[mips] Add support for accessing $gp as a named register.

[dsanders]

Summary

Mips Linux uses $gp to hold a pointer to thread info structure and accesses it
with a named register. This makes this work for LLVM.

The N32 ABI doesn't quite work yet since the frontend generates incorrect IR
for this case. It neglects to truncate the 64-bit GPR to a 32-bit value before
converting to a pointer. Given correct IR (as in the testcase in this patch),
it works correctly.

[atanasyan]

LGTM (with a nit).

lib/Target/Mips/MipsISelLowering.h

265

Missed override keyword?

[dsanders]

Thanks.

Missed override keyword?

Oops. Fixed in the commit (r225529).

lib/Target/Mips/MipsISelLowering.h

public:
     struct LTStr {
       bool operator()(const char *S1, const char *S2) const {
         return strcmp(S1, S2) < 0;
       }
     };
 
     void HandleByVal(CCState *, unsigned &, unsigned) const override;
 
+    unsigned getRegisterByName(const char* RegName, EVT VT) const;
+
   protected:
     SDValue getGlobalReg(SelectionDAG &DAG, EVT Ty) const;
 
     // This method creates the following nodes, which are necessary for
     // computing a local symbol's address:
     //
     // (add (load (wrapper $gp, %got(sym)), %lo(sym))
     template <class NodeTy>

lib/Target/Mips/MipsISelLowering.cpp

BuildMI(*BB, BB->begin(), DL,
           TII->get(Mips::PHI), MI->getOperand(0).getReg())
     .addReg(MI->getOperand(2).getReg()).addMBB(thisMBB)
     .addReg(MI->getOperand(3).getReg()).addMBB(copy0MBB);
 
   MI->eraseFromParent();   // The pseudo instruction is gone now.
 
   return BB;
 }
+
+// FIXME? Maybe this could be a TableGen attribute on some registers and
+// this table could be generated automatically from RegInfo.
+unsigned MipsTargetLowering::getRegisterByName(const char* RegName,
+                                               EVT VT) const {
+  // Named registers is expected to be fairly rare. For now, just support $28
+  // since the linux kernel uses it.
+  if (Subtarget.isGP64bit()) {
+    unsigned Reg = StringSwitch<unsigned>(RegName)
+                         .Case("$28", Mips::GP_64)
+                         .Default(0);
+    if (Reg)
+      return Reg;
+  } else {
+    unsigned Reg = StringSwitch<unsigned>(RegName)
+                         .Case("$28", Mips::GP)
+                         .Default(0);
+    if (Reg)
+      return Reg;
+  }
+  report_fatal_error("Invalid register name global variable");
+}

test/CodeGen/Mips/named-register-n32.ll

+; RUN: llc -march=mips64 -relocation-model=static -mattr=+noabicalls,-n64,+n32 < %s | FileCheck %s
+
+define i32* @get_gp() {
+entry:
+  %0 = call i64 @llvm.read_register.i64(metadata !0)
+  %1 = trunc i64 %0 to i32
+  %2 = inttoptr i32 %1 to i32*
+  ret i32* %2
+}
+
+; CHECK-LABEL: get_gp:
+; CHECK:           sll $2, $gp, 0
+
+declare i64 @llvm.read_register.i64(metadata)
+
+!llvm.named.register.$28 = !{!0}
+
+!0 = !{!"$28"}

test/CodeGen/Mips/named-register-n64.ll

+; RUN: llc -march=mips64 -relocation-model=static -mattr=+noabicalls < %s | FileCheck %s
+
+define i32* @get_gp() {
+entry:
+  %0 = call i64 @llvm.read_register.i64(metadata !0)
+  %1 = inttoptr i64 %0 to i32*
+  ret i32* %1
+}
+
+; CHECK-LABEL: get_gp:
+; CHECK:           move $2, $gp
+
+declare i64 @llvm.read_register.i64(metadata)
+
+!llvm.named.register.$28 = !{!0}
+
+!0 = !{!"$28"}

test/CodeGen/Mips/named-register-o32.ll

+; RUN: llc -march=mips -relocation-model=static -mattr=+noabicalls < %s | FileCheck %s
+
+define i32* @get_gp() {
+entry:
+  %0 = call i32 @llvm.read_register.i32(metadata !0)
+  %1 = inttoptr i32 %0 to i32*
+  ret i32* %1
+}
+
+; CHECK-LABEL: get_gp:
+; CHECK:           move $2, $gp
+
+declare i32 @llvm.read_register.i32(metadata)
+
+!llvm.named.register.$28 = !{!0}
+
+!0 = !{!"$28"}