[llvm] [BOLT] Retain certain local symbols (PR #184074)
YongKang Zhu via llvm-commits
llvm-commits at lists.llvm.org
Mon Mar 2 22:29:06 PST 2026
https://github.com/yozhu updated https://github.com/llvm/llvm-project/pull/184074
>From ebc6e2751392a857ade18d3280c8f7b7b8d4dac9 Mon Sep 17 00:00:00 2001
From: YongKang Zhu <yongzhu at fb.com>
Date: Wed, 18 Feb 2026 22:40:49 -0800
Subject: [PATCH 1/3] [BOLT] Retain certain local symbols
BOLT currently strips all STT_NOTYPE STB_LOCAL zero-sized symbols
that fall inside function bodies. Certain such symbols are named
labels (loop markers and subroutine entry points) or local function
symbols in hand-written assembly. We now keep them in local symbol
table in BOLT processed binaries for better symbolication.
---
bolt/lib/Rewrite/RewriteInstance.cpp | 43 +++++++++++++------
.../AArch64/compare-and-branch-inversion.S | 8 ++++
.../compare-and-branch-reorder-blocks.S | 2 +
bolt/test/AArch64/retain-local-symbols.s | 30 +++++++++++++
bolt/test/X86/avx512-trap.test | 3 +-
bolt/test/X86/dynamic-relocs-on-entry.s | 5 ++-
6 files changed, 74 insertions(+), 17 deletions(-)
create mode 100644 bolt/test/AArch64/retain-local-symbols.s
diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index e9bb5d81c80f9..f8bd956c3a8fe 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -5352,13 +5352,27 @@ void RewriteInstance::updateELFSymbolTable(
} else {
// Check if the function symbol matches address inside a function, i.e.
// it marks a secondary entry point.
+ // Also look up local NOTYPE symbols inside functions so we can
+ // update their addresses to reflect the output layout.
+ // Skip AArch64 marker symbols ($d, $x) inside functions —
+ // BOLT generates its own via addExtraSymbols.
+ auto IsAArch64MarkerSymbol = [&]() {
+ return BC->isAArch64() &&
+ (SymbolName->starts_with("$d") || SymbolName->starts_with("$x"));
+ };
+ const bool IsLocalLabel = Symbol.getType() == ELF::STT_NOTYPE &&
+ Symbol.getBinding() == ELF::STB_LOCAL &&
+ Symbol.st_size == 0 && !IsAArch64MarkerSymbol();
Function =
- (Symbol.getType() == ELF::STT_FUNC)
+ (Symbol.getType() == ELF::STT_FUNC || IsLocalLabel)
? BC->getBinaryFunctionContainingAddress(Symbol.st_value,
/*CheckPastEnd=*/false,
/*UseMaxSize=*/true)
: nullptr;
+ assert((!Function || !IsLocalLabel || !Function->isFolded()) &&
+ "Local label inside ICF-folded function");
+
if (Function && Function->isEmitted()) {
assert(Function->getLayout().isHotColdSplit() &&
"Adding symbols based on cold fragment when there are more than "
@@ -5366,6 +5380,12 @@ void RewriteInstance::updateELFSymbolTable(
const uint64_t OutputAddress =
Function->translateInputToOutputAddress(Symbol.st_value);
+ // Remove symbols that cannot be mapped to the output, e.g.
+ // data-in-code labels (jump tables) whose addresses BOLT
+ // does not track.
+ if (!OutputAddress)
+ continue;
+
NewSymbol.st_value = OutputAddress;
// Force secondary entry points to have zero size.
NewSymbol.st_size = 0;
@@ -5414,19 +5434,14 @@ void RewriteInstance::updateELFSymbolTable(
NewSymbol.st_shndx = getNewSectionIndex(Symbol.st_shndx);
}
- // Detect local syms in the text section that we didn't update
- // and that were preserved by the linker to support relocations against
- // .text. Remove them from the symtab.
- if (Symbol.getType() == ELF::STT_NOTYPE &&
- Symbol.getBinding() == ELF::STB_LOCAL && Symbol.st_size == 0) {
- if (BC->getBinaryFunctionContainingAddress(Symbol.st_value,
- /*CheckPastEnd=*/false,
- /*UseMaxSize=*/true)) {
- // Can only delete the symbol if not patching. Such symbols should
- // not exist in the dynamic symbol table.
- assert(!IsDynSym && "cannot delete symbol");
- continue;
- }
+ // Drop AArch64 marker symbols ($d, $x) inside functions —
+ // BOLT generates its own via addExtraSymbols.
