aec6ea0b757799..aa9609091e08c8 100644
--- a/llvm/include/llvm/BinaryFormat/Wasm.h
+++ b/llvm/include/llvm/BinaryFormat/Wasm.h
@@ -216,6 +216,7 @@ enum WasmSymbolType : unsigned {
enum WasmSegmentFlag : unsigned {
WASM_SEG_FLAG_STRINGS = 0x1,
WASM_SEG_FLAG_TLS = 0x2,
+ WASM_SEG_FLAG_RETAIN = 0x4,
};
// Kinds of tag attributes.
diff --git a/llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h b/llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
index 4a7c1ca4a57182..8eef45ce565deb 100644
--- a/llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
+++ b/llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
@@ -207,11 +207,14 @@ class TargetLoweringObjectFileCOFF : public TargetLoweringObjectFile {
class TargetLoweringObjectFileWasm : public TargetLoweringObjectFile {
mutable unsigned NextUniqueID = 0;
+ SmallPtrSet<GlobalObject *, 2> Used;
public:
TargetLoweringObjectFileWasm() = default;
~TargetLoweringObjectFileWasm() override = default;
+ void getModuleMetadata(Module &M) override;
+
MCSection *getExplicitSectionGlobal(const GlobalObject *GO, SectionKind Kind,
const TargetMachine &TM) const override;
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 2c5b0b3d0c44c7..0cc05a6846c208 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -2141,7 +2141,7 @@ static const Comdat *getWasmComdat(const GlobalValue *GV) {
return C;
}
-static unsigned getWasmSectionFlags(SectionKind K) {
+static unsigned getWasmSectionFlags(SectionKind K, bool Retain) {
unsigned Flags = 0;
if (K.isThreadLocal())
@@ -2150,11 +2150,22 @@ static unsigned getWasmSectionFlags(SectionKind K) {
if (K.isMergeableCString())
Flags |= wasm::WASM_SEG_FLAG_STRINGS;
+ if (Retain)
+ Flags |= wasm::WASM_SEG_FLAG_RETAIN;
+
// TODO(sbc): Add suport for K.isMergeableConst()
return Flags;
}
+void TargetLoweringObjectFileWasm::getModuleMetadata(Module &M) {
+ SmallVector<GlobalValue *, 4> Vec;
+ collectUsedGlobalVariables(M, Vec, false);
+ for (GlobalValue *GV : Vec)
+ if (auto *GO = dyn_cast<GlobalObject>(GV))
+ Used.insert(GO);
+}
+
MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal(
const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
// We don't support explict section names for functions in the wasm object
@@ -2178,16 +2189,18 @@ MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal(
Group = C->getName();
}
- unsigned Flags = getWasmSectionFlags(Kind);
+ unsigned Flags = getWasmSectionFlags(Kind, Used.count(GO));
MCSectionWasm *Section = getContext().getWasmSection(
Name, Kind, Flags, Group, MCContext::GenericSectionID);
return Section;
}
-static MCSectionWasm *selectWasmSectionForGlobal(
- MCContext &Ctx, const GlobalObject *GO, SectionKind Kind, Mangler &Mang,
- const TargetMachine &TM, bool EmitUniqueSection, unsigned *NextUniqueID) {
+static MCSectionWasm *
+selectWasmSectionForGlobal(MCContext &Ctx, const GlobalObject *GO,
+ SectionKind Kind, Mangler &Mang,
+ const TargetMachine &TM, bool EmitUniqueSection,
