[mips] Reduce number of instructions used for loading a global symbol's value

Now `lw/sw $reg, sym+offset` pseudo instructions for global symbol `sym`
are lowering into the following three instructions.
```
lw     $reg, %got(symbol)($gp)
addiu  $reg, $reg, offset
lw/sw  $reg, 0($reg)
```

It's possible to reduce the number of instructions by taking the offset
in account in the final `lw/sw` command. This patch implements that
optimization.
```
lw     $reg, %got(symbol)($gp)
lw/sw  $reg, offset($reg)
```

Differential Revision: https://reviews.llvm.org/D66553

llvm-svn: 369756

1 files changed, 13 insertions, 5 deletions

diff --git a/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp b/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
index 09dc73b061e..5e5f7279093 100644
--- a/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
+++ b/llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
@@ -3655,17 +3655,25 @@ void MipsAsmParser::expandMemInst(MCInst &Inst, SMLoc IDLoc, MCStreamer &Out,
     if (inPicMode()) {
       // FIXME:
       // a) Fix lw/sw $reg, symbol($reg) instruction expanding.
-      // b) If expression includes offset (sym + number), do not
-      //    encode the offset into a relocation. Take it in account
-      //    in the last load/store instruction.
       // c) Check that immediates of R_MIPS_GOT16/R_MIPS_LO16 relocations
       //    do not exceed 16-bit.
       // d) Use R_MIPS_GOT_PAGE/R_MIPS_GOT_OFST relocations instead
       //    of R_MIPS_GOT_DISP in appropriate cases to reduce number
       //    of GOT entries.
-      loadAndAddSymbolAddress(OffsetOp.getExpr(), TmpReg, Mips::NoRegister,
+      MCValue Res;
+      if (!OffsetOp.getExpr()->evaluateAsRelocatable(Res, nullptr, nullptr)) {
+        Error(IDLoc, "expected relocatable expression");
+        return;
+      }
+      if (Res.getSymB() != nullptr) {
+        Error(IDLoc, "expected relocatable expression with only one symbol");
+        return;
+      }
+
+      loadAndAddSymbolAddress(Res.getSymA(), TmpReg, Mips::NoRegister,
                               !ABI.ArePtrs64bit(), IDLoc, Out, STI);
-      TOut.emitRRI(Inst.getOpcode(), DstReg, TmpReg, 0, IDLoc, STI);
+      TOut.emitRRI(Inst.getOpcode(), DstReg, TmpReg, Res.getConstant(), IDLoc,
+                   STI);
     } else {
       const MCExpr *ExprOffset = OffsetOp.getExpr();
       MCOperand LoOperand = MCOperand::createExpr(