[mips][mc] Emit R_{MICRO}MIPS_JALR when expanding jal to jalr

When replacing jal with jalr, also emit '.reloc R_MIPS_JALR' (R_MICROMIPS_JALR
for micromips). The linker might then be able to turn jalr into a direct
call.
Add '-mips-jalr-reloc' to enable/disable this feature (default is true).

Differential revision: https://reviews.llvm.org/D55292

llvm-svn: 348760

1 files changed, 21 insertions, 3 deletions

diff --git a/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp b/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
index 79e0c001a63..78dfed68b92 100644
--- a/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
+++ b/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
@@ -39,6 +39,7 @@
 #include "llvm/MC/MCValue.h"
 #include "llvm/MC/SubtargetFeature.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -64,6 +65,11 @@ class MCInstrInfo;
 
 } // end namespace llvm
 
+static cl::opt<bool>
+EmitJalrReloc("mips-jalr-reloc", cl::Hidden,
+              cl::desc("MIPS: Emit R_{MICRO}MIPS_JALR relocation with jalr"),
+              cl::init(true));
+
 namespace {
 
 class MipsAssemblerOptions {
@@ -2065,9 +2071,21 @@ bool MipsAsmParser::processInstruction(MCInst &Inst, SMLoc IDLoc,
     JalrInst.addOperand(MCOperand::createReg(Mips::RA));
     JalrInst.addOperand(MCOperand::createReg(Mips::T9));
 
-    // FIXME: Add an R_(MICRO)MIPS_JALR relocation after the JALR.
-    // This relocation is supposed to be an optimization hint for the linker
-    // and is not necessary for correctness.
+    if (EmitJalrReloc) {
+      // As an optimization hint for the linker, before the JALR we add:
+      // .reloc tmplabel, R_{MICRO}MIPS_JALR, symbol
+      // tmplabel:
+      MCSymbol *TmpLabel = getContext().createTempSymbol();
+      const MCExpr *TmpExpr = MCSymbolRefExpr::create(TmpLabel, getContext());
+      const MCExpr *RelocJalrExpr =
+          MCSymbolRefExpr::create(JalSym, MCSymbolRefExpr::VK_None,
+                                  getContext(), IDLoc);
+
+      TOut.getStreamer().EmitRelocDirective(*TmpExpr,
+          inMicroMipsMode() ? "R_MICROMIPS_JALR" : "R_MIPS_JALR",
+          RelocJalrExpr, IDLoc, *STI);
+      TOut.getStreamer().EmitLabel(TmpLabel);
+    }
 
     Inst = JalrInst;
     ExpandedJalSym = true;