+ unsigned *NextUniqueID, bool Retain) {
StringRef Group = "";
if (const Comdat *C = getWasmComdat(GO)) {
Group = C->getName();
@@ -2212,7 +2225,7 @@ static MCSectionWasm *selectWasmSectionForGlobal(
(*NextUniqueID)++;
}
- unsigned Flags = getWasmSectionFlags(Kind);
+ unsigned Flags = getWasmSectionFlags(Kind, Retain);
return Ctx.getWasmSection(Name, Kind, Flags, Group, UniqueID);
}
@@ -2230,9 +2243,11 @@ MCSection *TargetLoweringObjectFileWasm::SelectSectionForGlobal(
else
EmitUniqueSection = TM.getDataSections();
EmitUniqueSection |= GO->hasComdat();
+ bool Retain = Used.count(GO);
+ EmitUniqueSection |= Retain;
return selectWasmSectionForGlobal(getContext(), GO, Kind, getMangler(), TM,
- EmitUniqueSection, &NextUniqueID);
+ EmitUniqueSection, &NextUniqueID, Retain);
}
bool TargetLoweringObjectFileWasm::shouldPutJumpTableInFunctionSection(
diff --git a/llvm/lib/MC/MCParser/WasmAsmParser.cpp b/llvm/lib/MC/MCParser/WasmAsmParser.cpp
index 97045495a60dec..b95ee33debc37b 100644
--- a/llvm/lib/MC/MCParser/WasmAsmParser.cpp
+++ b/llvm/lib/MC/MCParser/WasmAsmParser.cpp
@@ -115,6 +115,9 @@ class WasmAsmParser : public MCAsmParserExtension {
case 'S':
flags |= wasm::WASM_SEG_FLAG_STRINGS;
break;
+ case 'R':
+ flags |= wasm::WASM_SEG_FLAG_RETAIN;
+ break;
default:
return -1U;
}
diff --git a/llvm/lib/MC/MCSectionWasm.cpp b/llvm/lib/MC/MCSectionWasm.cpp
index e90f401b1efa1f..e3761820bb4c3e 100644
--- a/llvm/lib/MC/MCSectionWasm.cpp
+++ b/llvm/lib/MC/MCSectionWasm.cpp
@@ -70,6 +70,8 @@ void MCSectionWasm::printSwitchToSection(const MCAsmInfo &MAI, const Triple &T,
OS << 'S';
if (SegmentFlags & wasm::WASM_SEG_FLAG_TLS)
OS << 'T';
+ if (SegmentFlags & wasm::WASM_SEG_FLAG_RETAIN)
+ OS << 'R';
OS << '"';
diff --git a/llvm/lib/ObjectYAML/WasmYAML.cpp b/llvm/lib/ObjectYAML/WasmYAML.cpp
index 3b53788eddbabc..544a91d03dce01 100644
--- a/llvm/lib/ObjectYAML/WasmYAML.cpp
+++ b/llvm/lib/ObjectYAML/WasmYAML.cpp
@@ -561,6 +561,7 @@ void ScalarBitSetTraits<WasmYAML::SegmentFlags>::bitset(
#define BCase(X) IO.bitSetCase(Value, #X, wasm::WASM_SEG_FLAG_##X)
BCase(STRINGS);
BCase(TLS);
+ BCase(RETAIN);
#undef BCase
}
diff --git a/llvm/test/CodeGen/WebAssembly/no-strip.ll b/llvm/test/CodeGen/WebAssembly/no-strip.ll
new file mode 100644
index 00000000000000..e6206aebdce8e2
--- /dev/null
+++ b/llvm/test/CodeGen/WebAssembly/no-strip.ll
@@ -0,0 +1,22 @@
+; RUN: llc < %s --mtriple=wasm32-unknown-unknown | FileCheck %s
+
+ at llvm.used = appending global [
+ 5 x ptr
+] [
+ ptr @ga, ptr @gb, ptr @gc, ptr @gd, ptr @ge
+], section "llvm.metadata"
+
+; CHECK: .section .data.ga,"R",@
+ at ga = global i32 42
+; CHECK: .section .data.gb,"R",@
+ at gb = internal global i32 41
+; CHECK: .section .data..Lgc,"R",@
+ at gc = private global i32 40
+; CHECK: .section .rodata.gd,"R",@
+ at gd = constant i32 39
+
+; All sections with the same explicit name are flagged as retained if a part of them is retained.
