[X86] Fix x86-64 call *foo@tlsdesc(%rax) and support R_386_TLSGOTDESC R_386_TLS_DESC_CALL

D18885 emitted 5 bytes for call *foo@tlsdesc(%rax). It should use the
2-byte form instead and let R_X86_64_TLSDESC_CALL apply to the beginning
of the call instruction.

The 2-byte form was deliberately chosen to make ->LE and ->IE relaxation work:

    0:   48 8d 05 00 00 00 00    lea    0x0(%rip),%rax        # 7 <.text+0x7>
                         3: R_X86_64_GOTPC32_TLSDESC     a-0x4
    7:   ff 10                   callq  *(%rax)
                         7: R_X86_64_TLSDESC_CALL        a

=>

    0:   48 c7 c0 fc ff ff ff    mov    $0xfffffffffffffffc,%rax
    7:   66 90                   xchg   %ax,%ax

Also change the symbol type to STT_TLS when VK_TLSCALL or VK_TLSDESC is
seen.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D62512

llvm-svn: 361910

2 files changed, 21 insertions, 3 deletions

diff --git a/llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp b/llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
index e6939839d28..232a0659323 100644
--- a/llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
+++ b/llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp
@@ -271,6 +271,10 @@ static unsigned getRelocType32(MCContext &Ctx,
     assert(Type == RT32_32);
     assert(!IsPCRel);
     return ELF::R_386_GOTOFF;
+  case MCSymbolRefExpr::VK_TLSCALL:
+    return ELF::R_386_TLS_DESC_CALL;
+  case MCSymbolRefExpr::VK_TLSDESC:
+    return ELF::R_386_TLS_GOTDESC;
   case MCSymbolRefExpr::VK_TPOFF:
     assert(Type == RT32_32);
     assert(!IsPCRel);
diff --git a/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp b/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
index 8eb48fcb0bb..31d26d08a63 100644
--- a/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
+++ b/llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp
@@ -524,9 +524,23 @@ void X86MCCodeEmitter::emitMemModRMByte(const MCInst &MI, unsigned Op,
     // indirect register encoding, this handles addresses like [EAX].  The
     // encoding for [EBP] with no displacement means [disp32] so we handle it
     // by emitting a displacement of 0 below.
-    if (Disp.isImm() && Disp.getImm() == 0 && BaseRegNo != N86::EBP) {
-      EmitByte(ModRMByte(0, RegOpcodeField, BaseRegNo), CurByte, OS);
-      return;
+    if (BaseRegNo != N86::EBP) {
+      if (Disp.isImm() && Disp.getImm() == 0) {
+        EmitByte(ModRMByte(0, RegOpcodeField, BaseRegNo), CurByte, OS);
+        return;
+      }
+
+      // If the displacement is @tlscall, treat it as a zero.
+      if (Disp.isExpr()) {
+        auto *Sym = dyn_cast<MCSymbolRefExpr>(Disp.getExpr());
+        if (Sym && Sym->getKind() == MCSymbolRefExpr::VK_TLSCALL) {
+          // This is exclusively used by call *a@tlscall(base). The relocation
+          // (R_386_TLSCALL or R_X86_64_TLSCALL) applies to the beginning.
+          Fixups.push_back(MCFixup::create(0, Sym, FK_NONE, MI.getLoc()));
+          EmitByte(ModRMByte(0, RegOpcodeField, BaseRegNo), CurByte, OS);
+          return;
+        }
+      }
     }
 
     // Otherwise, if the displacement fits in a byte, encode as [REG+disp8].