+ if (IsAArch64MarkerSymbol() &&
+ BC->getBinaryFunctionContainingAddress(Symbol.st_value,
+ /*CheckPastEnd=*/false,
+ /*UseMaxSize=*/true)) {
+ assert(!IsDynSym && "cannot delete symbol");
+ continue;
}
}
}
diff --git a/bolt/test/AArch64/compare-and-branch-inversion.S b/bolt/test/AArch64/compare-and-branch-inversion.S
index f5903b2b5d25d..479775a2bbd64 100644
--- a/bolt/test/AArch64/compare-and-branch-inversion.S
+++ b/bolt/test/AArch64/compare-and-branch-inversion.S
@@ -30,8 +30,10 @@ immediate_increment:
# CHECK: <immediate_increment>:
# CHECK-NEXT: {{.*}} cblt x0, #0x1, 0x[[ADDR0:[0-9a-f]+]] <{{.*}}>
+# CHECK: <.exit0>:
# CHECK-NEXT: {{.*}} mov x0, #0x2 // =2
# CHECK-NEXT: {{.*}} ret
+# CHECK: <.cold0>:
# CHECK-NEXT: [[ADDR0]]: {{.*}} mov x0, #0x1 // =1
# CHECK-NEXT: {{.*}} ret
@@ -52,8 +54,10 @@ immediate_decrement:
# CHECK: <immediate_decrement>:
# CHECK-NEXT: {{.*}} cbhi x0, #0x0, 0x[[ADDR1:[0-9a-f]+]] <{{.*}}>
+# CHECK: <.exit1>:
# CHECK-NEXT: {{.*}} mov x0, #0x2 // =2
# CHECK-NEXT: {{.*}} ret
+# CHECK: <.cold1>:
# CHECK-NEXT: [[ADDR1]]: {{.*}} mov x0, #0x1 // =1
# CHECK-NEXT: {{.*}} ret
@@ -74,8 +78,10 @@ register_swap:
# CHECK: <register_swap>:
# CHECK-NEXT: {{.*}} cbgt x1, x0, 0x[[ADDR2:[0-9a-f]+]] <{{.*}}>
+# CHECK: <.exit2>:
# CHECK-NEXT: {{.*}} mov x0, #0x2 // =2
# CHECK-NEXT: {{.*}} ret
+# CHECK: <.cold2>:
# CHECK-NEXT: [[ADDR2]]: {{.*}} mov x0, #0x1 // =1
# CHECK-NEXT: {{.*}} ret
@@ -99,8 +105,10 @@ irreversible:
# CHECK: <irreversible>:
# CHECK-NEXT: {{.*}} cbgt x0, #0x3f, 0x[[ADDR3:[0-9a-f]+]] <{{.*}}>
# CHECK-NEXT: {{.*}} b 0x[[ADDR4:[0-9a-f]+]] <{{.*}}>
+# CHECK: <.exit3>:
# CHECK-NEXT: [[ADDR3]]: {{.*}} mov x0, #0x2 // =2
# CHECK-NEXT: {{.*}} ret
+# CHECK: <.cold3>:
# CHECK-NEXT: [[ADDR4]]: {{.*}} mov x0, #0x1 // =1
# CHECK-NEXT: {{.*}} ret
diff --git a/bolt/test/AArch64/compare-and-branch-reorder-blocks.S b/bolt/test/AArch64/compare-and-branch-reorder-blocks.S
index 464243d788c1f..0a5e75e9a4375 100644
--- a/bolt/test/AArch64/compare-and-branch-reorder-blocks.S
+++ b/bolt/test/AArch64/compare-and-branch-reorder-blocks.S
@@ -41,8 +41,10 @@ reorder_blocks:
# CHECK: <reorder_blocks>:
# CHECK-NEXT: {{.*}} cbgt x0, #0x0, 0x[[ADDR:[0-9a-f]+]] <{{.*}}>
+# CHECK: <.hot_exit>:
# CHECK-NEXT: {{.*}} mov x0, #0x2 // =2
# CHECK-NEXT: {{.*}} ret
+# CHECK: <.cold_exit>:
# CHECK-NEXT: [[ADDR]]: {{.*}} mov x0, #0x1 // =1
# CHECK-NEXT: {{.*}} ret
diff --git a/bolt/test/AArch64/retain-local-symbols.s b/bolt/test/AArch64/retain-local-symbols.s
new file mode 100644
index 0000000000000..b7c8e48d9e203
--- /dev/null
+++ b/bolt/test/AArch64/retain-local-symbols.s
@@ -0,0 +1,30 @@
+## Check that BOLT retains named local symbols (assembly labels) inside
+## functions and updates their addresses to reflect the output layout.