+; CHECK: .section dddd,"R",@
+ at ge = global i32 38, section "dddd"
+; CHECK: .section dddd,"R",@
+ at gg = global i32 37, section "dddd"
diff --git a/llvm/test/MC/WebAssembly/no-dead-strip.ll b/llvm/test/MC/WebAssembly/no-dead-strip.ll
index 9b550ec6cefbc2..6b3f090d9cab8f 100644
--- a/llvm/test/MC/WebAssembly/no-dead-strip.ll
+++ b/llvm/test/MC/WebAssembly/no-dead-strip.ll
@@ -1,21 +1,69 @@
-; RUN: llc -filetype=obj -wasm-keep-registers %s -o - | llvm-readobj --symbols - | FileCheck %s
+; RUN: llc < %s --mtriple=wasm32-unknown-unknown -filetype=obj -wasm-keep-registers -o - | obj2yaml - | FileCheck %s
-target triple = "wasm32-unknown-unknown"
-
- at llvm.used = appending global [1 x ptr] [ptr @foo], section "llvm.metadata"
+ at llvm.used = appending global [5 x ptr] [
+ ptr @foo, ptr @gv0, ptr @gv1, ptr @gv2, ptr @gv3
+], section "llvm.metadata"
define i32 @foo() {
entry:
ret i32 0
}
-; CHECK: Symbols [
-; CHECK-NEXT: Symbol {
-; CHECK-NEXT: Name: foo
-; CHECK-NEXT: Type: FUNCTION (0x0)
-; CHECK-NEXT: Flags [ (0x80)
-; CHECK-NEXT: NO_STRIP (0x80)
-; CHECK-NEXT: ]
-; CHECK-NEXT: ElementIndex: 0x0
-; CHECK-NEXT: }
-; CHECK-NEXT: ]
+; externally visible GV has NO_STRIP/RETAIN in both symtab entry and segment info
+ at gv0 = global i32 42
+; internal GV has NO_STRIP/RETAIN in both symtab entry and segment info
+ at gv1 = internal global i32 41
+; private GV has RETAIN in segment info only (no symtab entry)
+ at gv2 = private global i32 40
+; explicit section names
+ at gv3 = global i32 39, section "ddd.hello"
+ at gv4.not.used = global i64 38, section "ddd.hello"
+
+; CHECK: SymbolTable:
+; CHECK-NEXT: - Index: 0
+; CHECK-NEXT: Kind: FUNCTION
+; CHECK-NEXT: Name: foo
+; CHECK-NEXT: Flags: [ NO_STRIP ]
+; CHECK-NEXT: Function: 0
+; CHECK-NEXT: - Index: 1
+; CHECK-NEXT: Kind: DATA
+; CHECK-NEXT: Name: gv0
+; CHECK-NEXT: Flags: [ NO_STRIP ]
+; CHECK-NEXT: Segment: 0
+; CHECK-NEXT: Size: 4
+; CHECK-NEXT: - Index: 2
+; CHECK-NEXT: Kind: DATA
+; CHECK-NEXT: Name: gv1
+; CHECK-NEXT: Flags: [ BINDING_LOCAL, NO_STRIP ]
+; CHECK-NEXT: Segment: 1
+; CHECK-NEXT: Size: 4
+; CHECK-NEXT: - Index: 3
+; CHECK-NEXT: Kind: DATA
+; CHECK-NEXT: Name: gv3
+; CHECK-NEXT: Flags: [ NO_STRIP ]
+; CHECK-NEXT: Segment: 3
+; CHECK-NEXT: Size: 4
+; CHECK-NEXT: - Index: 4
+; CHECK-NEXT: Kind: DATA
+; CHECK-NEXT: Name: gv4.not.used
+; CHECK-NEXT: Flags: [ ]
+; CHECK-NEXT: Segment: 3
+; CHECK-NEXT: Offset: 8
+; CHECK-NEXT: Size: 8
+; CHECK-NEXT: SegmentInfo:
+; CHECK-NEXT: - Index: 0
+; CHECK-NEXT: Name: .data.gv0
+; CHECK-NEXT: Alignment: 2
+; CHECK-NEXT: Flags: [ RETAIN ]
+; CHECK-NEXT: - Index: 1
+; CHECK-NEXT: Name: .data.gv1
+; CHECK-NEXT: Alignment: 2
+; CHECK-NEXT: Flags: [ RETAIN ]
+; CHECK-NEXT: - Index: 2
+; CHECK-NEXT: Name: .data..Lgv2
+; CHECK-NEXT: Alignment: 2
+; CHECK-NEXT: Flags: [ RETAIN ]
+; CHECK-NEXT: - Index: 3
+; CHECK-NEXT: Name: ddd.hello
+; CHECK-NEXT: Alignment: 3
+; CHECK-NEXT: Flags: [ RETAIN ]
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