Add section type to support CFI jump table relaxation. #149259
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Created using spr 1.3.6-beta.1
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@llvm/pr-subscribers-mc Author: Peter Collingbourne (pcc) ChangesFor context see main pull request: #147424. Full diff: https://github.com/llvm/llvm-project/pull/149259.diff 5 Files Affected:
diff --git a/llvm/docs/Extensions.rst b/llvm/docs/Extensions.rst
index bad72c6ca8295..d8fb87b6998ad 100644
--- a/llvm/docs/Extensions.rst
+++ b/llvm/docs/Extensions.rst
@@ -581,6 +581,26 @@ This section stores pairs of (jump table address, number of entries).
This information is useful for tools that need to statically reconstruct
the control flow of executables.
+``SHT_LLVM_CFI_JUMP_TABLE`` Section (CFI jump table)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+This section contains the instructions that make up a `CFI jump table`_.
+It is expected to be ``SHF_ALLOC`` and may be laid out like a normal
+section. The ``SHT_LLVM_CFI_JUMP_TABLE`` section type gives the linker
+permission to modify the section in ways that would not normally be
+permitted, in order to optimize calls via the jump table.
+
+Each ``sh_entsize`` sized slice of a section of this type containing
+exactly one relocation may be considered to be a jump table entry
+that branches to the target of the relocation. This allows the linker
+to replace the jump table entry with the function body if it is small
+enough, or if the function is the last function in the jump table.
+
+A section of this type does not have to be placed according to its
+name. The linker may place the section in whichever output section it
+sees fit (generally the section that would provide the best locality).
+
+.. _CFI jump table: https://clang.llvm.org/docs/ControlFlowIntegrityDesign.html#forward-edge-cfi-for-indirect-function-calls
+
CodeView-Dependent
------------------
diff --git a/llvm/include/llvm/BinaryFormat/ELF.h b/llvm/include/llvm/BinaryFormat/ELF.h
index ebb257ab33821..b9e38801d880f 100644
--- a/llvm/include/llvm/BinaryFormat/ELF.h
+++ b/llvm/include/llvm/BinaryFormat/ELF.h
@@ -1159,6 +1159,7 @@ enum : unsigned {
SHT_LLVM_OFFLOADING = 0x6fff4c0b, // LLVM device offloading data.
SHT_LLVM_LTO = 0x6fff4c0c, // .llvm.lto for fat LTO.
SHT_LLVM_JT_SIZES = 0x6fff4c0d, // LLVM jump tables sizes.
+ SHT_LLVM_CFI_JUMP_TABLE = 0x6fff4c0e, // LLVM CFI jump table.
// Android's experimental support for SHT_RELR sections.
// https://android.googlesource.com/platform/bionic/+/b7feec74547f84559a1467aca02708ff61346d2a/libc/include/elf.h#512
SHT_ANDROID_RELR = 0x6fffff00, // Relocation entries; only offsets.
diff --git a/llvm/lib/MC/MCParser/ELFAsmParser.cpp b/llvm/lib/MC/MCParser/ELFAsmParser.cpp
index ec8b40261a6ca..c7c3df330fc94 100644
--- a/llvm/lib/MC/MCParser/ELFAsmParser.cpp
+++ b/llvm/lib/MC/MCParser/ELFAsmParser.cpp
@@ -571,7 +571,7 @@ bool ELFAsmParser::parseSectionArguments(bool IsPush, SMLoc loc) {
return TokError("expected end of directive");
}
- if (Mergeable)
+ if (Mergeable || TypeName == "llvm_cfi_jump_table")
if (parseMergeSize(Size))
return true;
if (Flags & ELF::SHF_LINK_ORDER)
@@ -637,6 +637,8 @@ bool ELFAsmParser::parseSectionArguments(bool IsPush, SMLoc loc) {
Type = ELF::SHT_LLVM_LTO;
else if (TypeName == "llvm_jt_sizes")
Type = ELF::SHT_LLVM_JT_SIZES;
+ else if (TypeName == "llvm_cfi_jump_table")
+ Type = ELF::SHT_LLVM_CFI_JUMP_TABLE;
else if (TypeName.getAsInteger(0, Type))
return TokError("unknown section type");
}
