[lld/mac] Add support for segment$start$ and segment$end$ symbols

These symbols are somewhat interesting in that they create non-existing segments, which as far as I know is the only way to create segments that don't contain any sections. Final part of part of PR50760. Like D106629, but for segments instead of sections. I'm not aware of anything that needs this in practice. Differential Revision: https://reviews.llvm.org/D106767
llvm · Jul 25, 2021 · 80caa1e · 80caa1e
1 parent afdeb43
commit 80caa1e
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Showing 5 changed files with 174 additions and 9 deletions.
diff --git a/lld/MachO/OutputSegment.cpp b/lld/MachO/OutputSegment.cpp
@@ -9,6 +9,7 @@
 #include "OutputSegment.h"
 #include "ConcatOutputSection.h"
 #include "InputSection.h"
+#include "Symbols.h"
 #include "SyntheticSections.h"
 
 #include "lld/Common/ErrorHandler.h"
@@ -145,6 +146,13 @@ void OutputSegment::sortOutputSections() {
   llvm::stable_sort(sections, compareByOrder<OutputSection *>(sectionOrder));
 }
 
+void OutputSegment::assignAddressesToStartEndSymbols() {
+  for (Defined *d : segmentStartSymbols)
+    d->value = addr;
+  for (Defined *d : segmentEndSymbols)
+    d->value = addr + vmSize;
+}
+
 void macho::sortOutputSegments() {
   llvm::stable_sort(outputSegments,
                     compareByOrder<OutputSegment *>(segmentOrder));

diff --git a/lld/MachO/OutputSegment.h b/lld/MachO/OutputSegment.h
@@ -10,7 +10,9 @@
 #define LLD_MACHO_OUTPUT_SEGMENT_H
 
 #include "OutputSection.h"
+#include "Symbols.h"
 #include "lld/Common/LLVM.h"
+#include "llvm/ADT/TinyPtrVector.h"
 
 #include <limits>
 #include <vector>
@@ -41,6 +43,7 @@ class OutputSegment {
 public:
   void addOutputSection(OutputSection *os);
   void sortOutputSections();
+  void assignAddressesToStartEndSymbols();
 
   const std::vector<OutputSection *> &getSections() const { return sections; }
   size_t numNonHiddenSections() const;
@@ -55,6 +58,9 @@ class OutputSegment {
   uint32_t initProt = 0;
   uint8_t index;
 
+  llvm::TinyPtrVector<Defined *> segmentStartSymbols;
+  llvm::TinyPtrVector<Defined *> segmentEndSymbols;
+
 private:
   std::vector<OutputSection *> sections;
 };

diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
@@ -203,6 +203,12 @@ enum class Boundary {
   End,
 };
 
+static Defined *createBoundarySymbol(const Undefined &sym) {
+  return symtab->addSynthetic(
+      sym.getName(), /*isec=*/nullptr, /*value=*/-1, /*isPrivateExtern=*/true,
+      /*includeInSymtab=*/false, /*referencedDynamically=*/false);
+}
+
 static void handleSectionBoundarySymbol(const Undefined &sym, StringRef segSect,
                                         Boundary which) {
   StringRef segName, sectName;
@@ -238,19 +244,19 @@ static void handleSectionBoundarySymbol(const Undefined &sym, StringRef segSect,
     inputSections.push_back(isec);
   }
 
-  Defined *boundarySym = symtab->addSynthetic(
-      sym.getName(), /*isec=*/nullptr, /*value=*/-1, /*isPrivateExtern=*/true,
-      /*includeInSymtab=*/false, /*referencedDynamically=*/false);
   if (which == Boundary::Start)
-    osec->sectionStartSymbols.push_back(boundarySym);
+    osec->sectionStartSymbols.push_back(createBoundarySymbol(sym));
   else
-    osec->sectionEndSymbols.push_back(boundarySym);
+    osec->sectionEndSymbols.push_back(createBoundarySymbol(sym));
 }
 
 static void handleSegmentBoundarySymbol(const Undefined &sym, StringRef segName,
                                         Boundary which) {
-  // FIXME
-  error("segment$start$ and segment$end$ symbols are not yet implemented");
+  OutputSegment *seg = getOrCreateOutputSegment(segName);
+  if (which == Boundary::Start)
+    seg->segmentStartSymbols.push_back(createBoundarySymbol(sym));
+  else
+    seg->segmentEndSymbols.push_back(createBoundarySymbol(sym));
 }
 
 void lld::macho::treatUndefinedSymbol(const Undefined &sym, StringRef source) {

diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
@@ -992,6 +992,7 @@ void Writer::finalizeAddresses() {
     addr = alignTo(addr, pageSize);
     seg->vmSize = addr - seg->addr;
     seg->fileSize = fileOff - seg->fileOff;
+    seg->assignAddressesToStartEndSymbols();
   }
 }
 

diff --git a/lld/test/MachO/start-end.s b/lld/test/MachO/start-end.s
@@ -1,11 +1,10 @@
 # REQUIRES: x86
 
