[mips][FastISel] Allow computation of addresses from constant expressions.

[vkalintiris]

Summary

Try to compute addresses when the offset from a memory location is a constant expression.

[dsanders]

Could you add a test case?

[vkalintiris]

Added a simple test-case.

[vkalintiris]

Modified the commit message to better reflect the change introduced with this patch.

[dsanders]

LGTM

llvm/trunk/lib/Target/Mips/MipsFastISel.cpp

bool MipsFastISel::computeAddress(const Value *Obj, Address &Addr) {
   if (const Instruction *I = dyn_cast<Instruction>(Obj)) {
     // Don't walk into other basic blocks unless the object is an alloca from
     // another block, otherwise it may not have a virtual register assigned.
     if (FuncInfo.StaticAllocaMap.count(static_cast<const AllocaInst *>(Obj)) ||
         FuncInfo.MBBMap[I->getParent()] == FuncInfo.MBB) {
       Opcode = I->getOpcode();
       U = I;
     }
-  } else if (isa<ConstantExpr>(Obj))
-    return false;
+  } else if (const ConstantExpr *C = dyn_cast<ConstantExpr>(Obj)) {
+    Opcode = C->getOpcode();
+    U = C;
+  }
   switch (Opcode) {
   default:
     break;
   case Instruction::BitCast: {
     // Look through bitcasts.
     return computeAddress(U->getOperand(0), Addr);
   }
   case Instruction::GetElementPtr: {

llvm/trunk/test/CodeGen/Mips/Fast-ISel/constexpr-address.ll

+; RUN: llc -march=mipsel -mcpu=mips32 -relocation-model=pic \
+; RUN:     -fast-isel=true -mips-fast-isel -fast-isel-abort=1 < %s | FileCheck %s
+; RUN: llc -march=mipsel -mcpu=mips32r2 -relocation-model=pic \
+; RUN:     -fast-isel=true -mips-fast-isel -fast-isel-abort=1 < %s | FileCheck %s
+
+@ARR = external global [10 x i32], align 4
+
+define void @foo() {
+; CHECK-LABEL: foo
+
+; CHECK-DAG:    lw      $[[ARR:[0-9]+]], %got(ARR)({{.*}})
+; CHECK-DAG:    addiu   $[[T0:[0-9]+]], $zero, 12345
+; CHECK:        sw      $[[T0]], 8($[[ARR]])
+
+entry:
+  store i32 12345, i32* getelementptr inbounds ([10 x i32], [10 x i32]* @ARR, i32 0, i32 2), align 4
+  ret void
+}