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

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Recommit r373168, which was reverted by r373242. This actually exposed a
boringssl bug which has been fixed for more than one month.

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.@tocbase .  For this
directive, suppressing .TOC. can be seen as a size optimization, but we
choose to drop it for simplicity and consistency.
This commit is contained in:
Fangrui Song 2019-09-29 15:26:12 +00:00
parent 5a753f8dae
commit 1aae2d93d5
3 changed files with 31 additions and 3 deletions
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3
lib/MC/ELFObjectWriter.cpp
View File

@ -615,9 +615,6 @@ bool ELFWriter::isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
return false;
}
if (Symbol.isUndefined() && !Symbol.isBindingSet())
return false;
if (Symbol.isTemporary())
return false;

22
test/MC/ELF/undef.s
View File

@ -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: ]

9
test/MC/ELF/weakref.s
View File

@ -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