[lld][WebAssembly] Do not emit relocs against dead symbols #129346
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When emitting relocs with linked output (i.e. --emit-relocs) skip relocs against dead symbols (which donot appear in the output) and do not emit them.
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@llvm/pr-subscribers-lld-wasm Author: Derek Schuff (dschuff) ChangesWhen emitting relocs with linked output (i.e. --emit-relocs) Full diff: https://github.com/llvm/llvm-project/pull/129346.diff 6 Files Affected:
diff --git a/lld/test/wasm/emit-relocs.s b/lld/test/wasm/emit-relocs.s
index bd136ba810b5e..385344cb23321 100644
--- a/lld/test/wasm/emit-relocs.s
+++ b/lld/test/wasm/emit-relocs.s
@@ -27,6 +27,12 @@ foo:
.int32 0
.size foo, 4
+.section .debug_info,"",@
+.p2align 2
+.int32 unused_function
+.int32 _start
+.int32 0
+
# CHECK: - Type: CODE
# CHECK-NEXT: Relocations:
# CHECK-NEXT: - Type: R_WASM_FUNCTION_INDEX_LEB
@@ -42,6 +48,15 @@ foo:
# CHECK-NEXT: Value: 1024
# CHECK-NEXT: Content: '00000000'
+# There should be a single relocation in this section (just the live symbol)
+# CHECK-NEXT: - Type: CUSTOM
+# CHECK-NEXT: Relocations:
+# CHECK-NEXT: - Type: R_WASM_FUNCTION_OFFSET_I32
+# CHECK-NEXT: Index: 0
+# CHECK-NEXT: Offset: 0x4
+# CHECK-NEXT: Name: .debug_info
+# CHECK-NEXT: Payload: FFFFFFFF0200000000000000
+
# CHECK: - Type: CUSTOM
# CHECK-NEXT: Name: linking
# CHECK-NEXT: Version: 2
diff --git a/lld/wasm/InputChunks.cpp b/lld/wasm/InputChunks.cpp
index ccdc92f5c8d71..0558f64aa7c42 100644
--- a/lld/wasm/InputChunks.cpp
+++ b/lld/wasm/InputChunks.cpp
@@ -167,6 +167,19 @@ void InputChunk::relocate(uint8_t *buf) const {
}
}
+static bool relocIsLive(const WasmRelocation& rel, ObjFile* file) {
+ return rel.Type == R_WASM_TYPE_INDEX_LEB ||
+ file->getSymbol(rel.Index)->isLive();
+}
+
+size_t InputChunk::getNumLiveRelocations() const {
+ size_t result = 0;
+ for (const WasmRelocation& rel : relocations) {
+ if (relocIsLive(rel, file)) result ++;
+ }
+ return result;
+}
+
// Copy relocation entries to a given output stream.
// This function is used only when a user passes "-r". For a regular link,
// we consume relocations instead of copying them to an output file.
