[LLD][ELF] Support OVERLAY symbol assignments

[CompilerWang]

This enables symbol assignment within OVERLAY linker script descriptions. This is a valuable addition to the OVERLAY syntax, as it allows defining symbols that can be used for alignment within overlay sections.

#158569

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[llvmbot]

@llvm/pr-subscribers-lld

@llvm/pr-subscribers-lld-elf

Author: None (CompilerWang)

Changes

This enables symbol assignment within OVERLAY linker script descriptions. This is a valuable addition to the OVERLAY syntax, as it allows defining symbols that can be used for alignment within overlay sections.

---

Full diff: https://github.com/llvm/llvm-project/pull/159895.diff

2 Files Affected:

• (modified) lld/ELF/ScriptParser.cpp (+7-2)
• (added) lld/test/ELF/linkerscript/overlay-symbol-assign.test (+31)

diff --git a/lld/ELF/ScriptParser.cpp b/lld/ELF/ScriptParser.cpp
index ddfa24d9cacf5..babc5cfbf43ae 100644
--- a/lld/ELF/ScriptParser.cpp
+++ b/lld/ELF/ScriptParser.cpp
@@ -996,8 +996,13 @@ OutputDesc *ScriptParser::readOverlaySectionDescription() {
       ctx.script->createOutputSection(readName(), getCurrentLocation());
   osd->osec.inOverlay = true;
   expect("{");
-  while (auto tok = till("}"))
-    osd->osec.commands.push_back(readInputSectionDescription(tok));
+  while (auto tok = till("}")) {
+    if (SymbolAssignment *assign = readAssignment(tok)) {
+      osd->osec.commands.push_back(assign);
+    } else {
+      osd->osec.commands.push_back(readInputSectionDescription(tok));
+    }
+  }
   osd->osec.phdrs = readOutputSectionPhdrs();
   return osd;
 }
diff --git a/lld/test/ELF/linkerscript/overlay-symbol-assign.test b/lld/test/ELF/linkerscript/overlay-symbol-assign.test
new file mode 100644
index 0000000000000..b48385cf22358
--- /dev/null
+++ b/lld/test/ELF/linkerscript/overlay-symbol-assign.test
@@ -0,0 +1,31 @@
+# REQUIRES: x86
+# RUN: echo 'nop; .section .small, "a"; .long 0; .section .big1, "a"; .quad 1; .section .big2, "a"; .quad 2;' \
+# RUN:   | llvm-mc -filetype=obj -triple=x86_64-unknown-linux - -o %t.o
+# RUN: ld.lld %t.o --script %s -o %t
+
+SECTIONS {
+  OVERLAY 0x1000 : AT ( 0x4000 ) {
+    .out.big { 
+      *(.big1)
+      . = ALIGN(0x100);
+      *(.big2)
+    }
+    .out.small { *(.small) }
+  }
+}
+
+## A variant of overlay.test with explicit program header assingment.
+## Check that we generate two program headers consistent with the overlay
+
+# RUN: llvm-readelf --sections -l %t | FileCheck %s
+
+# CHECK: Section Headers:
+# CHECK:      Name       Type     Address          Off    Size
+# CHECK:      .out.big   PROGBITS 0000000000001000 001000 000108
+# CHECK-NEXT: .out.small PROGBITS 0000000000001000 002000 000004
+
+
+# CHECK:      Program Headers:
+# CHECK:      Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
+# CHECK-NEXT: LOAD 0x001000 0x0000000000001000 0x0000000000004000 0x000108 0x000108 R   0x1000
+# CHECK-NEXT: LOAD 0x002000 0x0000000000001000 0x0000000000004108 0x000004 0x000004 R   0x1000
\ No newline at end of file

[CompilerWang]

@EugeneZelenko Could you please be a reviewer or add other reviewer to this PR? Thank you!

[CompilerWang]

@MaskRay Could you please be a reviewer or add other reviewer to this PR? Thank you!

[MaskRay]

delete blank lines at the end

[MaskRay]

just one blank line

[MaskRay]

Use RUN: rm -rf %t && split-file %s %t && cd %t pattern to prepare the input assembly file.
You can use #--- a.s for assembly and #--- 1.lds for the linker script.

[MaskRay]

also define a symbol sym = .? Does GNU ld accept it?