[MC] Error for .globl/.local which change the symbol binding and warn for .weak

GNU as let .weak override .globl since binutils-gdb
5ca547dc2399a0a5d9f20626d4bf5547c3ccfddd (1996) while MC lets the last
directive win (PR38921).

This caused an issue to Linux's powerpc port which has been fixed by
http://git.kernel.org/linus/968339fad422a58312f67718691b717dac45c399

Binding overriding is error-prone. This patch disallows a changed binding.
(https://sourceware.org/pipermail/binutils/2020-March/000299.html )

Our behavior regarding `.globl x; .weak x` matches GNU as. Such usage is
still suspicious but we issue a warning for now. We may upgrade it to an
error in the future.

Reviewed By: jhenderson, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D90108

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/MC/MCELFStreamer.cpp b/llvm/lib/MC/MCELFStreamer.cpp
index 5296e24..6b65314 100644
--- a/llvm/lib/MC/MCELFStreamer.cpp
+++ b/llvm/lib/MC/MCELFStreamer.cpp
@@ -225,17 +225,31 @@ bool MCELFStreamer::emitSymbolAttribute(MCSymbol *S, MCSymbolAttr Attribute) {
     break;
 
   case MCSA_Global:
+    // For `.weak x; .global x`, GNU as sets the binding to STB_WEAK while we
+    // traditionally set the binding to STB_GLOBAL. This is error-prone, so we
+    // error on such cases. Note, we also disallow changed binding from .local.
+    if (Symbol->isBindingSet() && Symbol->getBinding() != ELF::STB_GLOBAL)
+      getContext().reportError(SMLoc(), Symbol->getName() +
+                                            " changed binding to STB_GLOBAL");
     Symbol->setBinding(ELF::STB_GLOBAL);
     Symbol->setExternal(true);
     break;
 
   case MCSA_WeakReference:
   case MCSA_Weak:
+    // For `.global x; .weak x`, both MC and GNU as set the binding to STB_WEAK.
+    // We emit a warning for now but may switch to an error in the future.
+    if (Symbol->isBindingSet() && Symbol->getBinding() != ELF::STB_WEAK)
+      getContext().reportWarning(SMLoc(), Symbol->getName() +
+                                              " changed binding to STB_WEAK");
     Symbol->setBinding(ELF::STB_WEAK);
     Symbol->setExternal(true);
     break;
 
   case MCSA_Local:
+    if (Symbol->isBindingSet() && Symbol->getBinding() != ELF::STB_LOCAL)
+      getContext().reportError(SMLoc(), Symbol->getName() +
+                                            " changed binding to STB_LOCAL");
     Symbol->setBinding(ELF::STB_LOCAL);
     Symbol->setExternal(false);
     break;