@@ -179,6 +192,8 @@ void InputChunk::writeRelocations(raw_ostream &os) const {
<< " offset=" << Twine(off) << "\n");
for (const WasmRelocation &rel : relocations) {
+ if (!relocIsLive(rel, file))
+ continue;
writeUleb128(os, rel.Type, "reloc type");
writeUleb128(os, rel.Offset + off, "reloc offset");
writeUleb128(os, file->calcNewIndex(rel), "reloc index");
diff --git a/lld/wasm/InputChunks.h b/lld/wasm/InputChunks.h
index f545449e1246f..1fe78d76631f1 100644
--- a/lld/wasm/InputChunks.h
+++ b/lld/wasm/InputChunks.h
@@ -77,6 +77,7 @@ class InputChunk {
uint32_t getInputSectionOffset() const { return inputSectionOffset; }
size_t getNumRelocations() const { return relocations.size(); }
+ size_t getNumLiveRelocations() const;
void writeRelocations(llvm::raw_ostream &os) const;
bool generateRelocationCode(raw_ostream &os) const;
diff --git a/lld/wasm/OutputSections.cpp b/lld/wasm/OutputSections.cpp
index d679d1e676479..5038cd8ea965b 100644
--- a/lld/wasm/OutputSections.cpp
+++ b/lld/wasm/OutputSections.cpp
@@ -274,7 +274,7 @@ void CustomSection::writeTo(uint8_t *buf) {
uint32_t CustomSection::getNumRelocations() const {
uint32_t count = 0;
for (const InputChunk *inputSect : inputSections)
- count += inputSect->getNumRelocations();
+ count += inputSect->getNumLiveRelocations();
return count;
}
diff --git a/lld/wasm/OutputSections.h b/lld/wasm/OutputSections.h
index fcc8cf45dc73b..4b0329dd16cf2 100644
--- a/lld/wasm/OutputSections.h
+++ b/lld/wasm/OutputSections.h
@@ -41,6 +41,7 @@ class OutputSection {
virtual void writeTo(uint8_t *buf) = 0;
virtual void finalizeContents() = 0;
virtual uint32_t getNumRelocations() const { return 0; }
+ virtual uint32_t getNumLiveRelocations() const { return getNumRelocations(); }
virtual void writeRelocations(raw_ostream &os) const {}
std::string header;
diff --git a/lld/wasm/Symbols.cpp b/lld/wasm/Symbols.cpp
index a687fd6d6c4ef..09f110d0885f6 100644
--- a/lld/wasm/Symbols.cpp
+++ b/lld/wasm/Symbols.cpp
@@ -183,7 +183,7 @@ void Symbol::markLive() {
}
uint32_t Symbol::getOutputSymbolIndex() const {
- assert(outputSymbolIndex != INVALID_INDEX);
+ assert(outputSymbolIndex != INVALID_INDEX || !isLive());
return outputSymbolIndex;
}
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@llvm/pr-subscribers-lld Author: Derek Schuff (dschuff) ChangesWhen emitting relocs with linked output (i.e. --emit-relocs) Full diff: https://github.com/llvm/llvm-project/pull/129346.diff 6 Files Affected:
diff --git a/lld/test/wasm/emit-relocs.s b/lld/test/wasm/emit-relocs.s
index bd136ba810b5e..385344cb23321 100644
--- a/lld/test/wasm/emit-relocs.s
+++ b/lld/test/wasm/emit-relocs.s
@@ -27,6 +27,12 @@ foo:
.int32 0
.size foo, 4
+.section .debug_info,"",@
+.p2align 2
+.int32 unused_function
+.int32 _start
+.int32 0
+
# CHECK: - Type: CODE
# CHECK-NEXT: Relocations:
# CHECK-NEXT: - Type: R_WASM_FUNCTION_INDEX_LEB
@@ -42,6 +48,15 @@ foo:
# CHECK-NEXT: Value: 1024
# CHECK-NEXT: Content: '00000000'
+# There should be a single relocation in this section (just the live symbol)
+# CHECK-NEXT: - Type: CUSTOM
+# CHECK-NEXT: Relocations:
+# CHECK-NEXT: - Type: R_WASM_FUNCTION_OFFSET_I32
+# CHECK-NEXT: Index: 0
+# CHECK-NEXT: Offset: 0x4
+# CHECK-NEXT: Name: .debug_info
+# CHECK-NEXT: Payload: FFFFFFFF0200000000000000
+
# CHECK: - Type: CUSTOM
# CHECK-NEXT: Name: linking
# CHECK-NEXT: Version: 2
diff --git a/lld/wasm/InputChunks.cpp b/lld/wasm/InputChunks.cpp
index ccdc92f5c8d71..0558f64aa7c42 100644
--- a/lld/wasm/InputChunks.cpp
+++ b/lld/wasm/InputChunks.cpp
@@ -167,6 +167,19 @@ void InputChunk::relocate(uint8_t *buf) const {
}
}
+static bool relocIsLive(const WasmRelocation& rel, ObjFile* file) {
+ return rel.Type == R_WASM_TYPE_INDEX_LEB ||
+ file->getSymbol(rel.Index)->isLive();
+}
+
+size_t InputChunk::getNumLiveRelocations() const {
+ size_t result = 0;
+ for (const WasmRelocation& rel : relocations) {
+ if (relocIsLive(rel, file)) result ++;
+ }
+ return result;
+}
+
// Copy relocation entries to a given output stream.
