[MachO] Fix codegen of alias of alias.

[eugenis]

Summary

Fixes PR33316.

[pcc]

Looks like we will still crash if an alias that is referenced from an alias has a non-zero offset. Maybe that isn't so bad though, we should really be requiring aliases to be in canonical form (see D29781 and refs) in which case aliases of aliases would be invalid IR.

That wouldn't stop you from writing a .s that would crash MC, but unfortunately as far as I know it is already quite easy to crash MC right now :(

lib/MC/MCExpr.cpp

661

Please add a comment explaining what this is for.

test/CodeGen/X86/macho-alias-of-alias.ll

1 ↗

(On Diff #101658)

Can you convert this to a .s and move to test/MC?

[pcc]

LGTM, but Rafael should take a look as well.

test/MC/MachO/alias.ll

1 ↗

(On Diff #101818)

Rename to alias.s

6 ↗

(On Diff #101818)

Maybe check that we are producing a correct relocation here.

[eugenis]

r305012

lib/MC/MCExpr.cpp

if (Sym.isVariable() && SRE->getKind() == MCSymbolRefExpr::VK_None &&
         const MCSymbolRefExpr *A = Res.getSymA();
         const MCSymbolRefExpr *B = Res.getSymB();
         // FIXME: This is small hack. Given
         // a = b + 4
         // .long a
         // the OS X assembler will completely drop the 4. We should probably
         // include it in the relocation or produce an error if that is not
         // possible.
+        // Allow constant expressions.
         if (!A && !B)
           return true;
+        // Allows aliases with zero offset.
+        if (Res.getConstant() == 0 && (!A || !B))
+          return true;
       }
     }
 
     Res = MCValue::get(SRE, nullptr, 0);
     return true;
   }
 
   case Unary: {

test/MC/MachO/alias.s

+// RUN: llvm-mc -triple x86_64-apple-macosx10.12.0 %s -filetype=obj | llvm-readobj -r | FileCheck %s
+
+l_a:
+l_b = l_a
+l_c = l_b
+        .long l_c
+
+// CHECK:      Relocations [
+// CHECK-NEXT:   Section __text {
+// CHECK-NEXT:     0x0 0 2 1 X86_64_RELOC_UNSIGNED 0 l_c
+// CHECK-NEXT:   }
+// CHECK-NEXT: ]

test/MC/MachO/variable-exprs.s

 // CHECK-I386:     )
 // CHECK-I386:   }
 // CHECK-I386: ]
 // CHECK-I386: Relocations [
 // CHECK-I386:   Section __data {
 // CHECK-I386:     0x2C 0 2 0 GENERIC_RELOC_VANILLA 0 __data
 // CHECK-I386:     0x28 0 2 0 GENERIC_RELOC_VANILLA 0 __data
 // CHECK-I386:     0x24 0 2 1 GENERIC_RELOC_VANILLA 0 d3
-// CHECK-I386:     0x20 0 2 1 GENERIC_RELOC_VANILLA 0 d2
-// CHECK-I386:     0x1C 0 2 1 GENERIC_RELOC_VANILLA 0 d
+// CHECK-I386:     0x20 0 2 1 GENERIC_RELOC_VANILLA 0 d{{$}}
+// CHECK-I386:     0x1C 0 2 1 GENERIC_RELOC_VANILLA 0 d{{$}}
 // CHECK-I386:     0x18 0 2 n/a GENERIC_RELOC_VANILLA 1 0x5
 // CHECK-I386:     0x14 0 2 0 GENERIC_RELOC_VANILLA 0 __data
 // CHECK-I386:     0x10 0 2 0 GENERIC_RELOC_VANILLA 0 __data
 // CHECK-I386:     0x8 0 2 0 GENERIC_RELOC_VANILLA 0 __data
 // CHECK-I386:   }
 // CHECK-I386: ]
 // CHECK-I386: Symbols [
 // CHECK-I386:   Symbol {

 // CHECK-X86_64:     )
 // CHECK-X86_64:   }
 // CHECK-X86_64: ]
 // CHECK-X86_64: Relocations [
 // CHECK-X86_64:   Section __data {
 // CHECK-X86_64:     0x2C 0 2 1 X86_64_RELOC_UNSIGNED 0 g
 // CHECK-X86_64:     0x28 0 2 1 X86_64_RELOC_UNSIGNED 0 f
 // CHECK-X86_64:     0x24 0 2 1 X86_64_RELOC_UNSIGNED 0 d3
-// CHECK-X86_64:     0x20 0 2 1 X86_64_RELOC_UNSIGNED 0 d2
-// CHECK-X86_64:     0x1C 0 2 1 X86_64_RELOC_UNSIGNED 0 d
+// CHECK-X86_64:     0x20 0 2 1 X86_64_RELOC_UNSIGNED 0 d{{$}}
+// CHECK-X86_64:     0x1C 0 2 1 X86_64_RELOC_UNSIGNED 0 d{{$}}
 // CHECK-X86_64:     0x18 0 2 1 X86_64_RELOC_UNSIGNED 0 a
 // CHECK-X86_64:     0x14 0 2 1 X86_64_RELOC_UNSIGNED 0 e
 // CHECK-X86_64:     0x10 0 2 1 X86_64_RELOC_UNSIGNED 0 b
 // CHECK-X86_64:     0x8 0 2 1 X86_64_RELOC_UNSIGNED 0 b
 // CHECK-X86_64:   }
 // CHECK-X86_64: ]
 // CHECK-X86_64: Symbols [
 // CHECK-X86_64:   Symbol {