-## FIXME: Add tests for segment$start$foo, segment$end$foo once implemented.
-
 # RUN: rm -rf %t; split-file %s %t
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/main.s -o %t/main.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/foo.s -o %t/foo.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/seg.s -o %t/seg.o
 
 # RUN: %lld -lSystem %t/main.o %t/foo.o -o %t.out \
 # RUN:    -rename_section __FOO __bar __BAZ __quux \
@@ -171,6 +170,101 @@
 # CHECK: [[#%.16x, TEXTSTART]]
 # CHECK-NOT: [[#%.16x, TEXTEND]]
 
+###############################################################################
+## Test segment$start and segment$end.
+
+# RUN: %lld -lSystem %t/seg.o -o %t.seg.out \
+# RUN:    -rename_segment __FOO __BAZ \
+# RUN:    -rename_segment __WHAT __FOO \
+# RUN:    -u 'segment$start$__UFLAG_SEG' \
+# RUN:    -e 'segment$start$__TEXT'
+# RUN: llvm-objdump --macho --syms %t.seg.out > %t-seg-dump.txt
+## llvm-objdump can't dump segment names; use lld-otool for this.
+# RUN: llvm-otool -l %t.seg.out | grep -A6 LC_SEGMENT >> %t-seg-dump.txt
+# RUN: llvm-objdump --macho -d --no-symbolic-operands --no-show-raw-insn %t.seg.out >> %t-seg-dump.txt
+# RUN: llvm-objdump --macho --function-starts %t.out >> %t-seg-dump.txt
+# RUN: FileCheck %s --check-prefix=SEG < %t-seg-dump.txt
+
+# SEG-LABEL: SYMBOL TABLE:
+# SEG-NOT: segment$start$__TEXT
+# SEG-NOT: segment$end$__TEXT
+# SEG-NOT: segment$start$__FOO
+# SEG-NOT: segment$end$__FOO
+# SEG-NOT: segment$start$__BAZ
+# SEG-NOT: segment$end$__BAZ
+# SEG-NOT: segment$start$__WHAT
+# SEG-NOT: segment$end$__WHAT
+# SEG-NOT: segment$start$__UFLAG_SEG
+# SEG-NOT: segment$start$__UFLAG_SEG
+# SEG: segment$start$REGULAR
+# SEG: segment$end$REGULAR
+
+# SEG:           cmd LC_SEGMENT_64
+# SEG-NEXT:  cmdsize
+# SEG-NEXT:  segname __PAGEZERO
+
+# SEG:           cmd LC_SEGMENT_64
+# SEG-NEXT:  cmdsize
+# SEG-NEXT:  segname __TEXT
+# SEG-NEXT:   vmaddr 0x[[#%x, TEXTSTART:]]
+# SEG-NEXT:   vmsize 0x[[#%x, TEXTSIZE:]]
+
+# SEG:           cmd LC_SEGMENT_64
+# SEG-NEXT:  cmdsize
+# SEG-NEXT:  segname __BAZ
+# SEG-NEXT:   vmaddr 0x[[#%x, BAZSTART:]]
+# SEG-NEXT:   vmsize 0x[[#%x, BAZSIZE:]]
+
+# SEG:           cmd LC_SEGMENT_64
+# SEG-NEXT:  cmdsize
+# SEG-NEXT:  segname __FOO
+# SEG-NEXT:   vmaddr 0x[[#%x, FOOSTART:]]
+# SEG-NEXT:   vmsize 0x[[#%x, FOOSIZE:]]
+
+# SEG:           cmd LC_SEGMENT_64
+# SEG-NEXT:  cmdsize
+# SEG-NEXT:  segname __UFLAG_SEG
+# SEG-NEXT:   vmaddr 0x[[#%x, UFLAGSTART:]]
+# SEG-NEXT:   vmsize 0x0000000000000000
+
+# SEG:           cmd LC_SEGMENT_64
+# SEG-NEXT:  cmdsize
+# SEG-NEXT:  segname ASDF
+# SEG-NEXT:   vmaddr 0x[[#%x, ASDFSTART:]]
+# SEG-NEXT:   vmsize 0x0000000000000000
+
+# SEG: _main
+
+## segment$start$__TEXT / segment$end$__TEXT
+# SEG:      [[#%x, PC1:]]:
+# SEG-SAME: leaq [[#%d, TEXTSTART - PC1 - 7]](%rip), %rax
+# SEG-NEXT: [[#%x, PC2:]]:
+# SEG-SAME: leaq [[#%d, TEXTSTART + TEXTSIZE - PC2 - 7]](%rip), %rbx
+
+## segment$start$__FOO / segment$end$__FOO, which is renamed to __BAZ
+# SEG:      [[#%x, PC3:]]:
+# SEG-SAME: leaq [[#%d, BAZSTART - PC3 - 7]](%rip), %rax
+# SEG-NEXT: [[#%x, PC4:]]:
+# SEG-SAME: leaq [[#%d, BAZSTART + BAZSIZE - PC4 - 7]](%rip), %rbx
+
+## segment$start$__BAZ / segment$end$__BAZ
+# SEG:      [[#%x, PC5:]]:
+# SEG-SAME: leaq [[#%d, BAZSTART - PC5 - 7]](%rip), %rax
+# SEG-NEXT: [[#%x, PC6:]]:
+# SEG-SAME: leaq [[#%d, BAZSTART + BAZSIZE - PC6 - 7]](%rip), %rbx
+
+## segment$start$__WHAT / segment$end$__WHAT, which is renamed to __FOO
+# SEG:      [[#%x, PC7:]]:
+# SEG-SAME: leaq [[#%d, FOOSTART - PC7 - 7]](%rip), %rax
+# SEG-NEXT: [[#%x, PC8:]]:
+# SEG-SAME: leaq [[#%d, FOOSTART + FOOSIZE - PC8 - 7]](%rip), %rbx
+
+## segment$start$ASDF / segment$end$ASDF
+# SEG:      [[#%x, PC9:]]:
+# SEG-SAME: leaq [[#%d, ASDFSTART - PC9 - 7]](%rip), %rax
+# SEG-NEXT: [[#%x, PC10:]]:
+# SEG-SAME: leaq [[#%d, ASDFSTART - PC10 - 7]](%rip), %rbx
+
 #--- order.txt
 _otherfun
 _main
@@ -272,3 +366,53 @@ section$end$ACTUAL$symbol:
 .fill 1
 
