COFF: handle "undef - ." expressions

[• rafael]

Summary

This is another thing that the ELF implementation can do but is missing from COFF.

[ruiu]

LGTM

[• espindola]

306078

lib/MC/WinCOFFObjectWriter.cpp

if (SymB) {
     if (!B->getFragment()) {
       Asm.getContext().reportError(
           Fixup.getLoc(),
           Twine("symbol '") + B->getName() +
               "' can not be undefined in a subtraction expression");
       return;
     }
 
-    if (!A.getFragment()) {
-      Asm.getContext().reportError(
-          Fixup.getLoc(),
-          Twine("symbol '") + A.getName() +
-              "' can not be undefined in a subtraction expression");
-      return;
-    }
-
-    assert(&A.getSection() != &B->getSection() &&
-           "This doesn't need a relocation");
-
     // Offset of the symbol in the section
     int64_t OffsetOfB = Layout.getSymbolOffset(*B);
 
     // Offset of the relocation in the section
     int64_t OffsetOfRelocation =
         Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
 
     FixedValue = (OffsetOfRelocation - OffsetOfB) + Target.getConstant();

test/MC/COFF/bad-expr.s

 // RUN: not llvm-mc -filetype=obj -triple i386-pc-win32 %s 2>&1 | FileCheck %s
 
 // CHECK: symbol '__ImageBase' can not be undefined in a subtraction expression
-// CHECK: symbol '__ImageBase' can not be undefined in a subtraction expression
 
         .data
 _x:
         .long   _x-__ImageBase
-        .long   __ImageBase-_x

test/MC/COFF/cross-section-relative.s

t5:
   .align  16
 t6:
   .ascii  "\001\002"
   .zero 6
   .quad 256                     # 0x100
   .long g3-(t6+16)
   .zero 4
 
+.long foobar - .
 
 // READOBJ:  Section {
 // READOBJ:    Number: 5
 // READOBJ:    Name: .fix (2E 66 69 78 00 00 00 00)
 // READOBJ:    VirtualSize: 0x0
 // READOBJ:    VirtualAddress: 0x0
-// READOBJ:    RawDataSize: 56
+// READOBJ:    RawDataSize:
 // READOBJ:    PointerToRawData: 0xEC
-// READOBJ:    PointerToRelocations: 0x124
+// READOBJ:    PointerToRelocations:
 // READOBJ:    PointerToLineNumbers: 0x0
-// READOBJ:    RelocationCount: 6
+// READOBJ:    RelocationCount:
 // READOBJ:    LineNumberCount: 0
 // READOBJ:    Characteristics [ (0xC0500040)
 // READOBJ:      IMAGE_SCN_ALIGN_16BYTES (0x500000)
 // READOBJ:      IMAGE_SCN_CNT_INITIALIZED_DATA (0x40)
 // READOBJ:      IMAGE_SCN_MEM_READ (0x40000000)
 // READOBJ:      IMAGE_SCN_MEM_WRITE (0x80000000)
 // READOBJ:    ]
 // READOBJ:    SectionData (
 // READOBJ:      0000: 08000000 00000000 04000000 00000000  |................|
 // READOBJ:      0010: 00000000 00000000 04000000 00000000  |................|
 // READOBJ:      0020: 01020000 00000000 00010000 00000000  |................|
-// READOBJ:      0030: 04000000 00000000                    |........|
+// READOBJ:      0030: 04000000 00000000 04000000           |............|
 // READOBJ:    )
 // READOBJ:  }
 // READOBJ:  ]
 // READOBJ:  Relocations [
 // READOBJ:  Section (5) .fix {
 // READOBJ:    Relocation {
 // READOBJ:      Offset: 0x0
 // READOBJ:      Type: IMAGE_REL_AMD64_REL32 (4)

 // READOBJ:      Type: IMAGE_REL_AMD64_ADDR32NB (3)
 // READOBJ:      Symbol: g3
 // READOBJ:    }
 // READOBJ:    Relocation {
 // READOBJ:      Offset: 0x30
 // READOBJ:      Type: IMAGE_REL_AMD64_REL32 (4)
 // READOBJ:      Symbol: g3
 // READOBJ:    }
+// READOBJ:    Relocation {
+// READOBJ:      Offset: 0x38
+// READOBJ:      Type: IMAGE_REL_AMD64_REL32 (4)
+// READOBJ:      Symbol: foobar
+// READOBJ:    }