+
+# RUN: %clang %cflags %s -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.bolt -lite=false
+# RUN: llvm-nm -n %t.bolt | FileCheck %s
+
+# CHECK: T _start
+# CHECK: t loop_start
+# CHECK: t loop_end
+# CHECK: t helper
+
+ .text
+ .global _start
+ .type _start, %function
+_start:
+ mov x0, #10
+ bl helper
+loop_start:
+ sub x0, x0, #1
+ cbnz x0, loop_start
+loop_end:
+ ret
+
+helper:
+ add x0, x0, #1
+ ret
+
+## Force relocation mode.
+ .reloc 0, R_AARCH64_NONE
diff --git a/bolt/test/X86/avx512-trap.test b/bolt/test/X86/avx512-trap.test
index 93b02f4397cc8..8c98fa3016d57 100644
--- a/bolt/test/X86/avx512-trap.test
+++ b/bolt/test/X86/avx512-trap.test
@@ -8,7 +8,7 @@ RUN: llvm-objdump -d --disassemble-symbols=use_avx512 %t | \
RUN: FileCheck %s --check-prefix=CHECK-DIS-NO-TRAP
RUN: llvm-bolt %t --trap-avx512=1 -o %t.bolt --lite=0 2>&1 | FileCheck %s
-RUN: llvm-objdump -d --disassemble-symbols=use_avx512 %t.bolt | \
+RUN: llvm-objdump -d --disassemble-symbols=use_avx512,secondary_entry %t.bolt | \
RUN: FileCheck %s --check-prefix=CHECK-DIS
RUN: llvm-bolt %t --trap-avx512=0 -o %t.bolt --lite=0
@@ -20,6 +20,7 @@ CHECK: BOLT-WARNING: 1 function will trap on entry
## Check that we have two ud2 instructions - one per entry.
CHECK-DIS: use_avx512
CHECK-DIS-NEXT: ud2
+CHECK-DIS: <secondary_entry>:
CHECK-DIS-NEXT: ud2
## Check that we generate correct AVX-512
diff --git a/bolt/test/X86/dynamic-relocs-on-entry.s b/bolt/test/X86/dynamic-relocs-on-entry.s
index 4ec8ba4ad4460..477aece70f19e 100644
--- a/bolt/test/X86/dynamic-relocs-on-entry.s
+++ b/bolt/test/X86/dynamic-relocs-on-entry.s
@@ -5,13 +5,14 @@
# RUN: %clang %cflags -fPIC -pie %s -o %t.exe -nostdlib -Wl,-q
# RUN: llvm-bolt %t.exe -o %t.bolt > %t.out.txt
# RUN: llvm-readelf -r %t.bolt >> %t.out.txt
-# RUN: llvm-objdump --disassemble-symbols=chain %t.bolt >> %t.out.txt
+# RUN: llvm-objdump --disassemble-symbols=chain,Label %t.bolt >> %t.out.txt
# RUN: FileCheck %s --input-file=%t.out.txt
## Check if the new address in `chain` is correctly updated by BOLT
# CHECK: Relocation section '.rela.dyn' at offset 0x{{.*}} contains 1 entries:
# CHECK: {{.*}} R_X86_64_RELATIVE [[#%x,ADDR:]]
-# CHECK: [[#ADDR]]: c3 retq
+# CHECK: <Label>:
+# CHECK-NEXT: [[#ADDR]]: c3 retq
.text
.type chain, @function
chain:
>From 6443486e631fbbff8c4cf2efd1e98bae6af3fb8f Mon Sep 17 00:00:00 2001
From: YongKang Zhu <yongzhu at fb.com>
Date: Mon, 2 Mar 2026 10:34:51 -0800
Subject: [PATCH 2/3] Fix RISCV test failures
---
bolt/test/RISCV/mapping-syms-isa.test | 3 +--
bolt/test/RISCV/mapping-syms.s | 3 +--
2 files changed, 2 insertions(+), 4 deletions(-)
diff --git a/bolt/test/RISCV/mapping-syms-isa.test b/bolt/test/RISCV/mapping-syms-isa.test
index 22678af12f913..f9693058f00d1 100644
--- a/bolt/test/RISCV/mapping-syms-isa.test
+++ b/bolt/test/RISCV/mapping-syms-isa.test
@@ -13,6 +13,5 @@ CHECK: 00000002: ret
# Check .word is still present in output
CHECK-OBJDUMP: <_start>:
CHECK-OBJDUMP-NEXT: nop
-CHECK-OBJDUMP-NEXT: unimp
-CHECK-OBJDUMP-NEXT: unimp
+CHECK-OBJDUMP-NEXT: .word 0x00000000
CHECK-OBJDUMP-NEXT: ret
