[JITLink][ELF] Improve ELF section start/end symbol handling.

This commit adds section start and stop symbol handling to ELF/aarch64, and
fixes the section symbol prefixes (using `__start_` and `__stop_`, rather than
`__start` and `__end`). It also adds a testcase for handling of these symbols.

Author: lhames

llvm/include/llvm/ExecutionEngine/JITLink/JITLink.h

@@ -567,7 +567,7 @@ class Symbol {
   orc::ExecutorAddrDiff getOffset() const { return Offset; }
 
   void setOffset(orc::ExecutorAddrDiff NewOffset) {
-    assert(NewOffset < getBlock().getSize() && "Offset out of range");
+    assert(NewOffset <= getBlock().getSize() && "Offset out of range");
     Offset = NewOffset;
   }
 

llvm/lib/ExecutionEngine/JITLink/DefineExternalSectionStartAndEndSymbols.h

@@ -108,6 +108,25 @@ createDefineExternalSectionStartAndEndSymbolsPass(
       std::forward<SymbolIdentifierFunction>(F));
 }
 
+/// ELF section start/end symbol detection.
+inline SectionRangeSymbolDesc
+identifyELFSectionStartAndEndSymbols(LinkGraph &G, Symbol &Sym) {
+  constexpr StringRef StartSymbolPrefix = "__start_";
+  constexpr StringRef EndSymbolPrefix = "__stop_";
+
+  auto SymName = Sym.getName();
+  if (SymName.starts_with(StartSymbolPrefix)) {
+    if (auto *Sec =
+            G.findSectionByName(SymName.drop_front(StartSymbolPrefix.size())))
+      return {*Sec, true};
+  } else if (SymName.starts_with(EndSymbolPrefix)) {
+    if (auto *Sec =
+            G.findSectionByName(SymName.drop_front(EndSymbolPrefix.size())))
+      return {*Sec, false};
+  }
+  return {};
+}
+
 } // end namespace jitlink
 } // end namespace llvm
 

llvm/lib/ExecutionEngine/JITLink/ELF_aarch64.cpp

@@ -20,6 +20,8 @@
 #include "llvm/Object/ELFObjectFile.h"
 #include "llvm/Support/Endian.h"
 
+#include "DefineExternalSectionStartAndEndSymbols.h"
+
 #define DEBUG_TYPE "jitlink"
 
 using namespace llvm;
@@ -611,6 +613,11 @@ void link_ELF_aarch64(std::unique_ptr<LinkGraph> G,
     else
       Config.PrePrunePasses.push_back(markAllSymbolsLive);
 
+    // Resolve any external section start / end symbols.
+    Config.PostAllocationPasses.push_back(
+        createDefineExternalSectionStartAndEndSymbolsPass(
+            identifyELFSectionStartAndEndSymbols));
+
     // Add an in-place GOT/TLS/Stubs build pass.
     Config.PostPrunePasses.push_back(buildTables_ELF_aarch64);
   }

llvm/lib/ExecutionEngine/JITLink/ELF_x86_64.cpp

@@ -343,24 +343,6 @@ createLinkGraphFromELFObject_x86_64(MemoryBufferRef ObjectBuffer) {
       .buildGraph();
 }
 
-static SectionRangeSymbolDesc
-identifyELFSectionStartAndEndSymbols(LinkGraph &G, Symbol &Sym) {
-  constexpr StringRef StartSymbolPrefix = "__start";
-  constexpr StringRef EndSymbolPrefix = "__end";
-
-  auto SymName = Sym.getName();
-  if (SymName.starts_with(StartSymbolPrefix)) {
-    if (auto *Sec =
-            G.findSectionByName(SymName.drop_front(StartSymbolPrefix.size())))
-      return {*Sec, true};
-  } else if (SymName.starts_with(EndSymbolPrefix)) {
-    if (auto *Sec =
-            G.findSectionByName(SymName.drop_front(EndSymbolPrefix.size())))
-      return {*Sec, false};
-  }
-  return {};
-}
-
 void link_ELF_x86_64(std::unique_ptr<LinkGraph> G,
                      std::unique_ptr<JITLinkContext> Ctx) {
   PassConfiguration Config;

llvm/test/ExecutionEngine/JITLink/AArch64/ELF_section_start_and_stop_symbols.s

@@ -0,0 +1,43 @@
+# RUN: llvm-mc -triple=aarch64-unknown-linux-gnu -position-independent \
+# RUN:   -filetype=obj -o %t.o %s
+# RUN: llvm-jitlink -noexec -check %s %t.o
+
+	.text
+	.file	"elf_section_start_stop.c"
+	.globl	main
+	.p2align	2
+	.type	main,@function
+main:
+	adrp	x8, z
+	adrp	x9, y
+	ldr	w8, [x8, :lo12:z]
+	ldr	w9, [x9, :lo12:y]
+	sub	w0, w8, w9
+	ret
+.Lfunc_end0:
+	.size	main, .Lfunc_end0-main
+
+	.type	x,@object
+	.section	custom_section,"aw",@progbits
+	.globl	x
+	.p2align	2
+x:
+	.word	42
+	.size	x, 4
+
+# jitlink-check: *{8}z = (*{8}y) + 4
+
+	.type	y,@object
+	.data
+	.globl	y
+	.p2align	3, 0x0
+y:
+	.xword	__start_custom_section
+	.size	y, 8
+
+	.type	z,@object
+	.globl	z
+	.p2align	3, 0x0
+z:
+	.xword	__stop_custom_section
+	.size	z, 8