[llvm] Add section type to support CFI jump table relaxation. (PR #149259)

[Peter Collingbourne via llvm-commits]

https://github.com/pcc created https://github.com/llvm/llvm-project/pull/149259

For context see main pull request: #147424.


>From 19a2418bece082126f710ed5dec45e52fac03ffd Mon Sep 17 00:00:00 2001
From: Peter Collingbourne <peter at pcc.me.uk>
Date: Wed, 16 Jul 2025 23:09:42 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.6-beta.1
---
 llvm/docs/Extensions.rst                    | 20 ++++++++++++++++++++
 llvm/include/llvm/BinaryFormat/ELF.h        |  1 +
 llvm/lib/MC/MCParser/ELFAsmParser.cpp       |  4 +++-
 llvm/lib/Object/ELF.cpp                     |  1 +
 llvm/test/MC/AsmParser/llvm_section_types.s |  5 +++++
 5 files changed, 30 insertions(+), 1 deletion(-)

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,"", at llvm_lto
 .byte 1
+.section    .section9,"", at 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