[llvm] r373168 - [MC] Emit unused undefined symbol even if its binding is not set

[Fangrui Song via llvm-commits]

Author: maskray
Date: Sun Sep 29 08:26:12 2019
New Revision: 373168

URL: http://llvm.org/viewvc/llvm-project?rev=373168&view=rev
Log:
[MC] Emit unused undefined symbol even if its binding is not set

For the following two cases, we currently suppress the symbols. This
patch emits them (compatible with GNU as).

* `test2_a = undef`: if `undef` is otherwise unused.
* `.hidden hidden`: if `hidden` is unused. This is the main point of the
  patch, because omitting the symbol would cause a linker semantic
  difference.

It causes a behavior change that is not compatible with GNU as:

.weakref foo1, bar1

When neither foo1 nor bar1 is used, we now emit bar1, which is arguably
more consistent.

Another change is that we will emit .TOC. for .TOC. at tocbase .  For this
directive, suppressing .TOC. can be seen as a size optimization, but we
choose to drop it for simplicity and consistency.

Modified:
    llvm/trunk/lib/MC/ELFObjectWriter.cpp
    llvm/trunk/test/MC/ELF/undef.s
    llvm/trunk/test/MC/ELF/weakref.s

Modified: llvm/trunk/lib/MC/ELFObjectWriter.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/ELFObjectWriter.cpp?rev=373168&r1=373167&r2=373168&view=diff
==============================================================================
--- llvm/trunk/lib/MC/ELFObjectWriter.cpp (original)
+++ llvm/trunk/lib/MC/ELFObjectWriter.cpp Sun Sep 29 08:26:12 2019
@@ -615,9 +615,6 @@ bool ELFWriter::isInSymtab(const MCAsmLa
     return false;
   }
 
-  if (Symbol.isUndefined() && !Symbol.isBindingSet())
-    return false;
-
   if (Symbol.isTemporary())
     return false;
 

Modified: llvm/trunk/test/MC/ELF/undef.s
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/ELF/undef.s?rev=373168&r1=373167&r2=373168&view=diff
==============================================================================
--- llvm/trunk/test/MC/ELF/undef.s (original)
+++ llvm/trunk/test/MC/ELF/undef.s Sun Sep 29 08:26:12 2019
@@ -18,6 +18,8 @@
         .text
         movsd   .Lsym8(%rip), %xmm1
 
+.hidden hidden
+
 test2_a = undef
 test2_b = undef + 1
 
@@ -41,6 +43,17 @@ test2_b = undef + 1
 // CHECK-NEXT:     Section: .rodata.str1.1
 // CHECK-NEXT:   }
 // CHECK-NEXT:   Symbol {
+// CHECK-NEXT:     Name: hidden
+// CHECK-NEXT:     Value: 0x0
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Global
+// CHECK-NEXT:     Type: None
+// CHECK-NEXT:     Other [
+// CHECK-NEXT:       STV_HIDDEN
+// CHECK-NEXT:     ]
+// CHECK-NEXT:     Section: Undefined
+// CHECK-NEXT:   }
+// CHECK-NEXT:   Symbol {
 // CHECK-NEXT:     Name: sym6
 // CHECK-NEXT:     Value: 0x0
 // CHECK-NEXT:     Size: 0
@@ -49,4 +62,13 @@ test2_b = undef + 1
 // CHECK-NEXT:     Other: 0
 // CHECK-NEXT:     Section: Undefined
 // CHECK-NEXT:   }
+// CHECK-NEXT:   Symbol {
+// CHECK-NEXT:     Name: undef
+// CHECK-NEXT:     Value: 0x0
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Global
+// CHECK-NEXT:     Type: None
+// CHECK-NEXT:     Other: 0
+// CHECK-NEXT:     Section: Undefined
+// CHECK-NEXT:   }
 // CHECK-NEXT: ]

Modified: llvm/trunk/test/MC/ELF/weakref.s
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/ELF/weakref.s?rev=373168&r1=373167&r2=373168&view=diff
==============================================================================
--- llvm/trunk/test/MC/ELF/weakref.s (original)
+++ llvm/trunk/test/MC/ELF/weakref.s Sun Sep 29 08:26:12 2019
@@ -125,6 +125,15 @@ bar15:
 // CHECK-NEXT:     Section: .text
 // CHECK-NEXT:   }
 // CHECK-NEXT:   Symbol {
+// CHECK-NEXT:     Name: bar1
+// CHECK-NEXT:     Value: 0x0
+// CHECK-NEXT:     Size: 0
+// CHECK-NEXT:     Binding: Global
+// CHECK-NEXT:     Type: None
+// CHECK-NEXT:     Other: 0
+// CHECK-NEXT:     Section: Undefined
+// CHECK-NEXT:   }
+// CHECK-NEXT:   Symbol {
 // CHECK-NEXT:     Name: bar10
 // CHECK-NEXT:     Value: 0x28
 // CHECK-NEXT:     Size: 0