[PowerPC] Extend .reloc directive on PowerPC

When the compiler generates a GOT indirect load it must generate two loads. One
that loads the address of the element from the GOT and a second to load the
actual element based on the address just loaded from the GOT. However, the
linker can optimize these two loads into one load if it knows that it is safe
to do so. The compiler can tell the linker that the optimization is safe
by using the R_PPC64_PCREL_OPT relocation.

This patch extends the .reloc directive to allow the following setup

  pld 3, vec@got@pcrel(0), 1
.Lpcrel1=.-8
      ... More instructions possible here ...
.reloc .Lpcrel1,R_PPC64_PCREL_OPT,.-.Lpcrel1
  lwa 3, 4(3)

Reviewers: nemanjai, lei, hfinkel, sfertile, efriedma, tstellar, grosbach, MaskRay

Reviewed By: nemanjai, MaskRay

Differential Revision: https://reviews.llvm.org/D79625

Author: stefanp-synopsys

llvm/include/llvm/BinaryFormat/ELFRelocs/PowerPC64.def

@@ -97,6 +97,7 @@
 #undef R_PPC64_DTPREL16_HIGH
 #undef R_PPC64_DTPREL16_HIGHA
 #undef R_PPC64_REL24_NOTOC
+#undef R_PPC64_PCREL_OPT
 #undef R_PPC64_PCREL34
 #undef R_PPC64_GOT_PCREL34
 #undef R_PPC64_IRELATIVE
@@ -194,6 +195,7 @@ ELF_RELOC(R_PPC64_TPREL16_HIGHA,        113)
 ELF_RELOC(R_PPC64_DTPREL16_HIGH,        114)
 ELF_RELOC(R_PPC64_DTPREL16_HIGHA,       115)
 ELF_RELOC(R_PPC64_REL24_NOTOC,          116)
+ELF_RELOC(R_PPC64_PCREL_OPT,            123)
 ELF_RELOC(R_PPC64_PCREL34,              132)
 ELF_RELOC(R_PPC64_GOT_PCREL34,          133)
 ELF_RELOC(R_PPC64_IRELATIVE,            248)

llvm/lib/MC/MCObjectStreamer.cpp

@@ -665,6 +665,68 @@ void MCObjectStreamer::emitGPRel64Value(const MCExpr *Value) {
   DF->getContents().resize(DF->getContents().size() + 8, 0);
 }
 
+static Optional<std::pair<bool, std::string>>
+getOffsetAndDataFragment(const MCSymbol &Symbol, uint32_t &RelocOffset,
+                         MCDataFragment *&DF) {
+  if (Symbol.isVariable()) {
+    const MCExpr *SymbolExpr = Symbol.getVariableValue();
+    MCValue OffsetVal;
+    if(!SymbolExpr->evaluateAsRelocatable(OffsetVal, nullptr, nullptr))
+      return std::make_pair(false,
+                            std::string("symbol in .reloc offset is not "
+                                        "relocatable"));
+    if (OffsetVal.isAbsolute()) {
+      RelocOffset = OffsetVal.getConstant();
+      MCFragment *Fragment = Symbol.getFragment();
+      // FIXME Support symbols with no DF. For example:
+      // .reloc .data, ENUM_VALUE, <some expr>
+      if (!Fragment || Fragment->getKind() != MCFragment::FT_Data)
+        return std::make_pair(false,
+                              std::string("symbol in offset has no data "
+                                          "fragment"));
+      DF = cast<MCDataFragment>(Fragment);
+      return None;
+    }
+
+    if (OffsetVal.getSymB())
+      return std::make_pair(false,
+                            std::string(".reloc symbol offset is not "
+                                        "representable"));
+
+    const MCSymbolRefExpr &SRE = cast<MCSymbolRefExpr>(*OffsetVal.getSymA());
+    if (!SRE.getSymbol().isDefined())
+      return std::make_pair(false,
+                            std::string("symbol used in the .reloc offset is "
+                                        "not defined"));
+
+    if (SRE.getSymbol().isVariable())
+      return std::make_pair(false,
+                            std::string("symbol used in the .reloc offset is "
+                                        "variable"));
+
+    MCFragment *Fragment = SRE.getSymbol().getFragment();
+    // FIXME Support symbols with no DF. For example:
+    // .reloc .data, ENUM_VALUE, <some expr>
+    if (!Fragment || Fragment->getKind() != MCFragment::FT_Data)
+      return std::make_pair(false,
+                            std::string("symbol in offset has no data "
+                                        "fragment"));
+    RelocOffset = SRE.getSymbol().getOffset() + OffsetVal.getConstant();
+    DF = cast<MCDataFragment>(Fragment);
+  } else {
+    RelocOffset = Symbol.getOffset();
+    MCFragment *Fragment = Symbol.getFragment();
+    // FIXME Support symbols with no DF. For example:
+    // .reloc .data, ENUM_VALUE, <some expr>
+    if (!Fragment || Fragment->getKind() != MCFragment::FT_Data)
+      return std::make_pair(false,
+                            std::string("symbol in offset has no data "
+                                        "fragment"));
+    DF = cast<MCDataFragment>(Fragment);
+  }
+  return None;
+}
+
 Optional<std::pair<bool, std::string>>
 MCObjectStreamer::emitRelocDirective(const MCExpr &Offset, StringRef Name,
                                      const MCExpr *Expr, SMLoc Loc,
@@ -698,10 +760,17 @@ MCObjectStreamer::emitRelocDirective(const MCExpr &Offset, StringRef Name,
                           std::string(".reloc offset is not representable"));
 
   const MCSymbolRefExpr &SRE = cast<MCSymbolRefExpr>(*OffsetVal.getSymA());
-  if (SRE.getSymbol().isDefined()) {
-    // FIXME SRE.getSymbol() may not be relative to DF.
+  const MCSymbol &Symbol = SRE.getSymbol();
+  if (Symbol.isDefined()) {
+    uint32_t SymbolOffset = 0;
+    Optional<std::pair<bool, std::string>> Error;
+    Error = getOffsetAndDataFragment(Symbol, SymbolOffset, DF);
+
+    if (Error != None)
+      return Error;
+
     DF->getFixups().push_back(
-        MCFixup::create(SRE.getSymbol().getOffset() + OffsetVal.getConstant(),
+        MCFixup::create(SymbolOffset + OffsetVal.getConstant(),
                         Expr, Kind, Loc));
     return None;
   }