[llvm-objcopy][MachO] Fix the calculation of the output size

[alexander-shaposhnikov]

Summary

Virtual sections do not contribute to the final output size.
This diff fixes the corresponding calculations in the method MachOWriter::totalSize.

Test plan: make check-all

[smeenai]

LGTM

[MaskRay]

llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp

125

virtual section is a LLVM specific abstraction. Probably use zerofill?

[alexander-shaposhnikov]

llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp

125

@MaskRay - are you referring to the message inside assert(...) ?

[MaskRay]

llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp

125

Ah, yes. Though I think using "virtual" is fine as well, but in the Mach-O context it refers to specifically zerofill.

[alexander-shaposhnikov]

Update the message inside assert(...)

[MaskRay]

LGTM.

llvm/test/tools/llvm-objcopy/MachO/virtual-section.test

+## This test verifies that a virtual section does not contribute to the final
+## output size.
+
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-objcopy %t %t.copy
+# RUN: cmp %t %t.copy
+
+--- !mach-o
+FileHeader:
+  magic:           0xFEEDFACE
+  cputype:         0x00000007
+  cpusubtype:      0x00000003
+  filetype:        0x00000001
+  ncmds:           1
+  sizeofcmds:      124
+  flags:           0x00002000
+LoadCommands:
+  - cmd:             LC_SEGMENT
+    cmdsize:         124
+    segname:         __DATA
+    vmaddr:          4096
+    vmsize:          4096
+    fileoff:         152
+    filesize:        0
+    maxprot:         7
+    initprot:        7
+    nsects:          1
+    flags:           0
+    Sections:
+      - sectname:        __bss
+        segname:         __DATA
+        addr:            0x0000000000001000
+        size:            4096
+        offset:          0x00000000
+        align:           2
+        reloff:          0x00000000
+        nreloc:          0
+        flags:           0x00000001
+        reserved1:       0x00000000
+        reserved2:       0x00000000
+        reserved3:       0x00000000
+...

llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp

if (O.FunctionStartsCommandIndex) {
     if (LinkEditDataCommand.dataoff)
       Ends.push_back(LinkEditDataCommand.dataoff +
                      LinkEditDataCommand.datasize);
   }
 
   // Otherwise, use the last section / reloction.
   for (const LoadCommand &LC : O.LoadCommands)
     for (const std::unique_ptr<Section> &S : LC.Sections) {
+      if (S->isVirtualSection()) {
+        assert((S->Offset == 0) && "Zero-fill section's offset must be zero");
+        continue;
+      }
+      assert((S->Offset != 0) &&
+             "Non-zero-fill section's offset cannot be zero");
       Ends.push_back(S->Offset + S->Size);
       if (S->RelOff)
         Ends.push_back(S->RelOff +
                        S->NReloc * sizeof(MachO::any_relocation_info));
     }
 
   if (!Ends.empty())
     return *std::max_element(Ends.begin(), Ends.end());