// This function is used only when a user passes "-r". For a regular link,
// we consume relocations instead of copying them to an output file.
@@ -179,6 +192,8 @@ void InputChunk::writeRelocations(raw_ostream &os) const {
<< " offset=" << Twine(off) << "\n");
for (const WasmRelocation &rel : relocations) {
+ if (!relocIsLive(rel, file))
+ continue;
writeUleb128(os, rel.Type, "reloc type");
writeUleb128(os, rel.Offset + off, "reloc offset");
writeUleb128(os, file->calcNewIndex(rel), "reloc index");
diff --git a/lld/wasm/InputChunks.h b/lld/wasm/InputChunks.h
index f545449e1246f..1fe78d76631f1 100644
--- a/lld/wasm/InputChunks.h
+++ b/lld/wasm/InputChunks.h
@@ -77,6 +77,7 @@ class InputChunk {
uint32_t getInputSectionOffset() const { return inputSectionOffset; }
size_t getNumRelocations() const { return relocations.size(); }
+ size_t getNumLiveRelocations() const;
void writeRelocations(llvm::raw_ostream &os) const;
bool generateRelocationCode(raw_ostream &os) const;
diff --git a/lld/wasm/OutputSections.cpp b/lld/wasm/OutputSections.cpp
index d679d1e676479..5038cd8ea965b 100644
--- a/lld/wasm/OutputSections.cpp
+++ b/lld/wasm/OutputSections.cpp
@@ -274,7 +274,7 @@ void CustomSection::writeTo(uint8_t *buf) {
uint32_t CustomSection::getNumRelocations() const {
uint32_t count = 0;
for (const InputChunk *inputSect : inputSections)
- count += inputSect->getNumRelocations();
+ count += inputSect->getNumLiveRelocations();
return count;
}
diff --git a/lld/wasm/OutputSections.h b/lld/wasm/OutputSections.h
index fcc8cf45dc73b..4b0329dd16cf2 100644
--- a/lld/wasm/OutputSections.h
+++ b/lld/wasm/OutputSections.h
@@ -41,6 +41,7 @@ class OutputSection {
virtual void writeTo(uint8_t *buf) = 0;
virtual void finalizeContents() = 0;
virtual uint32_t getNumRelocations() const { return 0; }
+ virtual uint32_t getNumLiveRelocations() const { return getNumRelocations(); }
virtual void writeRelocations(raw_ostream &os) const {}
std::string header;
diff --git a/lld/wasm/Symbols.cpp b/lld/wasm/Symbols.cpp
index a687fd6d6c4ef..09f110d0885f6 100644
--- a/lld/wasm/Symbols.cpp
+++ b/lld/wasm/Symbols.cpp
@@ -183,7 +183,7 @@ void Symbol::markLive() {
}
uint32_t Symbol::getOutputSymbolIndex() const {
- assert(outputSymbolIndex != INVALID_INDEX);
+ assert(outputSymbolIndex != INVALID_INDEX || !isLive());
return outputSymbolIndex;
}
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✅ With the latest revision this PR passed the C/C++ code formatter. |
sbc100
approved these changes
Mar 1, 2025
lld/wasm/InputChunks.cpp
Outdated
| if (relocIsLive(rel, file)) | ||
| result++; | ||
| } | ||
| return result; |
There was a problem hiding this comment.
Is there a modern C++ way to write an accumulator like this?
There was a problem hiding this comment.
I think the alternative would be something like
return std::count_if(relocations.begin(), relocations.end(),
[&](const WasmRelocation &rel) {
return relocIsLive(rel, file);
});
That's... a little shorter, I guess?
sbc100
changed the title
jph-13
pushed a commit
to jph-13/llvm-project
that referenced
this pull request
Mar 21, 2025
When emitting relocs with linked output (i.e. --emit-relocs) skip relocs against dead symbols (which do not appear in the output) and do not emit them.
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When emitting relocs with linked output (i.e. --emit-relocs)
skip relocs against dead symbols (which do not appear in the output)
and do not emit them.