diff --git a/bolt/test/RISCV/mapping-syms.s b/bolt/test/RISCV/mapping-syms.s
index e8fdeb0c7572d..a29edaa80056e 100644
--- a/bolt/test/RISCV/mapping-syms.s
+++ b/bolt/test/RISCV/mapping-syms.s
@@ -15,8 +15,7 @@
/// Check .word is still present in output
// CHECK-OBJDUMP: <_start>:
// CHECK-OBJDUMP-NEXT: nop
-// CHECK-OBJDUMP-NEXT: unimp
-// CHECK-OBJDUMP-NEXT: unimp
+// CHECK-OBJDUMP-NEXT: .word 0x00000000
// CHECK-OBJDUMP-NEXT: ret
.text
.globl _start
>From 7ba89fa6e9e85ae6b319e9a6cb2a8d0d657263ad Mon Sep 17 00:00:00 2001
From: YongKang Zhu <yongzhu at fb.com>
Date: Mon, 2 Mar 2026 21:57:41 -0800
Subject: [PATCH 3/3] Use the existing marker check logic in BinaryContext, so
to consider RISC-V marker symbols as well. BOLT currently targets X86,
AArch64 and RISC-V only, and X86 doesn't have marker symbol.
---
bolt/include/bolt/Core/BinaryContext.h | 4 +++-
bolt/lib/Core/BinaryContext.cpp | 29 +++++++++++++++-----------
bolt/lib/Rewrite/RewriteInstance.cpp | 14 ++++++-------
bolt/test/RISCV/mapping-syms-isa.test | 3 ++-
bolt/test/RISCV/mapping-syms.s | 3 ++-
5 files changed, 31 insertions(+), 22 deletions(-)
diff --git a/bolt/include/bolt/Core/BinaryContext.h b/bolt/include/bolt/Core/BinaryContext.h
index 8741b3c077bdd..f8a0be0418433 100644
--- a/bolt/include/bolt/Core/BinaryContext.h
+++ b/bolt/include/bolt/Core/BinaryContext.h
@@ -887,9 +887,11 @@ class BinaryContext {
bool isRISCV() const { return TheTriple->getArch() == llvm::Triple::riscv64; }
- // AArch64-specific functions to check if symbol is used to delimit
+ // AArch64/RISC-V functions to check if symbol is used to delimit
// code/data in .text. Code is marked by $x, data by $d.
MarkerSymType getMarkerType(const SymbolRef &Symbol) const;
+ MarkerSymType getMarkerType(unsigned SymbolType, uint64_t SymbolSize,
+ StringRef SymbolName) const;
bool isMarker(const SymbolRef &Symbol) const;
/// Iterate over all BinaryData.
diff --git a/bolt/lib/Core/BinaryContext.cpp b/bolt/lib/Core/BinaryContext.cpp
index f0070b47ee52f..1f1e360c3c9f4 100644
--- a/bolt/lib/Core/BinaryContext.cpp
+++ b/bolt/lib/Core/BinaryContext.cpp
@@ -2039,35 +2039,40 @@ void BinaryContext::printCFI(raw_ostream &OS, const MCCFIInstruction &Inst) {
}
}
-MarkerSymType BinaryContext::getMarkerType(const SymbolRef &Symbol) const {
+MarkerSymType BinaryContext::getMarkerType(unsigned SymbolType,
+ uint64_t SymbolSize,
+ StringRef SymbolName) const {
// For aarch64 and riscv, the ABI defines mapping symbols so we identify data
// in the code section (see IHI0056B). $x identifies a symbol starting code or
// the end of a data chunk inside code, $d identifies start of data.
- if (isX86() || ELFSymbolRef(Symbol).getSize())
- return MarkerSymType::NONE;
-
- Expected<StringRef> NameOrError = Symbol.getName();
- Expected<object::SymbolRef::Type> TypeOrError = Symbol.getType();
-
- if (!TypeOrError || !NameOrError)
+ if (isX86() || SymbolSize)
return MarkerSymType::NONE;
- if (*TypeOrError != SymbolRef::ST_Unknown)
+ if (SymbolType != ELF::STT_NOTYPE)
return MarkerSymType::NONE;
- if (*NameOrError == "$x" || NameOrError->starts_with("$x."))