diff --git a/llvm/lib/Object/ELF.cpp b/llvm/lib/Object/ELF.cpp
index ff99dc25abe07..28a7040ea8d56 100644
--- a/llvm/lib/Object/ELF.cpp
+++ b/llvm/lib/Object/ELF.cpp
@@ -321,6 +321,7 @@ StringRef llvm::object::getELFSectionTypeName(uint32_t Machine, unsigned Type) {
STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_OFFLOADING);
STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_LTO);
STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_JT_SIZES)
+ STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_CFI_JUMP_TABLE)
STRINGIFY_ENUM_CASE(ELF, SHT_GNU_ATTRIBUTES);
STRINGIFY_ENUM_CASE(ELF, SHT_GNU_HASH);
STRINGIFY_ENUM_CASE(ELF, SHT_GNU_verdef);
diff --git a/llvm/test/MC/AsmParser/llvm_section_types.s b/llvm/test/MC/AsmParser/llvm_section_types.s
index 147b1499d2b88..1e644ced853b3 100644
--- a/llvm/test/MC/AsmParser/llvm_section_types.s
+++ b/llvm/test/MC/AsmParser/llvm_section_types.s
@@ -17,6 +17,8 @@
.byte 1
.section .section8,"",@llvm_lto
.byte 1
+.section .section9,"",@llvm_cfi_jump_table,1
+.byte 1
# CHECK: Name: .section1
# CHECK-NEXT: Type: SHT_LLVM_BB_ADDR_MAP
@@ -34,3 +36,6 @@
# CHECK-NEXT: Type: SHT_LLVM_OFFLOADING
# CHECK: Name: .section8
# CHECK-NEXT: Type: SHT_LLVM_LTO
+# CHECK: Name: .section9
+# CHECK-NEXT: Type: SHT_LLVM_CFI_JUMP_TABLE
+# CHECK: EntrySize: 1
|
|
@llvm/pr-subscribers-llvm-binary-utilities Author: Peter Collingbourne (pcc) ChangesFor context see main pull request: #147424. Full diff: https://github.com/llvm/llvm-project/pull/149259.diff 5 Files Affected:
diff --git a/llvm/docs/Extensions.rst b/llvm/docs/Extensions.rst
index bad72c6ca8295..d8fb87b6998ad 100644
--- a/llvm/docs/Extensions.rst
+++ b/llvm/docs/Extensions.rst
@@ -581,6 +581,26 @@ This section stores pairs of (jump table address, number of entries).
This information is useful for tools that need to statically reconstruct
the control flow of executables.
+``SHT_LLVM_CFI_JUMP_TABLE`` Section (CFI jump table)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+This section contains the instructions that make up a `CFI jump table`_.
+It is expected to be ``SHF_ALLOC`` and may be laid out like a normal
+section. The ``SHT_LLVM_CFI_JUMP_TABLE`` section type gives the linker
+permission to modify the section in ways that would not normally be
+permitted, in order to optimize calls via the jump table.
+
+Each ``sh_entsize`` sized slice of a section of this type containing
+exactly one relocation may be considered to be a jump table entry
+that branches to the target of the relocation. This allows the linker
+to replace the jump table entry with the function body if it is small
+enough, or if the function is the last function in the jump table.
+
+A section of this type does not have to be placed according to its
+name. The linker may place the section in whichever output section it
+sees fit (generally the section that would provide the best locality).
+
+.. _CFI jump table: https://clang.llvm.org/docs/ControlFlowIntegrityDesign.html#forward-edge-cfi-for-indirect-function-calls
+
CodeView-Dependent
------------------
diff --git a/llvm/include/llvm/BinaryFormat/ELF.h b/llvm/include/llvm/BinaryFormat/ELF.h
index ebb257ab33821..b9e38801d880f 100644
--- a/llvm/include/llvm/BinaryFormat/ELF.h
+++ b/llvm/include/llvm/BinaryFormat/ELF.h
@@ -1159,6 +1159,7 @@ enum : unsigned {
SHT_LLVM_OFFLOADING = 0x6fff4c0b, // LLVM device offloading data.
SHT_LLVM_LTO = 0x6fff4c0c, // .llvm.lto for fat LTO.
SHT_LLVM_JT_SIZES = 0x6fff4c0d, // LLVM jump tables sizes.
+ SHT_LLVM_CFI_JUMP_TABLE = 0x6fff4c0e, // LLVM CFI jump table.
