Pull in r195318 from upstream llvm trunk:

  The basic problem is that some mainstream programs cannot deal with the way
  clang optimizes tail calls, as in this example:

  int foo(void);
  int bar(void) {
  return foo();
  }

  where the call is transformed to:

   calll .L0$pb
  .L0$pb:
   popl  %eax
  .Ltmp0:
   addl  $_GLOBAL_OFFSET_TABLE_+(.Ltmp0-.L0$pb), %eax
   movl  foo@GOT(%eax), %eax
   popl  %ebp
   jmpl  *%eax                   # TAILCALL

  However, the GOT references must all be resolved at dlopen() time, and so this
  approach cannot be used with lazy dynamic linking (e.g. using RTLD_LAZY), which
  usually populates the PLT with stubs that perform the actual resolving.

  This patch changes X86TargetLowering::LowerCall() to skip tail call
  optimization, if the called function is a global or external symbol.

This fixes problems with loading X.org driver modules, which could occur
when X.org was compiled on i386 with tailcall optimization on, for which
ports r312583 was committed as a workaround.  After this change, the
workaround can be removed.

MFC after:	3 days

1 files changed, 8 insertions, 14 deletions

diff --git a/contrib/llvm/lib/Target/X86/X86ISelLowering.cpp b/contrib/llvm/lib/Target/X86/X86ISelLowering.cpp
index c3e69e04bbd2..1080925551ea 100644
--- a/contrib/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/contrib/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -2449,21 +2449,15 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
       RegsToPass.push_back(std::make_pair(unsigned(X86::EBX),
                DAG.getNode(X86ISD::GlobalBaseReg, DebugLoc(), getPointerTy())));
     } else {
-      // If we are tail calling and generating PIC/GOT style code load the
-      // address of the callee into ECX. The value in ecx is used as target of
-      // the tail jump. This is done to circumvent the ebx/callee-saved problem
-      // for tail calls on PIC/GOT architectures. Normally we would just put the
-      // address of GOT into ebx and then call target@PLT. But for tail calls
-      // ebx would be restored (since ebx is callee saved) before jumping to the
-      // target@PLT.
-
-      // Note: The actual moving to ECX is done further down.
+      // If we are tail calling a global or external symbol in GOT pic mode, we
+      // cannot use a direct jump, since that would make lazy dynamic linking
+      // impossible (see PR15086).  So pretend this is not a tail call, to
+      // prevent the optimization to a jump.
       GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee);
-      if (G && !G->getGlobal()->hasHiddenVisibility() &&
-          !G->getGlobal()->hasProtectedVisibility())
-        Callee = LowerGlobalAddress(Callee, DAG);
-      else if (isa<ExternalSymbolSDNode>(Callee))
-        Callee = LowerExternalSymbol(Callee, DAG);
+      if ((G && !G->getGlobal()->hasHiddenVisibility() &&
+          !G->getGlobal()->hasProtectedVisibility()) ||
+          isa<ExternalSymbolSDNode>(Callee))
+        isTailCall = false;
     }
   }