ELFObjectWriter: Allow one unique symver per symbol

[vlad.tsyrklevich]

Summary

Only allow a single unique .symver alias per symbol. This matches the
behavior of gas. I noticed that we ignored multiple mismatched symver
directives looking at https://reviews.llvm.org/D45798

[vlad.tsyrklevich]

Ping.

[pcc]

LGTM

lib/MC/ELFObjectWriter.cpp

423

Nit: I think line 423-427 can be written with one insert:

if (Renames.insert(std::make_pair(&Symbol, Alias)).first->second != Alias) report_fatal_error(...);

Maybe a little too terse though.

[shkhln]

Was this really necessary? For what it's worth, there is now an enhancement request on the Sourceware bug tracker regarding gas behavior: https://sourceware.org/bugzilla/show_bug.cgi?id=23840.

lib/MC/ELFObjectWriter.cpp

for (const std::pair<StringRef, const MCSymbol *> &P : Asm.Symvers) {
     if (!Symbol.isUndefined() && !Rest.startswith("@@@"))
       continue;
 
     // FIXME: produce a better error message.
     if (Symbol.isUndefined() && Rest.startswith("@@") &&
         !Rest.startswith("@@@"))
       report_fatal_error("A @@ version cannot be undefined");
 
+    if (Renames.count(&Symbol) && Renames[&Symbol] != Alias)
+      report_fatal_error(llvm::Twine("Multiple symbol versions defined for ") +
+                         Symbol.getName());
+
     Renames.insert(std::make_pair(&Symbol, Alias));
   }
 }
 
 static uint8_t mergeTypeForSet(uint8_t origType, uint8_t newType) {
   uint8_t Type = newType;
 
   // Propagation rules:

test/MC/ELF/multiple-different-symver.s

+// RUN: not llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o %t 2>&1 | FileCheck %s
+
+// CHECK: Multiple symbol versions defined for foo
+
+.symver foo, foo@1
+.symver foo, foo@2

test/MC/ELF/multiple-equiv-symver.s

+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s -o - 2>&1 | FileCheck %s
+
+// CHECK-NOT: Multiple symbol versions defined for foo
+
+.symver foo, foo@1
+.symver foo, foo@1