// Android's experimental support for SHT_RELR sections.
// https://android.googlesource.com/platform/bionic/+/b7feec74547f84559a1467aca02708ff61346d2a/libc/include/elf.h#512
SHT_ANDROID_RELR = 0x6fffff00, // Relocation entries; only offsets.
diff --git a/llvm/lib/MC/MCParser/ELFAsmParser.cpp b/llvm/lib/MC/MCParser/ELFAsmParser.cpp
index ec8b40261a6ca..c7c3df330fc94 100644
--- a/llvm/lib/MC/MCParser/ELFAsmParser.cpp
+++ b/llvm/lib/MC/MCParser/ELFAsmParser.cpp
@@ -571,7 +571,7 @@ bool ELFAsmParser::parseSectionArguments(bool IsPush, SMLoc loc) {
return TokError("expected end of directive");
}
- if (Mergeable)
+ if (Mergeable || TypeName == "llvm_cfi_jump_table")
if (parseMergeSize(Size))
return true;
if (Flags & ELF::SHF_LINK_ORDER)
@@ -637,6 +637,8 @@ bool ELFAsmParser::parseSectionArguments(bool IsPush, SMLoc loc) {
Type = ELF::SHT_LLVM_LTO;
else if (TypeName == "llvm_jt_sizes")
Type = ELF::SHT_LLVM_JT_SIZES;
+ else if (TypeName == "llvm_cfi_jump_table")
+ Type = ELF::SHT_LLVM_CFI_JUMP_TABLE;
else if (TypeName.getAsInteger(0, Type))
return TokError("unknown section type");
}
diff --git a/llvm/lib/Object/ELF.cpp b/llvm/lib/Object/ELF.cpp
index ff99dc25abe07..28a7040ea8d56 100644
--- a/llvm/lib/Object/ELF.cpp
+++ b/llvm/lib/Object/ELF.cpp
@@ -321,6 +321,7 @@ StringRef llvm::object::getELFSectionTypeName(uint32_t Machine, unsigned Type) {
STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_OFFLOADING);
STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_LTO);
STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_JT_SIZES)
+ STRINGIFY_ENUM_CASE(ELF, SHT_LLVM_CFI_JUMP_TABLE)
STRINGIFY_ENUM_CASE(ELF, SHT_GNU_ATTRIBUTES);
STRINGIFY_ENUM_CASE(ELF, SHT_GNU_HASH);
STRINGIFY_ENUM_CASE(ELF, SHT_GNU_verdef);
diff --git a/llvm/test/MC/AsmParser/llvm_section_types.s b/llvm/test/MC/AsmParser/llvm_section_types.s
index 147b1499d2b88..1e644ced853b3 100644
--- a/llvm/test/MC/AsmParser/llvm_section_types.s
+++ b/llvm/test/MC/AsmParser/llvm_section_types.s
@@ -17,6 +17,8 @@
.byte 1
.section .section8,"",@llvm_lto
.byte 1
+.section .section9,"",@llvm_cfi_jump_table,1
+.byte 1
# CHECK: Name: .section1
# CHECK-NEXT: Type: SHT_LLVM_BB_ADDR_MAP
@@ -34,3 +36,6 @@
# CHECK-NEXT: Type: SHT_LLVM_OFFLOADING
# CHECK: Name: .section8
# CHECK-NEXT: Type: SHT_LLVM_LTO
+# CHECK: Name: .section9
+# CHECK-NEXT: Type: SHT_LLVM_CFI_JUMP_TABLE
+# CHECK: EntrySize: 1
|
Created using spr 1.3.6-beta.1
MaskRay
approved these changes
Jul 18, 2025
Created using spr 1.3.6-beta.1
pcc
deleted the
users/pcc/spr/add-section-type-to-support-cfi-jump-table-relaxation
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llvm-sync bot
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Jul 18, 2025
For context see main pull request: #147424. Reviewers: MaskRay Reviewed By: MaskRay Pull Request: llvm/llvm-project#149259
mdfazlay
pushed a commit
to mdfazlay/llvm-project
that referenced
this pull request
Jul 18, 2025
For context see main pull request: llvm#147424. Reviewers: MaskRay Reviewed By: MaskRay Pull Request: llvm#149259
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For context see main pull request: #147424.