 .subsections_via_symbols
+
+#--- seg.s
+## Renamed to __BAZ,__bar by -rename_segment
+.section __FOO,__bar
+.space 1
+
+## Renamed to __FOO,__ever by -rename_segment
+.section __WHAT,__ever
+.space 1
+
+.text
+.globl segment$start$REGULAR
+segment$start$REGULAR:
+  retq
+
+.globl segment$end$REGULAR
+segment$end$REGULAR:
+  retq
+
+.globl _main
+_main:
+  movq segment$start$__TEXT@GOTPCREL(%rip), %rax
+  movq segment$end$__TEXT@GOTPCREL(%rip), %rbx
+
+  movq segment$start$__FOO@GOTPCREL(%rip), %rax
+  movq segment$end$__FOO@GOTPCREL(%rip), %rbx
+
+  movq segment$start$__BAZ@GOTPCREL(%rip), %rax
+  movq segment$end$__BAZ@GOTPCREL(%rip), %rbx
+
+  # (These two lines make ld64 crash linking the .o file.
+  # The rest of the test works with ld64 too.)
+  movq segment$start$__WHAT@GOTPCREL(%rip), %rax
+  movq segment$end$__WHAT@GOTPCREL(%rip), %rbx
+
+  # References to non-existing segments create that segment,
+  # without any sections in it.
+  movq segment$start$ASDF@GOTPCREL(%rip), %rax
+  movq segment$end$ASDF@GOTPCREL(%rip), %rbx
+
+  ## Non-undefined symbols don't create segments.
+  callq segment$start$REGULAR
+  callq segment$end$REGULAR
+
+  ret
+
+.section __TEXT,__fill
+.space 578
+
+.subsections_via_symbols