+ if (SymbolName == "$x" || SymbolName.starts_with("$x."))
return MarkerSymType::CODE;
// $x<ISA>
- if (isRISCV() && NameOrError->starts_with("$x"))
+ if (isRISCV() && SymbolName.starts_with("$x"))
return MarkerSymType::CODE;
- if (*NameOrError == "$d" || NameOrError->starts_with("$d."))
+ if (SymbolName == "$d" || SymbolName.starts_with("$d."))
return MarkerSymType::DATA;
return MarkerSymType::NONE;
}
+MarkerSymType BinaryContext::getMarkerType(const SymbolRef &Symbol) const {
+ Expected<StringRef> NameOrError = Symbol.getName();
+ if (!NameOrError)
+ return MarkerSymType::NONE;
+
+ return getMarkerType(ELFSymbolRef(Symbol).getELFType(),
+ ELFSymbolRef(Symbol).getSize(), *NameOrError);
+}
+
bool BinaryContext::isMarker(const SymbolRef &Symbol) const {
return getMarkerType(Symbol) != MarkerSymType::NONE;
}
diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index f8bd956c3a8fe..16c9012b1fee6 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -5354,15 +5354,15 @@ void RewriteInstance::updateELFSymbolTable(
// it marks a secondary entry point.
// Also look up local NOTYPE symbols inside functions so we can
// update their addresses to reflect the output layout.
- // Skip AArch64 marker symbols ($d, $x) inside functions —
+ // Skip AArch64/RISC-V marker symbols ($d, $x) inside functions —
// BOLT generates its own via addExtraSymbols.
- auto IsAArch64MarkerSymbol = [&]() {
- return BC->isAArch64() &&
- (SymbolName->starts_with("$d") || SymbolName->starts_with("$x"));
+ auto IsMarkerSymbol = [&]() {
+ return BC->getMarkerType(Symbol.getType(), Symbol.st_size,
+ *SymbolName) != MarkerSymType::NONE;
};
const bool IsLocalLabel = Symbol.getType() == ELF::STT_NOTYPE &&
Symbol.getBinding() == ELF::STB_LOCAL &&
- Symbol.st_size == 0 && !IsAArch64MarkerSymbol();
+ Symbol.st_size == 0 && !IsMarkerSymbol();
Function =
(Symbol.getType() == ELF::STT_FUNC || IsLocalLabel)
? BC->getBinaryFunctionContainingAddress(Symbol.st_value,
@@ -5434,9 +5434,9 @@ void RewriteInstance::updateELFSymbolTable(
NewSymbol.st_shndx = getNewSectionIndex(Symbol.st_shndx);
}
- // Drop AArch64 marker symbols ($d, $x) inside functions —
+ // Drop AArch64/RISC-V marker symbols ($d, $x) inside functions —
// BOLT generates its own via addExtraSymbols.
- if (IsAArch64MarkerSymbol() &&
+ if (IsMarkerSymbol() &&
BC->getBinaryFunctionContainingAddress(Symbol.st_value,
/*CheckPastEnd=*/false,
/*UseMaxSize=*/true)) {
diff --git a/bolt/test/RISCV/mapping-syms-isa.test b/bolt/test/RISCV/mapping-syms-isa.test
index f9693058f00d1..22678af12f913 100644
--- a/bolt/test/RISCV/mapping-syms-isa.test
+++ b/bolt/test/RISCV/mapping-syms-isa.test
@@ -13,5 +13,6 @@ CHECK: 00000002: ret
# Check .word is still present in output
CHECK-OBJDUMP: <_start>:
CHECK-OBJDUMP-NEXT: nop
-CHECK-OBJDUMP-NEXT: .word 0x00000000
+CHECK-OBJDUMP-NEXT: unimp
+CHECK-OBJDUMP-NEXT: unimp
CHECK-OBJDUMP-NEXT: ret
diff --git a/bolt/test/RISCV/mapping-syms.s b/bolt/test/RISCV/mapping-syms.s
index a29edaa80056e..e8fdeb0c7572d 100644
--- a/bolt/test/RISCV/mapping-syms.s
+++ b/bolt/test/RISCV/mapping-syms.s
@@ -15,7 +15,8 @@
/// Check .word is still present in output
// CHECK-OBJDUMP: <_start>:
// CHECK-OBJDUMP-NEXT: nop
-// CHECK-OBJDUMP-NEXT: .word 0x00000000
+// CHECK-OBJDUMP-NEXT: unimp
+// CHECK-OBJDUMP-NEXT: unimp
// CHECK-OBJDUMP-NEXT: ret
.text
.globl _start
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