[PowerPC] Revert r185476 and fix up TLS variant kinds

In the commit message to r185476 I wrote:

>The PowerPC-specific modifiers VK_PPC_TLSGD and VK_PPC_TLSLD
>correspond exactly to the generic modifiers VK_TLSGD and VK_TLSLD.
>This causes some confusion with the asm parser, since VK_PPC_TLSGD
>is output as @tlsgd, which is then read back in as VK_TLSGD.
>
>To avoid this confusion, this patch removes the PowerPC-specific
>modifiers and uses the generic modifiers throughout.  (The only
>drawback is that the generic modifiers are printed in upper case
>while the usual convention on PowerPC is to use lower-case modifiers.
>But this is just a cosmetic issue.)

This was unfortunately incorrect, there is is fact another,
serious drawback to using the default VK_TLSLD/VK_TLSGD
variant kinds: using these causes ELFObjectWriter::RelocNeedsGOT
to return true, which in turn causes the ELFObjectWriter to emit
an undefined reference to _GLOBAL_OFFSET_TABLE_.

This is a problem on powerpc64, because it uses the TOC instead
of the GOT, and the linker does not provide _GLOBAL_OFFSET_TABLE_,
so the symbol remains undefined.  This means shared libraries
using TLS built with the integrated assembler are currently
broken.

While the whole RelocNeedsGOT / _GLOBAL_OFFSET_TABLE_ situation
probably ought to be properly fixed at some point, for now I'm
simply reverting the r185476 commit.  Now this in turn exposes
the breakage of handling @tlsgd/@tlsld in the asm parser that
this check-in was originally intended to fix.

To avoid this regression, I'm also adding a different fix for
this problem: while common code now parses @tlsgd as VK_TLSGD,
a special hack in the asm parser translates this code to the
platform-specific VK_PPC_TLSGD that the back-end now expects.
While this is not really pretty, it's self-contained and
shouldn't hurt anything else for now.  One the underlying
problem is fixed, this hack can be reverted again.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185945 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/AsmParser/PPCAsmParser.cpp

@@ -190,6 +190,7 @@ class PPCAsmParser : public MCTargetAsmParser {
 
   const MCExpr *ExtractModifierFromExpr(const MCExpr *E,
                                         PPCMCExpr::VariantKind &Variant);
+  const MCExpr *FixupVariantKind(const MCExpr *E);
   bool ParseExpression(const MCExpr *&EVal);
 
   bool ParseOperand(SmallVectorImpl<MCParsedAsmOperand*> &Operands);
@@ -1006,6 +1007,57 @@ ExtractModifierFromExpr(const MCExpr *E,
   llvm_unreachable("Invalid expression kind!");
 }
 
+/// Find all VK_TLSGD/VK_TLSLD symbol references in expression and replace
+/// them by VK_PPC_TLSGD/VK_PPC_TLSLD.  This is necessary to avoid having
+/// _GLOBAL_OFFSET_TABLE_ created via ELFObjectWriter::RelocNeedsGOT.
+/// FIXME: This is a hack.
+const MCExpr *PPCAsmParser::
+FixupVariantKind(const MCExpr *E) {
+  MCContext &Context = getParser().getContext();
+
+  switch (E->getKind()) {
+  case MCExpr::Target:
+  case MCExpr::Constant:
+    return E;
+
+  case MCExpr::SymbolRef: {
+    const MCSymbolRefExpr *SRE = cast<MCSymbolRefExpr>(E);
+    MCSymbolRefExpr::VariantKind Variant = MCSymbolRefExpr::VK_None;
+
+    switch (SRE->getKind()) {
+    case MCSymbolRefExpr::VK_TLSGD:
+      Variant = MCSymbolRefExpr::VK_PPC_TLSGD;
+      break;
+    case MCSymbolRefExpr::VK_TLSLD:
+      Variant = MCSymbolRefExpr::VK_PPC_TLSLD;
+      break;
+    default:
+      return E;
+    }
+    return MCSymbolRefExpr::Create(&SRE->getSymbol(), Variant, Context);
+  }
+
+  case MCExpr::Unary: {
+    const MCUnaryExpr *UE = cast<MCUnaryExpr>(E);
+    const MCExpr *Sub = FixupVariantKind(UE->getSubExpr());
+    if (Sub == UE->getSubExpr())
+      return E;
+    return MCUnaryExpr::Create(UE->getOpcode(), Sub, Context);
+  }
+
+  case MCExpr::Binary: {
+    const MCBinaryExpr *BE = cast<MCBinaryExpr>(E);
+    const MCExpr *LHS = FixupVariantKind(BE->getLHS());
+    const MCExpr *RHS = FixupVariantKind(BE->getRHS());
+    if (LHS == BE->getLHS() && RHS == BE->getRHS())
+      return E;
+    return MCBinaryExpr::Create(BE->getOpcode(), LHS, RHS, Context);
+  }
+  }
+
+  llvm_unreachable("Invalid expression kind!");
+}
+
 /// Parse an expression.  This differs from the default "parseExpression"
 /// in that it handles complex \code @l/@ha \endcode modifiers.
 bool PPCAsmParser::
@@ -1013,6 +1065,8 @@ ParseExpression(const MCExpr *&EVal) {
   if (getParser().parseExpression(EVal))
     return true;
 
+  EVal = FixupVariantKind(EVal);
+
   PPCMCExpr::VariantKind Variant;
   const MCExpr *E = ExtractModifierFromExpr(EVal, Variant);
   if (E)